Welcome to this informational website designed to provide you with an in-depth look at the world of "chemical systems capable of storing and supplying electrical energy" - batteries; specifically, battery technology, charging and energy use. Primarily, this site content focuses on "wet cell" electric vehicle applications, but much of the information applies to batteries in general and will be helpful to anyone.

This site contains two technical volumes for your reference and information:

Battery Book 1. Lead Acid Traction Batteries
by Edward M. Marwell. Eugene P. Finger, and Eugene Sands
© Curtis Instruments 1981. All rights reserved.
Library of Congress Catalog card 81-65733; ISBN: 0-939488-00-0

While written in 1980, this technological primer is still as valid today as it was then. The general information of this book has become a widely read and popular, in-depth reference guide for science and industry. We hope that by publishing it on the web an even wider audience may benefit from this important educational volume.

Battery Book 2.
by Eugene P. Finger and Dr. David P. Boden
© Curtis Instruments 2007. All rights reserved.
This work focuses on constructing and modeling the lead acid battery.

Curtis Instruments, Inc. · Mount Kisco, N.Y. December 1980

The staff of Curtis Instruments. Inc ., assumes sole responsibility for the accuracy of the information in this book. In no way should it be assumed that the reviewers endorse the products referred to by company name and model in the text.

Preface

ENERGY AND FORK LIFT TRUCK BATTERIES

"Energy" is much in the news today, with good reason. Until the early 70s, the cost of electrical energy was not considered to be very important, and sometimes even went unstated in evaluating the cost of material handling operations. The "payback" of capital outlay and labor costs were the major considerations completely overshadowing the relatively miniscule costs of energy.

No longer, however, does capital outlay, or the cost of borrowed money, or of labor, etc., overshadow the cost of energy. Indeed, now, many engineering and purchasing decisions begin with an analysis of energy costs.

The reasons are obvious. Just as the shortages of gasoline and diesel fuel to operate combustion-engine vehicles have drastically increased their per-mile operating costs, so have the increased cost of generating and delivering electrical energy raised the cost of operating battery powered fork lift trucks. Every fork lift truck is a user of energy, whether that energy is derived from gasoline or from an electric grid fed by hydro-power, nuclear power, or fossil fuels. With all three categories of energy displaying strong tendencies to increasing costs in the foreseeable future, it is entirely reasonable to expect increasing concern for improving the overall efficiency with which battery-powered fork lift trucks are operated.

As developers and manufacturers of several proprietary instruments for monitoring the performance of batteries, we at Curtis have actively pursued the subject of efficiency in battery-powered vehicles.

An early example was our design of the battery state-of-charge indicator for NASA's Lunar Rover vehicle. The object in that case was to warn the astronauts so that they would not drive too far from their base station there being no means available for recharging the Rover's batteries,

A parallel program is our 933 Fuel Gage for battery-powered fork lift trucks. Here, the instrument is used to warn the driver when the truck's battery has reached the safe limit of discharge. Tens of thousands of these units are now in use on fork lift trucks throughout the world.

As we work with and listen to industry people using electric fork lift trucks, one theme emerges over and over again. "How can we minimize our energy costs?"

The purpose of this book is to assist those people in minimizing their energy costs:

• By helping them to correctly select batteries for their trucks.
• By showing them how to control the use of electrical energy in recharging their batteries.
• By helping them to avoid damaging batteries and trucks by over-discharging batteries.

The material in this book was compiled from numerous standard reference sources, from data published by manufacturers of lead acid traction batteries, from published technical papers, and from various engineering investigations carried on by Curtis as part of our ongoing study of batteries and their applications. There are, of course, no direct references to particular makes or models of truck, battery or charger. Of necessity we have generalized our examples to give them the widest possible application. Thus, where estimates of energy use, etc., are given, they are approximations based on our experience in the field and confirmed by reference to published product data. Note also that we have avoided placing specific values on capital equipment, labor, and energy.

Whenever feasible we have used nomenclature and abbreviations that conform to industry standards. In case of doubt we have relied on the standards promulgated by the IEEE/ANSI.

The text of this book has been reviewed for us by several well-known specialists in the field, and we would like to thank them individually:

Dr. David P. Boden, Vice-President Engineering and Technology, Douglas Battery Mfg. Co., Winston-Salem, N.C.

Larry E. Heisey, Battery Charging Engineer, Hobart Brothers Co., Troy, OH.

R. T. Josey, Mark C. Pope Associates, Inc., Smyrna, GA.

Service Engineering Department, Caterpillar Tractor Co. (Tow Motor), Mentor, OH.

Dr. William Reinmuth, Professor of Chemistry, Columbia University, NYC.

Section 1.

Energy, Work and the Storage Battery

Energy and Work

"Energy" is the ability to do "work," a definition that relates pretty well to the world of practical experience. Everyone knows that "it takes energy to get the work done." Energy can take many forms. In addition to human "energy," there are mechanical energy, heat energy, chemical energy, electrical energy, nuclear energy, etc.

Although each of these forms of energy is in a form slightly different from the others, all share the basic feature: each provides the ability to do work.

To turn the idea around, work is the process of "spending" energy, usually in a way that we humans call "useful," although it isn't strictly necessary that the work be useful in our sense of the word. Since nature doesn't care about usefulness, expending energy in any form at all is properly called work.

Potential energy is energy accumulated in a useful form but not yet used. A relevant example is the chemical energy accumulated in a charged storage battery. Connecting the battery to a charger - which, in turn, is connected via the power lines to a distant generating plant - stores, in chemical form, a small part of the energy output of the generating plant. It makes no difference whether the energy is generated in a hydro-electric or a steam-electric plant: the same small part of the energy output of the plant is stored, as potential energy in chemical form in the battery.

When the battery is connected to the circuits of an electric fork lift truck, its chemical energy is converted into electrical energy and released, a little at a time, to the truck, which converts it into mechanical energy in the form of useful, measurable work: moving heavy coils of wire from one end of the plant to another, stacking loaded pallets, and so on. Each expenditure of energy reduces the potential stored in the battery, and so less is then available. When all of the usable energy stored in the battery has been "used up" the battery is discharged. To get more work out of it we must recharge it by restoring its supply of energy.

Whatever the source of the energy at the power lines, it takes work to generate it, accumulate it, and transmit it, and more work to convert it into chemical form in the recharged battery. For practical purposes, all of these processes - from generating to charging - are part of the energy cost and are basic components of every electric truck operator's utility bill.

Energy and Power

Energy is the ability to do work and power is the rate at which the work is done. Lifting 55 pounds 10 feet in the air takes 550 foot-pounds of energy, and doing that much work in 1 second is 550 foot-pounds per second or 1 horsepower.

The term horsepower was invented in the 18th Century to create a practical unit for the rate of doing work. Presumably, it represents working "as fast as a horse," a rate we define as 550 foot-pounds per second. (Figure 1.)

Another unit used to represent the rate of doing work is the "watt." Because both the horsepower and the watt represent rates of doing work, they can be equated to one another, and it turns out that 1 horsepower is the same rate of work as approximately 750 watts*.

Where work and energy are concerned, only the total amount accumulated or spent matters ... not how quickly or slowly. Where power is concerned, however, the time factor enters. The faster a given amount of energy is spent, the higher the power rating; the slower the same amount of energy is spent, the lower the power rating.

• *Here and elsewhere. we have deliberately rounded values to simplify arithmetic.

Figure 1: Horsepower

Accumulating Energy

There are many ways to accumulate energy. For example, feeding our horse so that he can work - that is, deliver horsepower - is one way of accumulating energy, and so is charging a storage battery, which is actually called an accumulator in some other countries.

In accumulating energy, the important considerations are: the quantity of energy to be stored in the accumulator; the form in which the energy is accumulated, stored, and released for use; and the overall efficiency of the process of accumulating, storing, and releasing the energy.

In the lead acid storage battery, large quantities of energy are accumulated by chemical activity that is produced by the charging process. The energy is then released on demand in the convenient form of electric current.

Energy Efficiency

With charging and discharging batteries, as with all energy transfer processes, energy losses occur. The inequality between what is put into a system and what is drained from it is the system's energy efficiency. Generally, the energy efficiency of lead acid batteries is about 76%, meaning that 76% of the energy that was put into the battery during charging is all that is available for release during discharge. The energy efficiency is given as an approximate number since discharge rates and temperature can affect it.

The battery charger, which interfaces the battery with a source of AC power, also has an efficiency rating and, thus, is considered when calculating the overall efficiency of a battery system. A good charger is about 85% efficient, making for a combined charger/battery efficiency of about 65%, meaning that 65% of the electricity from the AC line fed into the battery is available as DC energy to a machine's components, such as controllers, motors, drive trains, etc., which are also not 100% efficient in their use of energy.

Another factor affecting the battery's efficiency is how and when it is charged. For a further discussion of charging regimens and their effect on energy efficiency and economy, see Section 4, Optimizing Energy Usage.

As an interesting note: only about 7% of the potential energy put into the power plant is available for actual work; for example, the lifting and moving of pallets from one place to another in a warehouse, when using an electric fork lift truck.

Figure 2: Energy Efficiency from Electric Generation Plant to Work Done by Electric Fork Lift Truck

Ohm's Law

Our use of electrical energy is dependent on the "flow" of electrical energy, the magnitude of the "force" propelling it and the "resistance" to its flow that naturally occurs in all materials through which it flows. The materials and the circuits they make up through which electrical energy flows, in effect, convert the energy into other forms of energy, such as heat and light, or work, such as turning a motor.

When a battery is used to power a fork lift truck, it operates with a "force" that affects the "flow" of its stored energy depending on the "load" - the power needs of the truck's network of "resistances."

The relationship between "flow", "force" and "resistance" is expressed mathematically in Ohm's Law, which was discovered by George Simon Ohm in the early 1800s. The Law states that E ("force" in volts) is equal to I ("flow" in amperes) times R ("resistance" in ohms). E = IR and its variants, I = E/R and R = E/l, comprise one of the basic tools electrical engineers have for designing electric powered machines.

• In sections of this book, current is discussed in conventional terms as flowing from the positive to the negative; while, in other sections, it is discussed in the electrochemist's terms as flowing from the negative to the positive.

Figure 3: The Concept of Electrical Current

Current and Amperes

Current flow is the means by which the battery releases its energy in electrical form. Current is the flow of electric charge from the positive terminal of the battery through the "load" (motors, pumps, controllers, etc.) of the truck, and back to the negative terminal of the battery. (Figure 3.)

The flow of current from the battery depletes the battery's stored charge. The rate of that depletion or current drain is measured in amperes. An ampere is the flow of 1 coulomb* per second. It is also defined as the amount of current that can be forced through a resistance of 1 ohm by an electrical potential of 1 volt.

The Volt

The volt is the unit of electrical potential, or pressure, that "forces" the current from the battery through the load and back to the battery. Since batteries are made up of many cells connected in series the total voltage of a battery, naturally, is the sum of the voltages of all its cells. A typical lead acid battery used in a fork lift truck may have 18 cells, nominally 2 volts each; the nominal battery voltage is therefore 36 volts.

The Watt

The watt is the electrical unit that defines the rate at which work is done, or energy is spent. Mathematically, we can say that Watts = Volts Amperes.

The bigger the current (amperes) and/or the voltage (volts), the faster the stored energy can be converted into work.

The Kilowatt-Hour

If the watt is the rate of doing work, then the total amount of work actually done is the product of watts and hours: watt-hours. If a battery delivers 1000 amperes at 24 volts for 2 hours, the total amount of energy delivered is

1000 x 24 = 24,000 watts x 2 hours = 48,000 watt-hours

To keep the significant digits from attracting too many zeros, we use the prefix "kilo" (k), which means 1000, and, sometimes, "mega" (M) for millions. The energy delivered in this example is therefore 48 kilowatt-hours.

• The coulomb is a unit of electrical energy that has been in use since before the discovery of the electron.
• It is actually made up of 6,000,000,000,000.000.000 electrons.

A rating of 1000 watt-hours (1 kWh) is equivalent to 1 horse working 11/3 hours. Where batteries are concerned, the kilowatt-hour rating at a stated discharge current is an accurate description of exactly how much energy the battery can deliver before it is discharged. Since electric bills are rendered on the basis of the number of kilowatt-hours of usage, it is often useful to make calculations about energy usage and efficiency directly in kilowatt-hours.

The Ampere-Hour

An ampere-hour is the total amount of electrical charge transferred when a current of 1 ampere flows for 1 hour. Therefore, the total usable charge stored in a battery can be stated in terms of ampere-hours - how long a current of a particular amperage can be drawn from the battery.

The ampere-hour rating accurately predicts the battery's capacity at a specified load current; batteries are therefore rated in ampere-hours at specified currents. A battery that can be discharged at 125 amperes for 6 hours before reaching its end-point voltage is rated at 125 amperes x 6 hours = 750 ampere-hours. Its "capacity" is therefore stated as "750 ampere-hours at the 6-hour discharge rate (at +25 ℃).

Battery Capacity

The term battery capacity relates to the amount of usable electrical energy stored in the battery. It is important to keep in mind that a manufacturer's rated capacity is given for 100% discharge of the battery. The recommended usable capacity, however, is generally 80% of the rated capacity to insure maximum battery life. For practical purposes, battery capacity is usually stated in ampere-hours because a particular number of ampere-hours of capacity is equatable to operating a given vehicle for a given length of time before its output voltage reaches the end point. A battery rated at 1200 ampere-hours is, therefore, thought of as maximally having 960 ampere-hours of usable capacity. Capacity is also affected by discharge rate and other variables that are discussed more thoroughly in the following pages.

Most manufacturers also provide capacity ratings in terms of kilowatt-hour specification when describing their batteries. As with ampere-hour ratings, the conditions under which kilowatt-hour specifications are determined must be specifically stated to be meaningful.

End Point Voltage (Final Voltage)

Today's lead acid traction battery has a downward-curving discharge characteristic, meaning that the voltage of the battery decreases gradually as it is discharged. The end point voltage is that which determines that the battery is discharged. Defining an end point voltage is an attempt to provide users with a cut-off beyond which the battery should not be used or damage to it and the equipment it is powering may occur.

Depending on the rate of current drain and the equipment, end point voltage can vary. However, when a usage pattern of the battery and equipment are predictable, as is pretty much the case with fork lift trucks, an end point voltage is quite meaningful.

The end point voltage for electric fork lift truck applications is selected by general agreement among battery and truck manufacturers. Still, for a 2 volt cell, there are differing opinions on just where the end point should be set with a normal range lying between 1.65 and 1.75 volts per cell (with the battery "loaded"). In some cases, especially at high discharge rates, this range is extended as low as 1.2 volts per cell by some manufacturers.

Battery manufacturers may suggest that their batteries can safely be discharged beyond the accepted range of end point voltages. No significant loss of battery life will be caused by such operation, they say. Further, they may even maintain that such operation is cost-effective as far as battery life is concerned.

Truck manufacturers, however, may take a different position. They say that allowing the battery voltage to fall appreciably below the specified end point may do irreparable damage to electrical equipment installed on the truck. They point out that operating at undervoltage can, for example, overheat motors causing ultimate failure and/or burn relay contacts.

From the users' point of view, though, one kind of damage may be as bad as another. A damaged battery must be replaced; a damaged pump motor or other component must be repaired or replaced. In either case, loss of the use of the truck - down time - is a problem that may be more severe than the physical damage. Users, therefore, may have more of an interest in establishing - and accurately detecting - the end point voltage than either of the two suppliers.

Section 2.

About Lead Acid Batteries

Introduction

There are two types of lead acid batteries generally used for vehicle applications ? the ordinary automotive battery (used for starting, lighting, and ignition) and the traction battery used to supply motive power for electric vehicles. Automotive batteries are designed for infrequent, very high current drains of short duration, and recharging begins as soon as the engine reaches operating speed. Traction batteries, on the other hand, are designed to be discharged continuously at relatively moderate current drains because there is no practical way to recharge the battery during operation. The stored charge of a traction battery, therefore, runs steadily down from its starting condition until the battery is recharged. A reasonable service life from such a battery might be considered as 1000 to 2000 cycles of discharge and charge; and typical life spans for industrial batteries, properly used and cared for in fork lift trucks, are about 5 years, sometimes even longer.

Figure 4 shows typical charge/discharge characteristics for batteries used in two common commercial applications - a taxi and a fork lift truck, both used in 2-shift operations.

How a Lead Acid Battery Is Made

The lead acid battery is made up of several identical cells, each of which contains two plates, one positive, the other negative. Both plates are immersed in an electrolyte that is a mixture of sulfuric acid and water.

Two types of cell construction are common: flat plate and tubular plate. The overall functions of the two types are identical, but their mechanical construction and performance differ slightly.

In a flat plate cell (Figure 5), each positive plate is a cast metallic lead frame which contains the lead dioxide active material. The negative plates contain spongy metallic lead active material within a similar grid structure. Positive and negative plate areas are usually identical.

In a tubular plate cell (Figure 5), the positive plates surround lead alloy spines. The lead dioxide is in close contact with the spine over its entire length, and is retained by a special sleeve. Negative plates are of spongy metallic lead in a grid form identical to those in flat plate cells.

Figure 4: Battery Charge/Discharge Cycles in Two Commercial Applications

Figure 5: Typical Flat Plate and Tubular Plate Cell Construction

In either case, the cell is filled with electrolyte, which is slightly heavier than water. The ratio between the weight of a given volume of electrolyte and the same volume of water is the specific gravity of the electrolyte.

Figure 5 shows how a typical industrial cell is assembled. In order to provide sufficient current output (amperes) each cell consists of many plates (for example, 11 positive and 12 negative). Because each positive plate is positioned between two negative plates, there is always one fewer positive than negative. The positive plates in each cell are connected in parallel to provide a positive bus of the required current output, which is connected to the positive terminal of the cell. Similarly, the negative plates are bussed and connected to the negative terminal.

The cells are connected by external metal straps that hook them into a series circuit ... a circuit in which the negative plates of one cell are connected to the positive plates of the next, so that the voltages of all cells are added to provide the total voltage of the battery. Typically the cells are numbered in sequence beginning with the cell containing the positive terminal of the battery (number 1) and ending with the cell containing the negative terminal. (Figure 6.) There can be any number of cells in a battery, but the numbers most commonly used are: 3, 6, 9, 12, 15, 16, 18, 20, 24, 30, 36, and 40.

Figure 6: Battery Cell Strapping and Numbering

Battery rating information is generally displayed in coded form, stamped into the lead of the first negative terminal or on a nameplate on the side of the battery. As an example, the code for a particular battery might read as follows:

The ratings for this battery are:
Voltage: 12 cells x 2 volts each = 24 volts
Capacity: (11 - 1) / 2 positive plates x 85 Ah each = 425 Ah.

Electrolyte

The electrolyte in a lead acid battery is a mixture of sulfuric acid and water. Sulfuric acid is a very active compound of hydrogen, sulfur, and oxygen. Its chemical formula is H₂S0₄. In water, the sulfuric acid molecules separate into two ions, hydrogen and "sulfate," the latter of which is made up of sulfur and oxygen atoms. Each sulfate ion contains two "excess" electrons and each therefore carries two negative electrical charges. Each hydrogen ion, having been stripped of one electron, carries one positive electrical charge.

Because sulfuric acid is highly reactive, it ionizes almost completely and so there are very few fully assembled molecules of sulfuric acid in the electrolyte at any instant. Furthermore, the ions are in constant motion, attracted and repelled by one another, by the water, and by any impurities in the mixture. This constant random motion eventually causes the ions to diffuse evenly throughout the electrolyte. If any force disturbs this even distribution, the random motion eventually restores it. However, since the electrolyte is contained in a complex structure of cells, redistribution takes a relatively long time. This fact turns out to play a key role in our ability to measure the exact state-of-charge of the battery at any instant, as will be shown later.

• In sections of this book. current is discussed in conventional terms as flowing from the positive to the negative; while. in other sections. it is discussed in the electrochemist's terms as flowing from the negative to the positive.

Figure 7: Schematic Representation of Reactions at Negative and Positive Plates

Producing an Open Circuit Voltage

The chemical reaction between the sulfate ions and the spongy lead of the negative plate produces lead sulfate, a compound that does not dissolve in water. This reaction frees two electrons and thereby produces a net negative electrical potential at the negative plate. (Figure 7.)

The presence of these free electrons slows down the chemical reaction at the negative plate because their negative charge repels other negatively charged sulfate ions. Fewer ions can then reach the negative plate to react with the spongy lead to form more lead sulfate. The overall reaction cannot continue very long, therefore, unless the excess electrons are permitted to leave the negative plate.

Meanwhile, at the positive plate, other sulfate ions react with the lead of the lead dioxide to produce lead sulfate; at the same time, the hydrogen ions of the acid react with the oxygen of the lead dioxide to form water. This combination of reactions produces a net positive potential at the positive plate. (Figure 7.) Here, too, the reaction can only continue as long as the electrical conditions are right. Within a short time, the supply of free electrons in the metal of the positive terminal is used up and no further chemical change can take place unless more are supplied.

The difference between the two potentials at the plates is the open circuit voltage or electromotive force (emf) of the cell. This emf (about 2.1 volts) will remain unchanged as long as no path is provided for the excess electrons to leave the negative plate and no source of electrons is provided for the positive plate. In this condition, there is little or no chemical activity in the cell, which means that a charged cell can be stored for a fairly long time without significant loss of energy. The open circuit voltage typically will drop by less than a millivolt (0.00lV) per day, during storage, if there is no loss of electrolyte - a process referred to as "self-discharging."

Producing Current

The available source of electrons to make up the deficit at the positive plate is, of course, the excess of free electrons at the negative plate. Since these free electrons are produced by the reaction between the acid and the lead, the total number of free electrons available is set by the amount of acid and lead available to react. A similar limitation exists for the positive plate; the total number of free electrons it can absorb is set by the amount of acid and lead dioxide available to react.

Since any flow of electrons is a transfer of charge, the total amount of charge stored in the cell is established by the total amounts of plate material and sulfuric acid available to react. The total amount of charge stored in the cell determines the capacity of the cell.

If a wire is connected between the two plates, the excess electrons instantaneously rush from negative to positive. This electron current * is very high because the wire is a short circuit between the terminals. If the wire is very thick (has no resistance at all), the total number of electrons transferred is determined only by the amount of electrolyte that has reacted ? and continues to react - with the two plates. The net charge transfer is 2 electrons per molecule of acid. Since the number of molecules of acid is inconceivably large, a gigantic current could flow between the shorted terminals, transferring nearly all of the cell's stored charge from one terminal to the other in a very short time.

If electrical resistance ... a load ... is connected between the terminals, then the current is limited by the resistance of the load, and the cell's charge is transferred from terminal to terminal, via the load, at a slower rate, i.e.; a smaller electron current. For a typical traction cell, the current can be hundreds of amperes. This current will flow as long as the load is connected and as long as there is active material left in the cell to sustain it.

Since no physical process is perfect, the electrolyte/plate reactions offer resistance to this internal current and therefore lose some of the transferred energy in the form of heat. The electrical effect of this internal resistance of the cell appears as a loss of potential (a voltage drop) at each plate. The cell's total voltage under load is therefore less than its open circuit voltage. The amount of energy lost to this internal resistance depends on the load current and on the concentration of acid in the cell ... especially the acid concentration at the positive plate. The larger the load current, the greater the loss of energy. Also, the lower the acid concentration at the plates, the higher the internal resistance of the cell.

• When discussing the electro chemical reactions in a battery, it is useful to refer to electron flow as current.

When current is produced by the cell, acid, lead dioxide and lead are converted to lead sulfate and water. Each acid molecule that reacts is no longer part of the electrolyte. This process, by reducing the concentration of acid in the water, gradually reduces the ability of the cell and leaves less energy in it.

In the design of batteries, the amounts of acid and plate-active materials are balanced so that the release of energy relates to the rate at which current is likely to be drawn. Batteries designed for low-rate applications, such as for storage in solar power systems, contain a larger amount of acid in proportion to plate-active material. They are designed to be plate-limited when used beyond their rated capacity. No plate materials will be available for releasing usable energy.

Batteries designed for high-rate applications, such as automotive ignition, etc., have a smaller amount of acid in proportion to plate-active material. They are designed to be acid-limited when used beyond their rated capacity.

As acid concentration becomes too low, a cell becomes incapable of releasing usable energy at the rate for which it was designed. Additional energy can only be drawn from it if the current rate is reduced. As it is driven to excessively low acid concentrations (through deep discharging), the coatings of lead sulfate produced by the chemical reactions at the plates will not reconvert. Upon charging, acid concentration is restored and plate coatings will again reconvert.

The traction battery used with fork lift trucks falls between the automotive and storage battery in its proportion of acid and plate-active material. It is generally considered to be acid-limited for rates exceeding the 6-hour capacity.

State-of-Charge

The cell's state-of-charge is determined by the amount of active material available to sustain a usable current flow through a load. At the outset, all of the active material is available and the cell is fully charged. When it can no longer produce usable current, the cell is fully discharged. At any point between these two extremes, the state-of-charge of the cell is expressed as a percentage of the total difference in charge between the fully charged and fully discharged states.

Since the state-of-charge is set by the availability of active material in the cell, it is conventional (but not alone sufficient) to define the cell's state-of-charge in terms of the specific gravity of the electrolyte. As defined above, specific gravity, a measure of density, is the ratio of the mass of the mixture of sulfuric acid and water in the electrolyte to pure water at a specified temperature. It is common to speak of, for example, 1300 SG in lieu of 1.300 specific gravity: a convenience simply achieved by multiplying 1.300 by 1000. For the purposes of this book, from this point on, specific gravity measurements shall be expressed in SG form. All SG measurements are corrected to + 25°C.

The relationship between state-of-charge and specific gravity is usually shown in a form similar to Figure 8. Note, however, that this illustration does not take into account the dynamic activity inside the cell while current is flowing. It shows only the long-term average relationship when the load has been disconnected and the sulfate ions have had a chance to diffuse evenly throughout the cell.

Figure 8: Stabilized SG for 2 Cell Types Vs State-of-Charge at the 6-Hour Rate

The time required for this diffusion process to be completed varies according to the rate, depth and length of discharge and is different in cells of different design. Figure 9 shows this effect as measured on a typical cell that has been discharged at a moderate rate. In this test, it took more than 16 hours for specific gravity to fully stabilize.

Since the lead sulfate forms at the plates, the specific gravity of the electrolyte is lowest near the plates and highest farther from them. Measuring specific gravity during or shortly after discharge actually provides false information about actual average specific gravity, with an error factor that depends on the depth and duration of the cell's recent discharges. *

Determining Battery Capacity

Battery capacity is determined through manufacturer testing. Manufacturers have test procedures which are utilized to establish the hour rate and ampere-hours of their batteries. Prior to making a capacity measurement, the battery is fully charged (typically 1290-1300 SG). Then it is connected to a load that draws a desired current. The battery's output current and its voltage are monitored continuously for the specified time. A conventional test setup is shown in Figure 10. In this case, the battery capacity was intended by its manufacturer to be 960 ampere-hours at the 6-hour rate; that is, the battery is designed to be capable of delivering 160 amperes for 6 hours. The final (end point) voltage is specified as 30.6 volts (1. 7 volts per cell). The resistance of the load in our hypothetical test setup is adjustable from 0.23 ohms to 0.19 ohms.

At the start of the test, the resistance is set to 0.23 ohms (160 amperes at 36.4 volts). As soon as the battery delivers some of its charge, its output voltage begins to fall. To keep the load current at 160 amperes, the load resistance must therefore be reduced slightly. This adjustment of the load resistance is continued until the battery output voltage reaches 1.7 volts per cell (load resistance of 0.19 ohms at 160 amperes). For this battery of 18 cells the end point voltage is 18 1.7 or 30.6 volt at 100% discharged.

• *In practice. the daily measure of specific gravity is made at the same point in the battery's operating sequence (for example, at the end of each shift). In this case, approximately the same conditions will have been reached when the measurement is made and the results will therefore be fairly consistent. Such measurements will, however, be offset from the true value of specific gravity by some unknown and uncompensated amount, which can be determined by letting the battery stabilize and remeasuring the specific gravity.

Figure 9: Time Required for SG to Stabilize During Discharge Rest Intervals

Figure 10: A Conventional Test Setup for Determining Battery Capacity

The end point voltage signifies, by general agreement, the practical, 100 % discharge of the cell. * The length of time it takes for this end point voltage to be reached is the "hours" part of the "ampere-hour" rating; the constant current, of course, is the "amperes" part.

In the U.S., traction batteries are usually specified at the 6-hour discharge rate. In other countries, a 5-hour rate is common. The rate is the constant current drain that depletes the battery's charge so that at the end of that many hours, the end point voltage across the load is only 1.7 volts per cell. For situations in which other discharge rates apply, manufacturers may specify other end point voltages ... some ranging as low as 1.2 volts at very high discharge rates or as high as 1.85 volt at very low discharge rates. A typical set of end point voltages is shown in Figure 11. In the U.S., traction battery data at various discharge rates is usually presented using 1.7 volts per cell as the 100% discharge end point.

Capacity and Discharge Rate

If we assume that the capacity of a typical 960 ampere-hour battery is unaffected by discharge rate, we would expect it to discharge in 3 hours with a current of 320 amperes (960 Ah divided by 320 A = 3 Hrs). Actually, at a current drain of 320 amperes, the final voltage of 1.7 volts per cell is reached after only about 2.5 hours. The capacity of the battery in ampere-hours and the discharge rate are not linearly related. For example, our typical battery delivered 160 amperes for 6 hours, which we call 100% capacity, but only 265 amperes for 3 hours, 17% less than might be expected, and 350 amperes for 2 hours, 27% less than expected. The point to keep in mind is that the heavier the continuous load on the battery, the less capacity it has. Figure 12 shows the manner in which discharge rate affects the capacities of two similarly rated batteries from two different manufacturers.

• *For all practical purposes, a cell is discharged only to 80% of its capacity because energy drawn from the cell after that point causes voltage to drop at a steep and rapid rate. In the world of lead acid traction batteries and fork lift trucks, a battery is considered discharged at 80 %. while at 100 % discharge it is well into the area of deep discharge. It would seem prudent to simply term the 80% level as 100%. but it is not the province of this book to alter any such widely used convention.
• For some street electric vehicle applications. voltages as low as 1.0 have been specified.

Figure 11: Typical End Point Voltage as a Function of Discharge Rate: (Valid when manufacturer rates battery with a current-dependent end point voltage, ([from Manufacturers\' data].)

Figure 12: How Battery Capacity Varies with Discharge Rate

Capacity and Temperature at Electrolyte

Another important factor that affects battery capacity is electrolyte temperature. Generally speaking, the higher the temperature the more rapidly any chemical action will proceed. The speed with which the acid combines with the plate materials is much higher when the electrolyte is hot. Conversely, when the electrolyte is cold, the reactions move slower.

At high temperatures, the faster chemical action at the plates permits more material to take part in the chemical reactions, which is roughly equivalent to having more material available to react. Since battery capacity ultimately depends on the amount of material available to react, increasing the temperature of the cell increases its capacity. *

This effect is so pronounced that at the freezing point of water, capacity at the 5-hour rate is only 65% of capacity at 80 °F. (See Figure 13.) For this reason, any specification of battery capacity must state the temperature at which the specification applies.

• *It is generally agreed in the battery industry that continuously high temperature can be related to grid deterioration of the plates. Considering 80°F as a normal temperature, for each 15°F of above normal, industry experts say that battery life will be reduced by half. A typical battery discharged al normal rates to 80% DOD at about 80°F (25°C) will show a raise of electrolyte temperature of about 12°F. To return the battery to normal, a cooling period of up to 12 hours may be necessary. Thus, manufacturers caution against using batteries two cycles per day. This does not allow time for cooling and results in reduced battery life.

State-of-Charge Measurements

The constant-current method outlined earlier (Figure 10) is the way in which batteries are evaluated at the factory to produce the specifications by which users select the correct battery

for any application. But once the battery has been selected and is installed on the truck, the user is interested in "state-of-charge" at any moment, as well as rated capacity. The standard capacity-measuring test is no help here because currents are constantly changing. Four techniques are used to measure state-of-charge:

• Specific gravity measurements
• Open circuit voltage measurements
• Measurement of battery voltage under load
• Ampere-hour measurements

Figure 13: How Battery Capacity may be Affected by Electrolyte Temperature.

Specific Gravity and Open Circuit Voltage

The open circuit voltage of a cell is a precise indicator of specific gravity when a cell is fully stabilized. And as such, the open circuit voltage is a precise measure of state-of-charge. Because open circuit voltage is determined solely by the concentrations of acid at the plates, it will not agree with specific gravity readings unless the acid is uniform everywhere in the cell. Then, measuring the open circuit voltage after stabilization is equivalent to measuring the specific gravity. This relationship is shown in Figure 14. The time required for stabilization can be hours, depending on the depth and duration of discharge and is different for cells of different design. Under laboratory conditions, Figure 14 is a valuable relationship; in practical applications, however, it is ambiguous at best. The unstabilized open circuit voltage will always read higher than at the equivalent point in Figure 14 if the cell has just been taken off the charger. Conversely, the unstabilized open circuit voltage will always be lower than at the equivalent point in Figure 14 if the cell has recently been discharged.

Figure 15 shows open circuit voltage of a typical cell measured at various times after disconnecting the load. In this test, the open circuit voltage rose rapidly but did not reach its stable value of 1.982 volts until more than 100 hours had elapsed. The peak of 1.990 volts reached after some 6 hours was not sustained.

Voltage under Load

Under test conditions like those shown in Figure 10, we can examine the way voltage under load is related to battery capacity. For example, let's assume that we are testing a traction battery with a capacity of 1050 Ah at the 6 hour rate.

At a moderate load of 200 amperes, we find that the voltage stays constant (within about 7%) for nearly 4 hours (actually 3.96 hours, as shown in Figure 16). Up to this point the battery has delivered 792 Ah, or 80% of its capacity.

If we repeat the test, but draw 400 amperes, the nominal voltage to 80% discharge holds constant to within about 8%, but for only about 1.6 hours (1.57 hours as shown in Figure 16). Up to this point the battery would only deliver 628 Ah, 80% of its capacity.

In either case, when the battery reaches 80% discharge, its voltage under load begins to fall rapidly, as is shown in Figure 16, and the fall-off rate gets steeper and steeper as the 100% discharge point is approached.

From Figure 16 you can see that the voltage under load ? when measured at a constant current - is highly predictable. Any change in voltage is determined by the number of ampere-hours drawn from the battery. Thus, the change in voltage under load is a measure of the charge withdrawn and, therefore, of the capacity remaining.

Of course, there are other factors to be taken into account. The first among these is that any measurement of battery characteristics is highly dependent on electrolyte temperature. The higher the temperature the greater the battery capacity, as shown in Figure 13.

Figure 14: How Stabilized Open Circuit Voltage Reflects Stabilized SG

Figure 15: Variation of Open Circuit Voltages as Cell Recovers After Load

The second factor is that our test measurement was made with a constant load, which does not reflect the real world at all. In fact, we know that interrupting or reducing the load long enough to allow some "recovery" actually increases the remaining capacity. Also an increase in the load reduces the amount of remaining capacity.

In either case, the measured voltage under load changes as the conditions change. If electrolyte temperature increases, so does voltage under load; if the load is interrupted and the battery "recovers," the measured voltage increases, and so on. There is no way to tell from the measured voltage what caused the change, but the voltage under load always decreases as capacity is withdrawn from the battery.

Ampere-hour Measurements

An ampere-hour meter integrates current in amperes with time in hours. Displaying ampere-hours of consumption, it can be used to indicate state-of-charge. Given the rated capacity of a battery, the state-of-charge can be calculated by subtracting ampere-hours consumed from rated capacity. This can be done by the Ah instrument and displayed directly as state-of-charge.

Figure 16: Cell Voltage at Two Constant Currents

Section 3.

Battery Charging

Introduction

A lead-acid battery can be discharged and recharged many times. In each cycle, the charging process stores energy in the battery in the form of potentially reactive compounds of sulfuric acid, lead and lead oxide. The discharge process is another chemical reaction among those components that release the stored charge in electrical form. Since no chemical or physical process can ever be 100% efficient, more energy is always used to charge the battery than can be recovered from it. Thus, determining the optimum conditions for battery charging grows in importance as the cost of energy increases.

How Energy Is Stored in the Cell

Forcing a direct current into the cell in the reverse direction replaces energy drawn from the cell during discharge. The effect on the electrolyte and the plates during this charging process is essentially the reverse of the discharge process. Lead sulfate at the plates and the water in the electrolyte are broken down into metallic lead, lead dioxide, hydrogen and sulfate ions. This re-creation of plate materials and sulfuric acid restores the original chemical conditions including, in time, the original specific gravity.

The amount of energy it takes to re-create the original specific gravity is, of course, at least the same as the energy produced by the chemical reactions during discharge. This energy is supplied by the charger in the same form that it was removed from the battery: as volts and ampere-hours (or kilowatt-hours). Thus, if the battery produced 36 kilowatt-hours during discharge, it takes at least 36 kilowatt-hours to recharge it, plus additional kilowatt-hours to make up for losses in the energy-transfer processes.

During the first few hours that an 80% discharged battery is on the charger, the charging current is relatively high. For example, in the first four hours of charging, about 70% of the ampere-hours previously withdrawn from the battery has been restored. (See Figure 17.) For the next three hours, as battery voltage approaches the charging voltage, the charging current through the electrolyte gradually decreases, so that from the end of the fourth hour until the end of the seventh, the state-of-charge increases by about 30%.

At this point, the number of ampere-hours returned to the battery is about the same as the number withdrawn, but the battery will still accept additional ampere-hours up to about 105% of the number withdrawn. Beyond about 105% (the nominal value for a "strong" battery) virtually all ampere-hours supplied to the battery are consumed in electrolysis and in heating the electrolyte. However, up to about this point the added ampere-hours serve mainly to make up for internal "coulombic" inefficiencies.

For the charge cycle as for the discharge cycle, stabilized specific gravity is a measure of the state-of-charge. Also, as during discharge, specific gravity does not respond instantly throughout the electrolyte. Instead, the specific gravity is highest at the plates, where sulfate ions are released and the greatest number of them are concentrated. Farther from the plates, specific gravity remains lower until the freed sulfate ions have diffused evenly throughout the electrolyte.

Specific gravity, therefore, lags well behind the state-of-charge of the battery, as shown in Figure 18. The maximum specific gravity lag is considerably greater in the charging process than in discharging. Starting at approximately 1140 SG (for a typical 80% discharged cell), after an hour on charge, the specific gravity rises 4 "points," only 3 % of the total rise of 150 points. But nearly 20% of the ampere-hours have been returned to the battery in that same hour.

By the end of the third hour, specific gravity has risen only a total of 32 points, to 1172 SG, or 21 % of the total rise, yet the returned charge is now about 50%. During hours 4, 5 and 6, specific gravity begins to catch up and, at the end of the sixth hour, specific gravity is 1278 SG, or 92% of its final value, compared to a returned charge of 95%.

Battery Chargers and Charging

The basic types of battery chargers available today are motor generator, ferroresonant and pulsed. Use of the correct charger is an important factor in maximizing the overall efficiency of the battery system. Used correctly, under proper conditions, a modern battery charger will routinely provide overall efficiencies on the order of 85% with a battery of 18-24 cells; 80% with 12 cells and 75% with a 6-cell battery.

Four methods exist to control the DC current and voltage supplied to a battery in the charging process: two-rate; voltage detect and time; taper; and pulsed.

In the two-rate method, charging begins at a high rate that is dropped to a much lower rate after 80-85% of the ampere-hours have been returned to the battery. This lower rate then tapers to a finish rate. The rate-change point coincides with the electrolytes gassing voltage, at which bubbling of hydrogen occurs. A voltage sensor/relay is commonly used to trigger the rate change.

Figure 17: How Ampere-Hours Are Returned to the Battery During an 8-Hour Charge

Figure 18: Lag of SG Measured During Charging Process Against Theoretical SG vs State-of-Charge

A variation of the two-rate method is the voltage detect and time method in which the gassing voltage triggers a timer which turns off the charger in a specified time after a finishing charge period.

In the taper method, the voltage starts at a high rate and steadily tapers downward as cell voltages rise to their charged levels.

The pulsed method involves supplying a burst of DC until a maximum voltage level is reached, at which time the supply is cut off. As the voltage decays and hits a minimum level, the supply is restored and so on, back and forth.

Ferroresonant Charger

Ferroresonant chargers are widely used in the U.S.A. to charge traction batteries. The ferroresonant charger is usually a fully automatic unit that produces a charge current that tapers steeply from a large initial value to the finish rate. A typical ferroresonant charger produces a current-voltage pattern like the one shown in Figure 19.

The internal voltage of the ferroresonant charger is essentially constant throughout the charge period, usually 8 hours. The output current, however, is limited by the battery voltage. At the beginning of the charge period, the battery voltage is considerably lower than the charging voltage and the maximum charging current flows. (This maximum current is usually set at from 16-26 amperes per 100 Ah of rated battery capacity.) As the battery is recharged, its voltage increases, gradually reducing the charging current to the finish rate of 2 to 5 amperes (7 for a battery near the end of its life) per 100 Ah of battery capacity.

Pulsed Chargers

Another type of charger, in wide use for traction batteries in Europe, operates on a different principle: pulsating direct current. In this case, the charger is periodically isolated from the battery terminals and battery open circuit voltage is automatically measured. If open circuit voltage is above a preset limit, the charger remains isolated; when open circuit voltage decays below that limit (as it always must), the charger is reconnected for another period of equal duration. Figure 20 shows this procedure.

Figure 19: Current Voltage Relationships in a Ferroresonant Charger

Figure 20: How a Pulsed Charger Operates

When the battery's state-of-charge is very low, charging current is connected almost 100% of the time. This is because the open circuit voltage is below the preset level or rapidly decays to it. However, as the battery's state-of-charge increases, it takes longer and longer for the open circuit voltage to decay to the preset limit.

The open circuit voltage, charging current and the pulse period duration are chosen so that when the battery is fully charged, the time for the open circuit voltage to decay is exactly the same as the pulse duration. When the charger controls sense this condition, the charger is automatically switched over to the finish rate current, in which short charging pulses are delivered periodically to the battery to maintain it at full charge.

Maximum Charge Rate

The maximum charge rate is set by the maximum allowable temperature rise in the battery's electrolyte and the requirement not to produce excessive gassing. A lead acid battery that has been normally discharged can absorb electrical energy very rapidly without overheating or excessive gassing. A practical temperature limit that is widely accepted is that the electrolyte should not rise above 46.1 °C (115 °F) with a starting electrolyte temperature of 29.4 °C (85 °F).

In the case of the battery that has been fully discharged, the charging current can safely start out as high as 1 ampere for every ampere-hour of battery capacity.

Studies have shown that if the charging rate in amperes is kept below a value equal to the number of ampere-hours lacking full charge, excessive temperatures and gassing will not occur. This is known as the "Ampere-hour Law". For instance, if 200 ampere-hours have been discharged, the charging rate may be anything less than 200 amperes, but must be reduced progressively so that the charging current in amperes is always less than the number of ampere-hours the battery lacks to be at 100% charge.

As a practical matter, the discharge state of a battery is not known; thus, chargers must utilize techniques which provide less than optimum charging rates.

The final charging voltage is limited by chemical considerations and temperature. The safe limit for the lead acid traction battery commonly used with fork lift trucks is generally agreed to be between about 2.40 and 2.55 volts per cell when charged at 25°C ambient temperature.

Finish Rate

The most common finish rate is approximately 5 amperes per 100 Ah of rated capacity, a rate low enough to avoid severe overcharging but high enough to complete the charging process in the eight hours normally available.

Equalizing Charges

By maintaining the finish rate for an extended period (up to 6 hours), a battery with cells at slightly varying voltages and/or depths of discharge can be equalized. The continued input of charge (overcharging) to the battery serves to "boil" off water in those cells of higher voltage and/or depths of discharge. Upon completion of the process, levels must be checked and water added as required to depleted cells. New batteries, referred to as low maintenance systems, may not permit adding of water and therefore are not designed for equalizing charges.

Terminating the Charge

Overcharging can materially shorten the life of a battery, and no amount of overcharging can increase battery capacity beyond its rated value. There are several "rules of thumb" that are followed in deciding when to end the charge:

• When the charge is complete, the voltage levels off and there is no further increase
• Charge current readings level off at the finish rate
• The battery gasses freely
• The specific gravity reaches a stable value.

Gassing

Hydrogen bubbles are produced at the negative plates and oxygen at the positive plates during charging. After the battery reaches full charge almost all added energy goes into this gassing. The gassing process begins in the range of 2.30 to 2.38 volts per cell, depending on cell chemistry and construction. After full charge, gassing releases about 1 cubic foot of hydrogen per cell for each 63 ampere-hours supplied. Since a 4 % concentration of hydrogen in air is explosive, ventilation of battery rooms is required for safety.

Energy Efficiency in the Charging Process

Nothing is free, least of all energy. Since the charging process can never be 100% efficient, we must be careful about how energy is used in this process.

The charging process converts energy supplied by the local utility to kilowatt-hours stored in the battery which is subsequently available for transfer to a load ... in our use, an electric fork lift truck. Two major components are involved in this process: the battery itself and the charger. The charger interfaces with the power line on one side and with the battery on the other. The battery, in turn, interfaces with the charger on its input side and with the truck on its output side.

How the Charger Affects Energy Efficiency

Energy is consumed in the charging process. While most of the charging energy goes into restoring the original chemical conditions in the cell, some is lost in the battery, and some is lost in the charger, mainly as heat.

We define the efficiency of the charger as the efficiency with which power line energy is supplied to the battery in usable form. This efficiency varies not only from type to type and from manufacturer to manufacturer, but also may vary from unit to unit. Let's explore this with measurements in a "typical" case (charging a 36V, 1200 ampere-hour battery).

• During the typical 8-hour charging period, the charger supplies energy to the battery at a rate that depends on the battery's state-of-charge at any instant. This accumulation of energy is shown in Figure 21 as curve C. Note that this cumulative curve rises steeply and then gradually becomes flatter until at the end of 8 hours it is almost completely horizontal.
• The charger draws energy from the power line to provide the battery operating energy. This energy is shown as curve L in Figure 21. The shape of this curve is similar to that of the curve C, but L is always higher than C because the charger takes more energy from the line than it delivers to the battery.
• Working from these two curves, it is possible to determine the actual charger efficiency up to any time on charge. All we need to do is measure the heights of the two curves at the desired time mark, divide C by L, and multiply by 100. The overall charge efficiency E, is determined by the values at the end of the 8-hour period.
• In a typical case, the charger draws approximately 50 kilowatt-hours from the line and delivers about 42 of them to the battery, when recharging our typical battery from 80% discharge. Thus, the charger is about 84 % efficient over a standard 8-hour charge period when recharging an 80% discharged battery.

How the Battery Affects Energy Efficiency

The battery accepts only part of the energy supplied by the charger; furthermore, it also delivers only part of that energy to the load. The efficiency with which the battery releases the energy supplied it by the charger can be demonstrated in a manner similar to that used to determine charger efficiency.

• During the 8-hour charge period, our typical battery accepts 42 kilowatt-hours of energy from the charger, as shown in Figure 21.
• To understand clearly the battery's efficiency as part of the total efficiency of the electric truck system, it must be remembered that the amperes are being delivered to the battery at the charger voltage which, for a typical 36 volt battery, might be an average of 40-42 volts, while the battery discharge is at an average of 33-35 volts. Even if the total ampere-hours of charge and discharge were the same (and normally we charge an additional 5%) the kilowatt-hours (which is the total energy) would vary by the difference of the average voltage during charge and the average voltage during discharge. Consequently the battery efficiency is much less than may be thought if one only considers the ampere-hours charged and discharged.
• The overall battery efficiency is determined by comparing the 32 kilowatt-hours delivered from the battery* with the 42 kilowatt-hours delivered to it: 32 - 42 100 = 76%.

Figure 21: How the Charger Affects Efficiency

Overall System Efficiency

The overall system efficiency is the efficiency with which the power line energy (50 kilowatt-hours) is converted to energy delivered to the load (32 kilowatt-hours): 32 - 50 100 = 64 %. This figure duplicates the overall system efficiency calculated from the two factors, charger efficiency and battery efficiency: 84% 76% = 64%.

How Depth of Discharge Affects System Efficiency

Efficiency is affected by the depth of discharge of a battery when it's placed on charge. Any battery that is less than 80% discharged forces the charger to become a waster of energy. Significant amounts of power line energy are converted into small amounts of useful battery charge. In the case of a battery that has been essentially idle during the previous shift (less than 10% discharge), blindly placing it on charge for eight hours will waste nearly half of the energy delivered to the battery.

The three graphs of Figure 22 show, in an 8-hour charge period, the hour-by-hour cumulative line energy input (L) to the charger and the charger energy input to the battery (C) for each of three cases: a battery placed on charge at 80% discharged; another at 40% and a third at 20% discharged.

The dramatic effect on charger and battery efficiency is obvious when the data from Figure 22 are presented in Table 1 and Table 2.

*Battery au/put (kWh) equals average voltage limes Ah delivered to 80% DOD.

Figure 22: Effect of Depth of Discharge (DOD) on System Efficiency

The Combined Effect

In Table 3, the overall system efficiency, the product of charger and battery efficiencies, is shown for each of the above three cases.

From this examination, it becomes quite clear that if a fixed, 8-hour charging routine is to be followed, the overall efficiency with which energy is used is determined mostly by the state-of-charge of the battery when it goes to the charger as shown in Figure 23.

Figure 23: Overall Energy Efficiency in Charging a Battery Discharged to Various Depths (8-Hour Charge)

Section 4.

Optimizing Energy Usage

Beginning with Conclusions

After only a brief study of Figure 23, it appears that the optimum battery selection is one that results in 80% discharge by the end of a pre-established work program. Said another way, the battery should have a rated capacity of 125% of the energy it is expected to deliver during discharge. It makes no difference whether the intention is to charge the battery at the end of each workshift or whether the battery is to be charged at the completion of a particular task. The conclusion remains the same: the battery should be placed on charge when it has been 80% discharged.

Selecting the Correct Battery Capacity

The selection of battery capacity for a given truck and application becomes more significant with each increase in the capital cost of batteries and charging stations as well as with each increase in the cost of energy.

Each industrial truck application presents a separate battery-selection problem with more involved than the size or lifting ability of the truck. Since trucks may be used for different purposes, each application presents a specific work profile that can be thought of as a series of rapidly varying current drains.

In any application, this battery drain varies from instant to instant during the entire time the truck is in use. As shown in Figure 24, every task the truck performs represents a different battery drain ... that can, for example, range from 5 amperes, while steering, to 1000 amperes, motor in-rush current on the pump motor.

Figure 25 shows a hypothetical operation for a typical truck-lifting loads and transferring them to nearby locations. Our typical truck takes 6 separate steps to complete these operations, and each step drains energy from the battery.

Of course, there is no simple way to calculate in advance exactly how much energy will be needed to perform any single step of an operation, let alone a whole day's work. The only real solution is to measure the number of ampere-hours used by the truck when it performs each step. This is done with an ampere-hour meter installed on the truck. *

With the Power Prover Ampere-Hour Meter installed on the test truck, the driver performs the stipulated sequence of steps and the meter readings show the total ampere-hours and the amperes used in the operation. The number used in each step of the operation can be monitored by recording the meter readings while the truck operates.

• *The Curtis Model 1020 Power Prover Ampere-Hour Meter is an accurate, easy-to-install ampere-hour meter widely used for this purpose.

A procedure of this kind that includes representative operations performed by the truck provides a simple and accurate basis for selecting battery capacity ... or for verifying assumptions about ampere-hour requirements.

Ways to Measure State-of-Charge on the Fork Lift Truck

The reason for measuring the state-of-charge as the battery is in use is twofold: to protect the battery from deeply discharging and, thereby, internal damage; and to protect the truck's electrical components from the negative effects of low voltages, a consequence of deeply discharged batteries. At average discharge rates of 8 hours or less, a measurement which accounts for the remaining capacity as a function of discharge rate can serve to protect both the battery and the truck.

An ampere-hour meter provides useful information that greatly simplifies specifying the correct battery and measures ampere-hours used, but not the rate at which it is used. And the rate at which it is used is crucial in measuring the state-of-charge because battery capacity is different at different rates of discharge.

Aside from ampere-hour metering, three basic measures have been used in industry to determine battery state-of-charge: specific gravity, open circuit voltage, voltage under load.

Specific Gravity: A Static Measure

Not only are specific gravity measurements not convenient to make during the work period, but their value is limited because it takes time for the specific gravity to stabilize after the battery load is disconnected. Although a convenient measure of overall battery condition, specific gravity measurements give no valid indication of the discharge history that produced the reading. Any given reading of stabilized specific gravity can either be the result of heavy discharge for a short period or of prolonged discharge at a very light load. This effect is shown in Figure 26, in which the specific gravity and the open circuit are plotted against % of discharge for currents from 25 amperes to 800 amperes for a 1200 ampere-hour battery rated at 6 hours. Any specific gravity line, for example, 1165 SG, intersects a number of discharge lines. 1l65SG, in particular, corresponds to 80% discharge at the 6-hour rate (200 amperes). However, if actual operation is at 600 amperes, the truck motor will have been subjected to repeated, excessively low voltages (less than 1.7 volts per cell) for a significant amount of time.

Of course, if the truck has been used in essentially the same manner during each workshift, then the specific gravity (unstabilized) measured at the end of the shift will be pretty much the same from day-to-day. If there were any sudden change in the reading, it would be informative, but would not reveal anything about the state-of-charge except in a general way.

Figure 24: Relative Current Drain for Typical Industrial Trucks

Figure 25: A Hypothetical Industrial Truck Operation

Open Circuit Voltage: Another Static Measure

Since specific gravity and open circuit voltage are directly related, a similar line of reasoning shows that unstabilized open circuit voltage is also not a valid measure of battery condition. In Figure 26, any of the constant voltage lines can represent any number of battery discharge histories. Hence the open circuit voltage is not a useful measure of state-of-charge during operations. Note that battery manufacturers always determine capacity by measuring voltage while the load is connected. Disconnecting the load and immediately measuring open circuit voltage reveals nothing about the state-of-charge.

Voltage Under Load: A Dynamic Measure

When the truck operates, it presents varying electrical loads to the battery. As soon as the battery is "loaded" the open circuit voltage drops abruptly to the initial value of voltage under load. As long as the load current stays constant, the voltage under load slowly decreases as the battery discharges. If we keep track of the average voltage under load, we can tell how fast the battery is being discharged at any instant. Voltage under load measurements account for the rate at which a battery is discharged, whereas specific gravity and open circuit voltage reveal nothing about rate.

Figures 27 and 28 illustrate aspects of voltage under load. In Figure 27, voltage under load is shown as a measure of state-of-charge for 5 constant currents from 0-100% discharged. In Figure 28, varying current rates, based on the work procedure illustrated in Figure 24, are shown as a magnified micro-section tracked by an instrument with appropriate electronic computing circuitry.

Figure 26: Stabilized SG and Open Circuit Voltage as Measures of State-of-Charge for Various Discharge Rates

Since time always moves from left to right in Figure 27, the net effect of many different loads is to move the measurement point steadily toward the right, always along one load line or another. On the fork lift truck, there are many more load line variations. Every interval, during which the measured voltage follows a given load line, contributes a particular percentage of discharge.

One way to accomplish voltage averaging in an instrument is to continuously compare the varying battery voltage with a reference voltage which changes as a function of battery state-of-charge. Whenever the battery voltage is less than the reference voltage, the time below the reference voltage is measured and stored in the instrument's memory. The output of the memory sets the value of the reference voltage and represents the state-of-charge of the battery. It is displayed on a meter ("fuel" gage) located right on the truck in the driver's view.

Figure 28 shows how the reference moves in response to the battery voltage and how the "fuel" gage displays the state-of-charge.

State-of-Charge-Based Charging Can Save Energy

Some plant managers feel that trucks must be available without interruption throughout a workshift. Thus it becomes necessary to provide each fork lift truck with a fully charged battery at the start of each shift and to return the battery for charging at shift end. In any two- or three-shift operation, this practice normally requires at least twice as many batteries as trucks plus a charging station for each truck. For substantial fleets this means large capital investments, considerable floor space, significant personnel requirements, and additional energy costs.

This practice of workshift-based charging is especially prevalent in manufacturing plants where stopping the assembly line for any reason cannot be tolerated. At one automotive manufacturing site, at which a stalled line would be excessively costly, management has initiated a program in which fully charged batteries are installed on 150 trucks in 40 minutes at the end of every workshift.

Figure 27: Voltage Under Load as the Measure of State-of-Charge

Figure 28: A Practical Application of Voltage Under Load as a Measure of State-of-Charge

For other than such above noted critical requirements, with the use of a reliable and economical means of monitoring battery state-of-charge on the truck, another approach to charge scheduling has emerged. It is no longer necessary to provide each truck with a fully charged battery at the start of each shift and to work the battery through to shift end. Rather, if there is still significant charge left in the battery at the end of the shift, the battery can be left on the truck and worked until the need for charging is indicated. Then, and only then, the truck returns for a freshly charged battery. Its discharged battery is then placed on charge and, 8 hours later, is ready for use again.

As shown earlier, if an 8-hour charge period is used (as is generally the case), the optimum discharge point is 80%. Figure 29 shows how rapidly the energy requirements rise when batteries are charged for 8 hours after having been discharged less than 80%. Working each battery to the 80% discharge point before returning it for charging permits the most effective use of energy in the system and provides significant savings in energy costs.

Since each of the batteries reaches the 80% discharge point at a time that depends on the way it is worked, most will work longer than one 8-hour shift. Since a new battery isn't required for each truck at the end of every shift, it isn't necessary to have at least one replacement battery per truck, nor is it necessary to have at least one charger for each truck. Fewer batteries and chargers mean less capital investment, less space used for charging, and fewer people to do the work.

Figure 29: How Energy Requirements are Affected by Depth of Discharge

Further, when a battery is placed on charge it draws maximum current from the line. Placing all of the fleet's batteries on charge at one time, therefore, creates a large demand, which is reflected in the cost of energy in the form of higher utility peak demand charges. However, starting batteries on charge at different times throughout the entire shift reduces this demand and, therefore, the cost of energy.

How A "Typical" Fleet Can Save Energy

To dramatize the energy saved by charging batteries under optimum conditions, we have prepared data for a hypothetical 20-truck fleet operating for a 5-day week of 2 shifts per day. The data cover one month of 20 working days and show the following:

That it is possible to significantly reduce the amount of energy required to operate the fleet

That by appropriate fleet management it is possible to greatly reduce the magnitude of peak power demand

That this modified fleet operation holds the promise of reducing the capital and labor costs associated with industrial trucks.

Our data and calculations are not derived from operating a real fleet. They do, however, suggest how to reduce the cost of operation of any real fleet of trucks.

In this fleet, all trucks are equipped with the same battery type: a 36 volt, 1200 ampere-hour unit. In the original operating mode, each truck started each shift with a fully charged battery. We have divided the batteries into six classes (A-F) based on their average state-of-charge at the end of a typical shift. The classes, the number of batteries in each class, and the discharge data are listed in Section I of Table 4.

Section 2 of Table 4 shows the energy used by each battery (kilowatt-hours are calculated at the average output voltage). Section 3 then totals these per-battery figures by battery class. The overall efficiency is calculated by dividing the output energy (kWh) by the AC input energy and multiplying by 100. Since there are 40 shifts in our 20-day month, the totals are multiplied by 40 to obtain an estimate of energy used by the entire fleet over a full month of operation. A very substantial 31.6 megawatt-hours is used ... but at only 55% overall efficiency to produce the required 17.4 megawatt-hours of work delivered by the batteries.

By the simple expedient of returning each truck for a fresh battery when its "fuel" gage reads between 75% and 80% discharged, we can sharply reduce this energy waste. Since overall system efficiency is 67% for batteries that are charged after being 80% discharged, we can reduce the power line energy needed for our hypothetical fleet to 25.9 megawatt-hours if each battery is 80% discharged before being recharged. This means that about 5.7 megawatt-hours can be saved each month.

Reducing Peak Power Demand Costs*

If the 80% discharge point for our typical battery is 960 ampere-hours, then when that battery is discharged to 832 ampere-hours, or 69.3% (as in Class B), there is still a fraction of a workshift left in the battery. In fact, a Class B battery will actually last 1.15 shifts. Each of the other classes of battery will last correspondingly longer into the second shift, as shown in Table 5.

Table 5 shows how battery charges can be spread out across the two workshifts because individual batteries will reach 80% discharge at different times, depending on individual work profiles. With time, the spread will grow even more random, so that charge starts will be evenly spread throughout the two shifts. As shown in Figure 30, the net effect is to reduce the peak power demand below its absolute maximum of nearly 220 kilowatts. (The maximum peak power demand occurs when all 20 batteries are placed on charge at once.) For example, splitting the batteries into two groups of 10 cuts the peak demand to about 198 kilowatts, a 10% reduction. And in the optimum case, in which one battery starts on charge every 24 minutes throughout the shift, peak demand falls to 130 kilowatts, a reduction of over 40%.

• *Peak Power Demand: The use of electricity is made up of varying periods of high and low demand. Power companies build and provide sufficient generating capacity to meet periods of highest demand. To help defray the large capital outlay made to build and maintain maximum generating capacity the electric company charges customers what is referred to as a "demand charge". The charge is levied as an added cost of electricity. Two meters are used to monitor the consumption of electricity by commercial and industrial customers: one to track kWh's and one to monitor highest average kilowatts of demand, The demand meter reads 15 minute intervals of which the two highest are used to make up an average half-hour for the monthly demand charge. Thus, it behooves a customer to limit concentration of demand for power. In the electric fork lift truck environment, this means not putting all batteries on charge al the same time or charging batteries at periods of low demand (nighttime).

Figure 31 is a more general chart that shows the effect of different charging schedules on peak demand for any size fleet. To use the chart, calculate the maximum peak demand ? the peak demand when 100% of the batteries are placed on charge at the same time. Then decide how many batteries you will be charging in each group and the interval between groups. (The interval is equal to 8 hours multiplied by the percent placed on charge. For example, if 5% are to be started at once, the interval is 5% of 8 hours, or 24 minutes.)

Figure 31 shows that when 5% are placed on charge every 24 minutes, the peak demand is only 59% of maximum. If your peak demand were (for example) 300 kW, the state-of-charge-based charging procedure would reduce the peak to about 177 kW. Table 6 shows the dollar impact of workshift-based vs state-of-charge-based charging.

Figure 30: Reduction of Peak Power Demand with Slate-of-Charge Charging Schedule

Capital and Labor Savings

Since each battery can last for at least one shift, and most last for part of a second, it is no longer necessary to have two batteries for every truck. The total of 40 batteries previously required for our fleet of 20 trucks is reduced to only 35, a 12% saving in capital investment. We reach this conclusion by evaluating the number of batteries required per shift as shown in Table 7.

Figure 31: Effect of Alternative Charging Schedules on Peak Power Demand

From the reduction in the required number of batteries there follows, naturally, a reduction in the number of chargers required, and in the number of square feet of space devoted to charging, changing, and maintaining of batteries.

There also follows from the reduction in the number of batteries and chargers a 30% reduction in the total number of battery charges required during the year's operation of the fleet.

Under the workshift-based procedure, 200 battery charges per week supported 20 trucks, 2 shifts per day. This amounted to 10,400 battery charges per year as shown in Table 8.

In the state-of-charge-based procedure, made possible by on-board monitoring of battery capacity, there are only 35 batteries (instead of 40) to support the fleet. Thus, in 52 weeks, only 6812 charges are required for the fleet, a net reduction of 3588 charges per year.

Every time a battery is removed from a truck and charged a certain amount of labor is involved. In addition to the business of lifting, emplacing, and breaking and making connections, there is also the labor of checking specific gravity, topping off electrolyte level, cleaning, etc., all of which must be done every time a battery is charged, regardless of its state-of-charge when it is removed from the truck.

A net reduction of 3588 charges per year (more than 1.7 per workshift for our typical double-shift, 20-truck fleet) is certainly a labor saving worth examining on its own merits.

Further, since only a small number of trucks arrive at the charging station at any given time, owing to the fact that their batteries seldom reach full discharge at the same instant, there is a considerable reduction in waiting time as compared to the workshift-based procedure previously followed. Trucks (and drivers), therefore, are more productive on the average, since less time is spent standing idle, waiting for fresh batteries.

Section 5.

WEAR AND TEAR

Overworking the Battery

Overworking the battery can have a detrimental effect on its performance and life. For example, if the truck is worked well beyond the normal rating of its battery ... for example, by repeatedly lifting very large loads very fast for a long time ... it is possible for the battery voltage to fall below the manufacturer's specified end point. While it is true that letting the battery recover (for a time that may extend from minutes to hours, depending on the depth of discharge) will bring it back to a useful state-of-charge, it is also true that repeated heavy discharges of this kind can damage the plates by overheating, sulfation and cell (polarity) reversal.

The makers of trucks and their electrical components offer another set of objections to overworking the battery. Operating at lower-than-specified voltage can do irreparable damage to relays, SCRs (Silicon Controlled Rectifier), contactors, motors, etc.

Causes and Effects

Damage to a battery and/or a truck caused by deep discharge is the result of failure to detect the 80% discharge point of the battery and its continued use. In the case of component failure, inadequate maintenance is often at fault.

The use of a reliable, accurate and repeatable "fuel" gage on the truck will always prevent both battery and truck damage because the "fuel" gage will always detect the 80% recommended discharge limit. A properly designed "fuel" gage with a lift lockout will actually prevent the driver from working the truck past this limit and will force him to return for battery charging.

To get the most out of traction batteries, every truck should be equipped with a reliable, accurate, repeatable "fuel" gage and controller; operating procedures should be arranged so that batteries are placed on charge only when 80% discharged; chargers should be maintained in good operating condition; and a regular routine of inspection and preventive maintenance should be followed. To do less is to waste energy, time, and money.

Epilog

Curtis Instruments, Inc. has undertaken to produce and make available this text as a part of its effort to more completely understand batteries and their use with electric fork lift trucks and other industrial electric vehicles, and to share that understanding with others related to the material handling industry.

In the future, we hope to distribute supplements to this book in the form of "application notes" addressing such areas as battery manufacturers' rating systems, how they compare and concepts of standardization of graphical displays for test data; a universal energy units conversion table; etc.

We welcome reader comments, additions, corrections (if any), etc. Keeping the information accessible and flowing will have a most definite positive impact on all our related industries.

1.0 Types of Lead-Acid Batteries

We are dependent on lead-acid batteries for many uses in our lives that can be subdivided into three broad categories: engine-starting, motive power and standby power.

1.1 Engine Starting

The most common use of engine-starting batteries is in automobiles and trucks. They provide energy for starting, lighting and fuel ignition. Other uses occur in lawn mowers, snowmobiles, boats and all-terrain vehicles. The features of the battery for these applications are, for example:

• discharge at very high rates and at temperatures ranging from -20^°F to 200^°F

• a sufficient reserve capacity to operate vehicle electrical systems when charger fails and to power off-key drains

• thousands of engine starts (resulting in shallow discharge/charge cycles)

• low self-discharge rates so they operate after long periods of non-use

• charging by an alternator

• three-to-five-year life.

An engine-starting battery must be capable of high bursts of power. All types of polarization must be minimized to produce the highest voltage possible. Therefore, engine-starting batteries are designed with a very large area of working electrode surface. This reduces current density, which in turn reduces activation polarization during discharge. Large surface area is achieved by designing batteries with large numbers of very thin (as thin as 1 millimeter) electrodes. Resistive polarization is minimized by reducing the spacing between plates, reducing metallic conductive paths and using heavy-duty plate-connecting straps. These design features also reduce concentration polarization since the diffusion gradient from the bulk electrode to the reacting surface is reduced.

To provide reserve power in case of charger failure and to operate electrical components with the engine off, the engine-starting battery must contain sufficient active material (lead dioxide, lead and sulfuric acid) in the plates. This necessitates a design compromise requiring the use of thicker plates and a larger amount of electrolyte. The result is an increase in polarization and a reduction in power. Design invariably involves a trade-off between the highest cranking power and adequate reserve capacity.

1.2 Motive Power

The function of this type of battery, also called traction battery, is to propel an electric vehicle (EV). EVs are widely used in the material handling industry for supplying energy to fork lift trucks. Other uses include electric golf cars, mining vehicles, airport baggage handling tugs, sweepers/scrubbers and wheelchairs. These applications require the battery to be capable of:

• moderate discharge rates (three to six hours of discharge before recharging)

• high capacities up to thousands of ampere-hours

• cycling at high depths of discharge (over 80% of capacity removed before recharging)

• operation over a wide range of temperatures from 0^°F to 100^°F

• controlled charging

• five-year life (approximately 1500 cycles).

The principal requirements of traction batteries are high capacity and long life. Since capacity is the fuel that powers an EV, the higher the capacity, the more work that can be done before the need to recharge. For greater capacity, a battery is designed with thick electrodes containing large amounts of active material. To obtain increased active material, the density is increased to a higher level than that used in automotive batteries. Electrodes are considerably thicker than those in automotive batteries. Typical motive-power plates can be 6-7 millimeters thick. Thick electrodes give batteries longer life. Since the principal mechanism that causes these batteries to wear out is grid corrosion, using thick grids also extends life. Since discharge rates are moderate to low and the electrodes are made from thick lead-alloy grids, activation and resistive polarization are not major components of the voltage drop during discharge. This is primarily caused by concentration polarization due to the increased distance for ionic migration and diffusion. However, remember that during deep discharge, the amount of lead sulfate in the electrodes is increased significantly, causing an increase in resistance as discharge progresses. Eventually, active materials in the plates or sulfuric acid are consumed, causing a rapid increase in polarization and a reduction in the voltage at the end of discharge.

The battery is both the energy source and the fuel. Thus, it is important to know the depth of discharge or how much fuel remains. Since the life of a motive-power battery is proportional to depth of discharge, knowing how much capacity has been removed permits recharging before damaging the battery by over discharging.

1.3 Standby Power

Most of us never see a standby-power battery, though they are a major segment of the battery industry. Their use is growing at a faster rate than engine-starting and motive-power batteries. They are mostly used as components in larger systems and housed away from public view in dedicated rooms and cabinets. They function to provide energy when the main power is interrupted, i.e., during power outages. For example, we take for granted that our telephones operate during a blackout. In the United States and in other developed countries, entire telephone systems are supported by batteries that can supply power for up to several hours. Standby power permits making emergency calls in spite of power outages. The principal areas where standby-power batteries are used are:

• telecommunications

• uninterruptible power systems (UPS)

• switchgear and control operation

• emergency lighting

• security.

Of these, telecommunications and uninterruptible power systems are the largest segments in the United States with 48% ($310 million) and 27% ($174 million), respectively, of the total standby-power battery sales (1998 figures).

Each of these applications has different electrical requirements which, in turn, require different battery designs. However, they have in common the following features:

• wide range of discharge rates

• shallow cycling except in emergency situations

• narrow temperature range of operation

• float charging

• up to 20-year-life expectancy.

Standby batteries spend most of their life being charged at a rate that is just sufficient to maintain the battery in a state of full charge. This is known as float charging. It is also important that these batteries have low gassing rates during float charging so that water loss is minimized. A low gassing rate is achieved by controlling the polarization of the positive and negative electrodes to a value just sufficient to maintain full charge while at the same time minimizing electrolysis of water. The gassing rate is also reduced by the use of calcium, calcium/tin or low antimony-alloy grids. These alloys have low corrosion rates, thereby prolonging battery life.

The lead-calcium alloy grids in the positive plates slowly degrade by intergranular corrosion. In this process, corrosion takes place between the metallic grains and produces corrosion products that progressively push grains apart. This causes the alloy to expand and the plates to grow larger with time. To allow for this, the battery designer suspends the positive plates on a plastic bar attached to the negative plates, thereby providing space for the positive plate to expand. This design feature allows the grid to expand without placing strain on the cover seal.

Another feature of standby batteries is the use of flame-retardant vent plugs. These are necessary to prevent sparks or arcs from outside the battery communicating with the gases inside the cells and causing an explosion.

1.4 Valve-Regulated Lead-Acid Batteries

As we have already seen, conventional lead-acid batteries evolve hydrogen and oxygen when they are charged. These gases are a result of the electrolysis of water inside the battery and, therefore, water is consumed and must be replaced. Water replacement must be carried out frequently enough to avoid drying out the battery, a job that comprises a major activity of battery maintenance. Also, these gases are an explosion hazard and carry with them a fine mist of sulfuric acid that can be deposited on external conductors, resulting in corrosion. Since oxygen and hydrogen gases emitted from cells can form explosive mixtures, the battery room or enclosure has to be ventilated.

For many years, the battery industry has pursued a battery design that eliminates this maintenance and reduces the work and cost of owning batteries. The primary objective has been to develop a way to recombine the hydrogen and oxygen back into water inside the battery, eliminating the need to add water. The objective has been achieved and batteries that employ recombination principles are now widely available.

This new lead-acid battery design is the valve-regulated or recombination-type battery. Operating on the gas-recombining principle, it gets its name because it is fitted with a pressure-release valve that maintains a certain oxygen pressure inside the battery. In these batteries, oxygen generated inside the cell during charging is recombined within the cell to re-form water.

1.4.1 Principle of Operation

Normally, when a lead-acid battery is overcharged, oxygen is evolved at the positive plate and hydrogen is evolved at the negative plate. These gases are vented from the battery and the water that is consumed in producing them is replenished periodically from an outside source during normal maintenance. In the valve-regulated battery, a method has been found to recombine the gases inside the cell, thereby avoiding gas emission and the need to add water during the life of the battery.

Some years ago, it was discovered that if oxygen gas diffused to the negative plate, it would react with the negative sponge lead and be consumed. However, the amount of oxygen that could effectively reach the negative plate was severely restricted by the separators and the electrolyte. These formed a barrier to the diffusion of oxygen so that it was easier for the gas to escape from the cell than to migrate to the negative plate. With recently discovered and instituted design changes that promote diffusion of oxygen, virtually all of it can reach the negative plate and be recombined to water.

Oxygen will react at the negative plate in the presence of sulfuric acid as quickly as it can diffuse to the lead surface according to the following reaction:

Pb + H₂SO₄ + -O₂ = PbSO₄ + H₂O

Thus, the oxygen that diffuses to the negative is converted to water. As a result of this reaction, no water is emitted from the cell and, therefore, no water needs to be added. For this reason, these batteries are sometimes referred to as maintenance-free, although other forms of routine maintenance are still required.

There are two distinct designs of recombination battery currently in use: absorbedelectrolyte and gelled electrolyte.

1.4.2 Absorbed Electrolyte

The separator is replaced by a layer of porous glass mat. The cells are sealed with a valve to keep the cell pressurized at 2-5 pounds per square inch. The cell is filled with just enough electrolyte to wet the plates and partially wet the separator, thus creating an electrolyte-starved condition. Because the separator is not completely saturated with electrolyte, oxygen gas generated at the positive plate can diffuse through it and migrate to the negative plate. The pressure valve keeps the gas inside the cell long enough for diffusion to take place. As the oxygen is reduced at the negative plate, the negative-plate lead is oxidized to lead sulfate. This prevents the negative plate from becoming fully charged. Therefore, it does not start to evolve hydrogen. In some designs, an excess of negative active material is included to ensure the negative plate does not become fully charged. This provides additional protection against hydrogen evolution. Since the container of the cell or battery is held under pressure, it must be made of a material that will not distort.

1.4.3 Gelled Electrolyte

In the gelled electrolyte design, the plates are separated by conventional separators. The cell is filled with a gel composed of sulfuric acid and silica. After the gel is added to the cell, it hardens in a manner similar to gelatin so that it is immobilized. In time, the gel gradually dries out, creating very small cracks and fissures. These act as channels for oxygen to diffuse from the positive plate to the negative electrode.

Though the way in which transport of oxygen to the negative plate is different between the absorbed and gelled electrolyte designs, the principle of operation is the same for both. With the exception of the separator and gel, both types of batteries are constructed the same way.

1.4.4 Construction of Valve-Regulated Batteries

Grids

The recombination reaction in a valve-regulated battery is very sensitive to poisoning by low levels of impurities in the grid, active materials and electrolyte. For this reason, it is important that the alloys used to make the grids contain very low levels of metallic impurities. Virtually all valve-regulated batteries have positive grids made from either lead-calcium-tin alloys or pure lead while the negative grids are cast from lead-calcium alloy.

Plates

The plates are made in a manner similar to conventional automotive and industrial batteries. They are coated with a paste made from leady oxide, water and sulfuric acid. Like conventional batteries, the negative plates contain expander. After pasting, the plates are cured and dried using methods yet to be described. Since the amount of compression of the glass mat in an absorbed electrolyte design is very important to achieve exactly the right amount of wetting, it is important also to control the thickness of the plates very carefully.

Assembly

The assembly methods of valve-regulated batteries differ substantially from those of automotive and industrial batteries. In the absorbed-electrolyte type, the plates are clad in glass felt which acts both as separator and electrolyte absorber. This glass material is highly porous and has the ability to absorb a considerable amount of sulfuric acid. The amount of acid the glass absorbs can be adjusted by altering its compression - the greater the compression, the less the electrolyte absorbed. The correct compression is important to ensure that the cell contains enough electrolyte to sustain the discharge reaction of the battery while making sure that it is not saturated with acid. Mat oversaturation can prevent the diffusion of the oxygen gas from the positive plate to the negative plate.

In the absorbed-electrolyte design, the glass-clad plates are stacked to produce the required capacity and then the element is compressed so that it can be inserted into the container. The cover is cemented or welded in place and the electrolyte is then added. The assembled cell or battery can then be formed in a manner similar to that used for motive-power or standby batteries.

In the gelled-electrolyte type, the cell element is stacked in the standard manner with separators between the positive and negative plates. It is then inserted into the container and the cover is installed. The cell then can be filled with either gelled or liquid electrolyte. If liquid electrolyte is used, the cell is formed, drained and then refilled with gel. Alternatively, the battery can be filled with gelled sulfuric acid and then formed. Cell formation is done more quickly with the liquid electrolyte process than with the gel-filled. However, it does involve the extra step and added cost.

1.4.5 The Advantages of Valve-Regulated Batteries

Reduced Maintenance

Without the need to add water, most routine maintenance required on flooded batteries is eliminated. Periodic cleaning and servicing are greatly reduced due to the elimination of water spills on top of the battery and the associated corrosion of terminals.

Elimination of Gassing

No gases are evolved from the battery because they are recombined inside the container. Thus, there is no need to ventilate the battery compartment or room. These batteries are intrinsically safer than flooded types. The acidic spray emitted from vents of flooded batteries during charging that can cause corrosion of battery terminals and affect adjacent electronic circuitry does not occur.

Operation in Any Orientation

Electrolyte immobilization prevents acid spills. These batteries can be used on their side, a big advantage in installations where space is limited.

1.4.6 The Limitations of Valve-Regulated Batteries

Cycle Life

At the time of this writing, valve-regulated batteries have not been developed for applications requiring a large number of deep cycles. With deep-discharge cycling, these batteries display a rapid decline in capacity, known as premature capacity loss (PCL). Two reasons for PCL are proffered:

•  formation of a barrier layer of corrosion products between the active material and the grid that cannot be removed by charging. The layer progressively and eventually builds up enough to reduce capacity below a useful level;
•  loss of cohesion of the active material that gradually reduces inter-particle contact and thus capacity. Recent work has shown that increased compression on the electrodes reduces the effect of these structural changes, slowing down the capacity loss during cycling.

Another interesting finding, though not fully understood, is that rapid recharging, maybe resulting in better preservation of the structure of the active material, considerably reduces the capacity loss.

Unfortunately, lead-antimony alloys, which are widely used in batteries designed for cycling, cannot be used in valve-regulated batteries. They release antimony ions into the electrolyte that deposit on the negative electrode and increase hydrogen evolution.

These batteries do, however, have better cycle life than conventional flooded lead-calcium batteries. Therefore, they can be used satisfactorily in limited-cycle-life applications.

Float Life

Though these batteries are often used in float applications, they do not match the 20-year life of flooded types. While still in dispute, a ten-year life is likely the maximum achievable for valve-regulated batteries. A major reason for this is the difficulty in maintaining the proper degree of polarization of the electrodes during float charging. When the cell is charged, the current that would normally polarize the negative electrode and keep it charged is fully consumed in reducing the oxygen that has migrated from the positive electrode. Consequently, the negative electrode is depolarized and will be at its open circuit potential. Under this condition, it gradually self-discharges due to local action and loses capacity. However, if the float current is increased to assure that the negative remains fully charged, it will produce hydrogen gas that will cause loss of water and eventual failure by drying out. This knife edge situation makes it almost impossible to control the polarization of both electrodes to the correct degree during float charging. Impurities in the active material, grids and electrolyte can also affect the polarization of the negative electrode and either increase hydrogen evolution or self-discharge. Work is under way to develop charging strategies to overcome the problem. Catalysts are being looked at that would recombine hydrogen and oxygen, thus allowing the negative to be adequately polarized without risking dry-out.

1.4.7 Applications of Valve-Regulated Batteries

Valve-regulated batteries are becoming more widely used for standby applications and are finding limited use in cycling applications such as motive power. The major standby uses include:

•  telephone systems
•  uninterruptible power systems
•  burglar and fire alarms
•  emergency lighting.

The major attraction is reduced maintenance for the user.

The most popular motive-power application, at present, is for wheelchairs. The spill-proof properties and lack of gassing are the most attractive features.

Learn the easy solution of the Rubik's Cube and impress your friends with your amazing new skill.

1.0 Types of Lead-Acid Batteries

We are dependent on lead-acid batteries for many uses in our lives that can be subdivided into three broad categories: engine-starting, motive power and standby power.

1.1 Engine Starting

The most common use of engine-starting batteries is in automobiles and trucks. They provide energy for starting, lighting and fuel ignition. Other uses occur in lawn mowers, snowmobiles, boats and all-terrain vehicles. The features of the battery for these applications are, for example:

• discharge at very high rates and at temperatures ranging from -20^°F to 200^°F
• a sufficient reserve capacity to operate vehicle electrical systems when charger fails and to power off-key drains
• thousands of engine starts (resulting in shallow discharge/charge cycles)
• low self-discharge rates so they operate after long periods of non-use
• charging by an alternator
• three-to-five-year life.

An engine-starting battery must be capable of high bursts of power. All types of polarization must be minimized to produce the highest voltage possible. Therefore, engine-starting batteries are designed with a very large area of working electrode surface. This reduces current density, which in turn reduces activation polarization during discharge. Large surface area is achieved by designing batteries with large numbers of very thin (as thin as 1 millimeter) electrodes. Resistive polarization is minimized by reducing the spacing between plates, reducing metallic conductive paths and using heavy-duty plate-connecting straps. These design features also reduce concentration polarization since the diffusion gradient from the bulk electrode to the reacting surface is reduced.

To provide reserve power in case of charger failure and to operate electrical components with the engine off, the engine-starting battery must contain sufficient active material (lead dioxide, lead and sulfuric acid) in the plates. This necessitates a design compromise requiring the use of thicker plates and a larger amount of electrolyte. The result is an increase in polarization and a reduction in power. Design invariably involves a trade-off between the highest cranking power and adequate reserve capacity.

1.2 Motive Power

The function of this type of battery, also called traction battery, is to propel an electric vehicle (EV). EVs are widely used in the material handling industry for supplying energy to fork lift trucks. Other uses include electric golf cars, mining vehicles, airport baggage handling tugs, sweepers/scrubbers and wheelchairs. These applications require the battery to be capable of:

• moderate discharge rates (three to six hours of discharge before recharging)
• high capacities up to thousands of ampere-hours
• cycling at high depths of discharge (over 80% of capacity removed before recharging)
• controlled charging
• five-year life (approximately 1500 cycles).

The principal requirements of traction batteries are high capacity and long life. Since capacity is the fuel that powers an EV, the higher the capacity, the more work that can be done before the need to recharge. For greater capacity, a battery is designed with thick electrodes containing large amounts of active material. To obtain increased active material, the density is increased to a higher level than that used in automotive batteries. Electrodes are considerably thicker than those in automotive batteries. Typical motive-power plates can be 6-7 millimeters thick. Thick electrodes give batteries longer life. Since the principal mechanism that causes these batteries to wear out is grid corrosion, using thick grids also extends life. Since discharge rates are moderate to low and the electrodes are made from thick lead-alloy grids, activation and resistive polarization are not major components of the voltage drop during discharge. This is primarily caused by concentration polarization due to the increased distance for ionic migration and diffusion. However, remember that during deep discharge, the amount of lead sulfate in the electrodes is increased significantly, causing an increase in resistance as discharge progresses. Eventually, active materials in the plates or sulfuric acid are consumed, causing a rapid increase in polarization and a reduction in the voltage at the end of discharge.

The battery is both the energy source and the fuel. Thus, it is important to know the depth of discharge or how much fuel remains. Since the life of a motive-power battery is proportional to depth of discharge, knowing how much capacity has been removed permits recharging before damaging the battery by over discharging.

1.3 Standby Power

Most of us never see a standby-power battery, though they are a major segment of the battery industry. Their use is growing at a faster rate than engine-starting and motive-power batteries. They are mostly used as components in larger systems and housed away from public view in dedicated rooms and cabinets. They function to provide energy when the main power is interrupted, i.e., during power outages. For example, we take for granted that our telephones operate during a blackout. In the United States and in other developed countries, entire telephone systems are supported by batteries that can supply power for up to several hours. Standby power permits making emergency calls in spite of power outages. The principal areas where standby-power batteries are used are:

• telecommunications
• uninterruptible power systems (UPS)
• switchgear and control operation
• emergency lighting
• security.

Of these, telecommunications and uninterruptible power systems are the largest segments in the United States with 48% ($310 million) and 27% ($174 million), respectively, of the total standby-power battery sales (1998 figures).

Each of these applications has different electrical requirements which, in turn, require different battery designs. However, they have in common the following features:

• wide range of discharge rates
• shallow cycling except in emergency situations
• narrow temperature range of operation
• float charging
• up to 20-year-life expectancy.

Standby batteries spend most of their life being charged at a rate that is just sufficient to maintain the battery in a state of full charge. This is known as float charging. It is also important that these batteries have low gassing rates during float charging so that water loss is minimized. A low gassing rate is achieved by controlling the polarization of the positive and negative electrodes to a value just sufficient to maintain full charge while at the same time minimizing electrolysis of water. The gassing rate is also reduced by the use of calcium, calcium/tin or low antimony-alloy grids. These alloys have low corrosion rates, thereby prolonging battery life.

The lead-calcium alloy grids in the positive plates slowly degrade by intergranular corrosion. In this process, corrosion takes place between the metallic grains and produces corrosion products that progressively push grains apart. This causes the alloy to expand and the plates to grow larger with time. To allow for this, the battery designer suspends the positive plates on a plastic bar attached to the negative plates, thereby providing space for the positive plate to expand. This design feature allows the grid to expand without placing strain on the cover seal.

Another feature of standby batteries is the use of flame-retardant vent plugs. These are necessary to prevent sparks or arcs from outside the battery communicating with the gases inside the cells and causing an explosion.

1.4 Valve-Regulated Lead-Acid Batteries

As we have already seen, conventional lead-acid batteries evolve hydrogen and oxygen when they are charged. These gases are a result of the electrolysis of water inside the battery and, therefore, water is consumed and must be replaced. Water replacement must be carried out frequently enough to avoid drying out the battery, a job that comprises a major activity of battery maintenance. Also, these gases are an explosion hazard and carry with them a fine mist of sulfuric acid that can be deposited on external conductors, resulting in corrosion. Since oxygen and hydrogen gases emitted from cells can form explosive mixtures, the battery room or enclosure has to be ventilated.

For many years, the battery industry has pursued a battery design that eliminates this maintenance and reduces the work and cost of owning batteries. The primary objective has been to develop a way to recombine the hydrogen and oxygen back into water inside the battery, eliminating the need to add water. The objective has been achieved and batteries that employ recombination principles are now widely available.

This new lead-acid battery design is the valve-regulated or recombination-type battery. Operating on the gas-recombining principle, it gets its name because it is fitted with a pressure-release valve that maintains a certain oxygen pressure inside the battery. In these batteries, oxygen generated inside the cell during charging is recombined within the cell to re-form water.

1.4.1 Principle of Operation

Normally, when a lead-acid battery is overcharged, oxygen is evolved at the positive plate and hydrogen is evolved at the negative plate. These gases are vented from the battery and the water that is consumed in producing them is replenished periodically from an outside source during normal maintenance. In the valve-regulated battery, a method has been found to recombine the gases inside the cell, thereby avoiding gas emission and the need to add water during the life of the battery.

Some years ago, it was discovered that if oxygen gas diffused to the negative plate, it would react with the negative sponge lead and be consumed. However, the amount of oxygen that could effectively reach the negative plate was severely restricted by the separators and the electrolyte. These formed a barrier to the diffusion of oxygen so that it was easier for the gas to escape from the cell than to migrate to the negative plate. With recently discovered and instituted design changes that promote diffusion of oxygen, virtually all of it can reach the negative plate and be recombined to water.

Oxygen will react at the negative plate in the presence of sulfuric acid as quickly as it can diffuse to the lead surface according to the following reaction:

Pb + H₂SO₄ + -O2 = PbSO₄ + H₂O

Thus, the oxygen that diffuses to the negative is converted to water. As a result of this reaction, no water is emitted from the cell and, therefore, no water needs to be added. For this reason, these batteries are sometimes referred to as maintenance-free, although other forms of routine maintenance are still required.

There are two distinct designs of recombination battery currently in use: absorbed electrolyte and gelled electrolyte.

1.4.2 Absorbed Electrolyte

The separator is replaced by a layer of porous glass mat. The cells are sealed with a valve to keep the cell pressurized at 2-5 pounds per square inch. The cell is filled with just enough electrolyte to wet the plates and partially wet the separator, thus creating an electrolyte-starved condition. Because the separator is not completely saturated with electrolyte, oxygen gas generated at the positive plate can diffuse through it and migrate to the negative plate. The pressure valve keeps the gas inside the cell long enough for diffusion to take place. As the oxygen is reduced at the negative plate, the negative-plate lead is oxidized to lead sulfate. This prevents the negative plate from becoming fully charged. Therefore, it does not start to evolve hydrogen. In some designs, an excess of negative active material is included to ensure the negative plate does not become fully charged. This provides additional protection against hydrogen evolution. Since the container of the cell or battery is held under pressure, it must be made of a material that will not distort.

1.4.3 Gelled Electrolyte

In the gelled electrolyte design, the plates are separated by conventional separators. The cell is filled with a gel composed of sulfuric acid and silica. After the gel is added to the cell, it hardens in a manner similar to gelatin so that it is immobilized. In time, the gel gradually dries out, creating very small cracks and fissures. These act as channels for oxygen to diffuse from the positive plate to the negative electrode.

Though the way in which transport of oxygen to the negative plate is different between the absorbed and gelled electrolyte designs, the principle of operation is the same for both. With the exception of the separator and gel, both types of batteries are constructed the same way.

1.4.4 Construction of Valve-Regulated Batteries

Grids

The recombination reaction in a valve-regulated battery is very sensitive to poisoning by low levels of impurities in the grid, active materials and electrolyte. For this reason, it is important that the alloys used to make the grids contain very low levels of metallic impurities. Virtually all valve-regulated batteries have positive grids made from either lead-calcium-tin alloys or pure lead while the negative grids are cast from lead-calcium alloy.

Plates

The plates are made in a manner similar to conventional automotive and industrial batteries. They are coated with a paste made from leady oxide, water and sulfuric acid. Like conventional batteries, the negative plates contain expander. After pasting, the plates are cured and dried using methods yet to be described. Since the amount of compression of the glass mat in an absorbed electrolyte design is very important to achieve exactly the right amount of wetting, it is important also to control the thickness of the plates very carefully.

Assembly

The assembly methods of valve-regulated batteries differ substantially from those of automotive and industrial batteries. In the absorbed-electrolyte type, the plates are clad in glass felt which acts both as separator and electrolyte absorber. This glass material is highly porous and has the ability to absorb a considerable amount of sulfuric acid. The amount of acid the glass absorbs can be adjusted by altering its compression - the greater the compression, the less the electrolyte absorbed. The correct compression is important to ensure that the cell contains enough electrolyte to sustain the discharge reaction of the battery while making sure that it is not saturated with acid. Mat oversaturation can prevent the diffusion of the oxygen gas from the positive plate to the negative plate.

In the absorbed-electrolyte design, the glass-clad plates are stacked to produce the required capacity and then the element is compressed so that it can be inserted into the container. The cover is cemented or welded in place and the electrolyte is then added. The assembled cell or battery can then be formed in a manner similar to that used for motive-power or standby batteries.

In the gelled-electrolyte type, the cell element is stacked in the standard manner with separators between the positive and negative plates. It is then inserted into the container and the cover is installed. The cell then can be filled with either gelled or liquid electrolyte. If liquid electrolyte is used, the cell is formed, drained and then refilled with gel. Alternatively, the battery can be filled with gelled sulfuric acid and then formed. Cell formation is done more quickly with the liquid electrolyte process than with the gel-filled. However, it does involve the extra step and added cost.

1.4.5 The Advantages of Valve-Regulated Batteries

Reduced Maintenance

Without the need to add water, most routine maintenance required on flooded batteries is eliminated. Periodic cleaning and servicing are greatly reduced due to the elimination of water spills on top of the battery and the associated corrosion of terminals.

Elimination of Gassing

No gases are evolved from the battery because they are recombined inside the container. Thus, there is no need to ventilate the battery compartment or room. These batteries are intrinsically safer than flooded types. The acidic spray emitted from vents of flooded batteries during charging that can cause corrosion of battery terminals and affect adjacent electronic circuitry does not occur.

Operation in Any Orientation

Electrolyte immobilization prevents acid spills. These batteries can be used on their side, a big advantage in installations where space is limited.

1.4.6 The Limitations of Valve-Regulated Batteries

Cycle Life

At the time of this writing, valve-regulated batteries have not been developed for applications requiring a large number of deep cycles. With deep-discharge cycling, these batteries display a rapid decline in capacity, known as premature capacity loss (PCL). Two reasons for PCL are proffered:

• formation of a barrier layer of corrosion products between the active material and the grid that cannot be removed by charging. The layer progressively and eventually builds up enough to reduce capacity below a useful level;
• loss of cohesion of the active material that gradually reduces inter-particle contact and thus capacity. Recent work has shown that increased compression on the electrodes reduces the effect of these structural changes, slowing down the capacity loss during cycling.

Another interesting finding, though not fully understood, is that rapid recharging, maybe resulting in better preservation of the structure of the active material, considerably reduces the capacity loss.

Unfortunately, lead-antimony alloys, which are widely used in batteries designed for cycling, cannot be used in valve-regulated batteries. They release antimony ions into the electrolyte that deposit on the negative electrode and increase hydrogen evolution.

These batteries do, however, have better cycle life than conventional flooded lead-calcium batteries. Therefore, they can be used satisfactorily in limited-cycle-life applications.

Float Life

Though these batteries are often used in float applications, they do not match the 20-year life of flooded types. While still in dispute, a ten-year life is likely the maximum achievable for valve-regulated batteries. A major reason for this is the difficulty in maintaining the proper degree of polarization of the electrodes during float charging. When the cell is charged, the current that would normally polarize the negative electrode and keep it charged is fully consumed in reducing the oxygen that has migrated from the positive electrode. Consequently, the negative electrode is depolarized and will be at its open circuit potential. Under this condition, it gradually self-discharges due to local action and loses capacity. However, if the float current is increased to assure that the negative remains fully charged, it will produce hydrogen gas that will cause loss of water and eventual failure by drying out. This knife edge situation makes it almost impossible to control the polarization of both electrodes to the correct degree during float charging. Impurities in the active material, grids and electrolyte can also affect the polarization of the negative electrode and either increase hydrogen evolution or self-discharge. Work is under way to develop charging strategies to overcome the problem. Catalysts are being looked at that would recombine hydrogen and oxygen, thus allowing the negative to be adequately polarized without risking dry-out.

1.4.7 Applications of Valve-Regulated Batteries

Valve-regulated batteries are becoming more widely used for standby applications and are finding limited use in cycling applications such as motive power. The major standby uses include:

• telephone systems
• uninterruptible power systems
• burglar and fire alarms
• emergency lighting.

The major attraction is reduced maintenance for the user.

The most popular motive-power application, at present, is for wheelchairs. The spill-proof properties and lack of gassing are the most attractive features.

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2.0 Electrode Materials

Although a number of basic materials are used to produce all types of lead-acid batteries, significant differences exist among battery types in both materials and design. Batteries are built to satisfy specific application-based demands. Each particular duty cycle prescribes its own design, materials and methods of construction. For example, engine-starting batteries use lead-calcium alloys for their high conductivity needed for high cranking rates. Motive-power batteries use lead-antimony alloys for their renowned cycling capability. Different separator and container materials meet the needs of a wide variety of performance life and environmental conditions.

2.1 Lead

Lead is the basic raw material used to produce the lead oxide and alloys used to make lead-acid batteries. The industry standard for pig lead is ASTM Designation B29-79(84), which covers two grades used to produce oxide for active material or for mixing with other elements to produce alloys. The specifications are shown in

Table 2-1.

• Table 2-1: Chemical requirements for ASTM designation B29-79(84)

The only difference between these designations is the permissible concentrations of silver and the aggregate of silver and copper. Both elements increase evolution of hydrogen from the negative electrode. Therefore, corroding lead or lead of even greater purity is generally used in battery types where low water loss is required.

Most battery manufacturers use their own specifications and can specify lower limits and additional elements not included in Table 2-1. The Battery Council International (BCI) has also developed specifications for Standard Reference Materials used by lead-acid battery manufacturers. They include antimony alloys and calcium alloys with and without tin. They are shown in Table 2-2.

• Table 2-2: Chemical analyses of BCI standard reference materials

The BCI standards provide general guidance to the industry. However, as in the case for lead metal, battery manufacturers frequently use their own specifications for alloys. Standards have also been developed for battery containers and separators.

2.2 Lead Oxides as Starting Material

Lead oxides have been used in lead-acid batteries as starting materials for plates since the early days, particularly lead monoxide (PbO) and red lead (Pb₃O₄). It was customary to use a mixture of lead monoxide and red lead in the positive plates and pure lead monoxide in the negative plates. In contrast to today, these oxides contained very little free lead, as most battery producers then believed that it lowered the quality of the product.

The development of two processes, Ball Mill and Barton, for oxide production resulted in a dramatic change in the oxides used for battery manufacture. Both processes considerably reduced the cost of lead monoxide, but yielded a material containing a significant amount of free lead. The material is generally called leady oxide. Prompted by the reduced cost, battery companies developed paste formulations and new processes, i.e., curing, that permitted leady oxide use. Starting in the 1930s, leady oxide became the material of choice. Its principle advantages are:

• low production cost
• usable for both positive and negative electrodes
• usable for all types of lead-acid batteries.

Some disadvantages of leady oxide exist, for instance, no batch-to-batch exact repeatability in the manufacturing process. This causes variability in the preparation of pastes and thereby variability in plates. Free lead remains in the plates following pasting and subsequently has to be removed during the curing process. This extends the curing time, increases the inventory of plates and, perhaps most seriously, prevents development of continuous paste mixing-pasting-drying-assembly processes.

With the advent of valve-regulated batteries and battery applications that demand a high degree of reproducibility among cells, non-leady oxides, particularly pure lead monoxide and red lead, have had an upsurge in use. These materials can be made to exact chemical compositions, eliminating most of the variability associated with leady oxide. They also reduce curing time and allow continuous plate-making technology. In addition, red lead reduces formation time and improves initial capacity.

An intriguing development of the late 1990s was the use of basic lead sulfates or partially sulfated oxides to make plates. These materials have the same benefits as non-leady oxides and also reduce the amount of heat developed when the cells are filled with electrolyte. The need for cooling is thereby reduced, as is the time between filling with electrolyte and placing on formation.

Leady oxide still remains the most widely used active material for battery plate production. The Barton and Ball Mill processes are still used to make leady oxide.

Barton processing begins with feeding molten lead and air into a large pot equipped with a rotating paddle to agitate the lead. Represented by the equation Pb + O₂ → PbO, the formed lead oxide is drawn off by an air stream, classified with a cyclone and blown to a hopper. Often, it is also passed through attrition mills to produce different particle size distributions for different battery applications.

Varying the conditions under which the reactor operates permits controlling the degree of lead oxidation, particle size distribution and the ratio of αPbO to βPbO in the product. For example, high-temperature-reactor operation reduces the amount of free lead in the oxide and increases the amount of βPbO. By reducing temperature, free lead is increased, as is the ratio of αPbO to βPbO. Air-flow rate through the reactor also affects characteristics of the oxide. Increased air flow increases both free lead and oxide particle size as shown in Table 2-3. Conversely, decreasing air flow reduces both free lead content and particle size. The Barton system is extremely versatile, which is why it is so widely used by the battery industry as shown in Table 2-4.

Attrition mill oxide, also leady oxide, is more commonly produced in Europe and Asia. Barton oxide is more common in the United States. In the attrition mill process, lead balls or ingots are tumbled in a rotating drum. The lead is abraded through friction and oxidized by air passed through the drum. The oxide produced this way is generally finer than Barton oxide. It has a greater acid absorption value, usually in the range of 220-240 mg/g. The oxide contains no β modification, since the temperature of the reaction in the attrition mill is only about 100^°C.

• Table 2-3: Variable conditions and effects in the Barton process

• Table 2-4: Barton process leady oxide characteristics for different battery applications most important to battery manufacturers

2.3 Pure Lead Monoxide

Pure lead monoxide is becoming widely used by battery manufacturers, particularly of valve-regulated batteries. The most important way it differs from leady oxide is its absence of free lead. Particle size is greater and acid absorption lower than required for automotive batteries, but is about equivalent to the leady oxide used in industrial batteries. The absence of free lead improves the reproducibility of paste mixing and curing. As noted above, with leady oxide some free lead is oxidized during paste mixing and more oxidation occurs during pasting and curing. Since the amount of oxidation is variable in each step, inconsistent paste characteristics result; therefore, there is reduced reproducibility in the finished plates. The result is further variability in the cells made of these plates. This is a major problem in valve-regulated batteries where cell-to-cell voltage repeatability is important for proper long-term operation under oxygen recombination conditions. Another advantage of lead monoxide is that no lead-oxidation-elimination step is required in the curing process. This reduces time and permits design of process conditions for producing the optimum ratio of tribasic and tetrabasic lead sulfates in the plates.

• Table 2-5: Typical characteristics of battery grade lead monoxide

2.4 Red Lead

Red lead is beneficial in improving the electrochemical performance of lead-acid batteries. The lead is in a higher oxidation state, at a ratio of lead:oxygen = 1.33, than in lead monoxide and its electrical conductivity is also higher. Batteries made with red lead in the positive paste can be formed quicker. They also have an initial capacity higher than those made from leady oxide pastes.

Red lead is normally used in one of two ways: a high-percentage red lead (~80% % by weight Pb₃O₄) can be blended into a paste mixture to produce the desired amount in the finished paste, or a lower-percentage red lead (~25% % by weight Pb₃O₄) can be substituted for leady oxide. Either way, paste-mixing and plate-pasting processes are very similar to those with leady oxide. Particle size decreases and acid absorption increases as the percentage of Pb₃O₄ in the red lead is increased. Thus, if a paste mix is produced containing 25% by weight Pb₃O₄, a better result is generally obtained when it is added as the higher percentage material due to its higher reactivity.

• Table 2-6: Typical characteristics for battery-grade red lead

2.5 Oxide Production Methods

Oxide production methods for leady oxide, Pb0 and red lead are illustrated in Figure 2-1. Due to its low cost and flexibility, most lead oxide in the United States is produced by the Barton process. Barton pot oxide is also used as the feedstock for producing pure lead monoxide (Pb0) and red lead.

As shown in Figure 2-1, lead of the required purity is melted in a kettle and then fed into a reactor vessel equipped with a rotating paddle. Air is drawn through the reactor and leady oxide is drawn off by the air stream and classified by conventional methods. By careful control of temperature, air flow and paddle speed, oxide of various characteristics can be produced. Generally, the temperature will influence the amount of free lead and the alpha:beta ratio in the oxide. The velocity of the air flow will influence the amount of free lead and particle size of the material.

Pure Pb0 and red lead are produced in calcining furnaces, in which the raw material is agitated while being heated at the optimum temperature for oxidation to the required product. For Pb0, the temperature is held at 600^°C. For red lead, a temperature of 450^°C to 500^°C is used. For red lead, the furnace is discharged when the desired amount of Pb₃O₄ is reached. Oxide with 25% by weight red lead can be made in 5-6 hours, while an oxide with >80% Pb₃O₄ may require 16-18 hours of calcining.

 The most important physical property of the oxide is particle size distribution, influencing both battery performance and life. Though the Barton process is capable of a great degree of flexibility through process parameter adjustments, a system with a hammer mill provides greater control of particle size. In a hammer mill, a variable-speed rotating shaft is fitted with a number of hammers. An air stream is drawn through the mill and controlled by a damper setting. Oxide dwell time in the mill can be varied depending on air velocity, while the amount of grinding is controlled by the number of hammers and rotation speed. The oxide being fed to the mill can be ground to a wide range of particle size distributions. Process settings have been established that allow a high degree of control over median particle size and amount of fine particle in the product. Through a combination of equipment and process, oxide particle size distribution can meet the desired specifications of the battery manufacturer.

Figure 2.1: Schematic diagram of process for production of leady oxide, lead monoxide and red lead

2.6 Lead Sulfates as Starting Material

Other active materials than lead oxides are used by some battery manufacturers as the starting material for the active mass. In the late 1990s, battery designs included electrodes wound into a spiral. These batteries have a very high plate-surface area, resulting in delivery of very high power output. A problem with this type of construction is the heating that occurs when cells are filled with electrolyte. A reaction between sulfuric acid and unreacted oxide in the paste results. The manufacturer may need to install cooling systems to remove the heat before the battery is formed. In extreme cases, the reaction is so rapid that the specific gravity of the electrolyte drops to a dangerously low level, leading to dissolution of the lead sulfate. When the cells are placed on formation and the specific gravity of the electrolyte again rises, the lead sulfate can precipitate in the separator and short circuit the cell.

The problem can be reduced or eliminated by use of pastes composed of lead sulfate and water. Depending on battery application, tribasic lead sulfate, tetrabasic lead sulfate or a mixture can be used. The paste can be made from pure lead sulfates or oxide sulfated to a higher than usual degree. Cells made with highly sulfated plates evolve less heat when they are filled with electrolyte and do not require cooling. Short circuiting problems are also eliminated.

2.7 Sulfuric Acid

Sulfuric Acid plays a dual role in a lead-acid battery, both as a reactant and an electrolyte, i.e., an ionic conductor. It is important that the acid used in the electrolyte is pure and does not contain any impurities that can affect battery performance. The battery industry considers sulfuric acid that meets Federal Specification O-S-801-b to be acceptable for electrolyte manufacture. The impurity limits in Federal Specification O-S-801-b are shown in Table 2-7.

• Table 2-7: Maximum impurity limits for sulfuric acid in lead-acid battery electrolyte

The concentration of sulfuric acid that is used in the various processes for production of lead-acid batteries can span a wide range. Thus, it is usually purchased as 93.19% H₂SO₄ (1.835 s.g.) and then diluted to the required strength for either the process or the battery type.

2.8 Grids

For the majority of lead-acid batteries, the positive electrode controls performance and life. Battery failures are generally caused by degradation of the positive plate. Based on surveys of failed automobile batteries, the predominant causes of failure have been:

• grid corrosion
• grid growth
• active material shedding.

In the late 1990s, some negative-plate-based battery failure has been identified in valve-regulated batteries in float service. Though not fully understood, the phenomenons most likely cause relates to self-discharge resulting from insufficient polarization while charging.

As noted above, positive plates have to meet different missions depending on battery application. This has resulted in considerable differences among the materials, design and processing of these plates for automotive, industrial and standby applications.

Positive grids in all batteries act as both support structure and electrical conductor. They must be capable of withstanding all handling operations during production, such as casting, trimming, pasting, stacking and welding. They must also have adequate electrical conductivity to reduce Ohmic losses during operation and resist corrosion so that adequate service life will be obtained. Positive grids or, more properly, support structures can be made from pure lead or from lead alloys. While the vast majority of batteries have some kind of grid structure to hold the paste, in the newer spiral-wound designs the paste is applied to a lead foil instead of a grid.

Grids can be made in a variety of ways, for example:

• book mold casting

• continuous casting

• expanded metal

• stamping.

In book mold casting, the grid is cast in a mold directly from molten lead alloy. In continuous casting, the molten alloy is dispensed onto a rotating wheel and engraved with the grid pattern. In expanded metal and stamping, the grid is formed mechanically from a cast or rolled alloy strip. Book mold casting is more suited to small production runs as the molds on the casting machines can be changed easily and quickly. The other methods, requiring longer set-up times, suit long production runs of the same grid.

2.9 Grid Alloys

Grid alloys used most widely are lead-antimony (Pb-Sb), lead-calcium (Pb-Ca) and lead-calcium-tin (Pb-Ca-Sn). The alloying metals increase the mechanical strength of the lead, increase its creep resistance and improve its castability. These alloys are less corrosion-resistant than pure lead. Thus, other additives are used to modify the grain structure and reduce corrosion. Lead-antimony alloys are generally used in batteries that require deep discharges and long cycle lives. Lead-calcium(tin) alloys are used where the duty cycle requires high-rate discharges and shallow cycling. Some applications of the two classes of alloys are shown in Table 2-8.

• Table 2-8: Applications of lead-antimony and lead-calcium(tin) alloys

Lead-calcium alloys are used in lead-acid battery grids, as calcium has proven to be a beneficial hardening agent. Figure 2-2 is a phase diagram to aid understanding of the hardening process. A phase diagram shows the different phases that occur in a mixture of metals as functions of the relative amounts of the metals and the temperature. Lead-calcium alloys are strengthened by precipitation of a calcium-rich intermetallic compound from a supersaturated solid solution. Supersaturation occurs when the molten alloy is cooled rapidly, for example, during grid casting. Considering the region marked α in Figure 2-2, the strengthening results from a dispersion of the second phase throughout the matrix. As revealed in the diagram, above 0.07% Ca, Pb₃Ca crystals are formed. Below 0.07% Ca, Pb₃Ca crystallites are found in the alloy, contributing to the strength by impeding grain boundary mobility.

Figure 2.2: Lead-calcium phase diagram

Lead-antimony alloys are also used in lead-acid batteries, as antimony is another effective hardening agent for lead. The phase diagram for the lead-antimony binary system is shown in Figure 2-3. The diagram reveals that for antimony concentrations lower than 3.5%, the alloys are age-hardenable and can be strengthened by formation of supersaturated solutions. This occurs when the alloy is cooled rapidly during grid casting. In this respect, antimony and calcium alloys display similarities both are hardenable by the formation of supersaturated solutions. In antimony alloys, precipitation of antimony occurs from the supersaturated solution. Between antimony concentrations of 3.5% and 11.1%, a two-phase structure of the α plus eutectic is formed when cooled to room temperature. The eutectic acts as a stiffening framework for the solid solution. As the amount of antimony is increased, increasing amounts of eutectic (α +β) are formed up to the eutectic composition of 11.1% antimony. At this point, the alloy reaches its maximum strength. This framework of eutectic in the solid solution can cause the alloy to become brittle and prone to cracking.

Figure 2.3: Lead-antimony phase diagram

When tin is added to lead-antimony alloys, it increases the strength by forming an antimony-tin precipitate in the solid solution.

Age hardening refers to the fact that hardening by precipitation of phases from a supersaturated solution is a time-dependent process hardness increases progressively with time after casting. The newly cast grids are soft and flexible, requiring aging for a certain amount of time before they are strong enough to be pasted.

It is common to age harden lead-calcium grids for up to three days and lead-antimony grids for up to two days before pasting. Figure 2-4 shows the ultimate tensile strength of a 0.08% calcium alloy after various aging times.

Figure 2.4: Aging characteristics of lead-0.08% calcium alloys

The air-cooled alloys reach about 80% of their fully aged strength within 24 hours of casting and then slowly increase in strength over time, reaching a level of about 5,000 psi. If the grids are quenched in water, they are considerably harder, reaching a fully aged hardness of about 6,300 psi. This is probably due to a greater degree of supersaturation in the solid solution that results in increased precipitation and strengthening.

The effect of adding tin to the alloy is shown in Figure 2-5.

Figure 2.5: Aging characteristics of cast, air cooled, lead-calcium-tin alloys

Alloy compositions used by battery manufacturers are different for each application.

Automotive battery-grid alloys, since the 1970s, have been the lead-calcium(tin) type almost without exception: a result of the introduction of maintenance-free designs where it is either difficult or impossible to add water. Thus, alloys that minimized water loss during charging were required. For some years, alloys containing a lower amount of antimony were used, but since the 1970s they have been gradually phased out in favor of lead-calcium(tin) systems. Table 2-9 lists examples of widely used low-antimony and lead-calcium alloys. The first calcium alloys did not incorporate aluminum. It was added in the late 1970s to prevent calcium loss in the melt pot. Positive plate-grid alloys have also been developed with higher tin levels to provide improved conductivity of the grid/active material interface. An example is shown in Table 2-10.

• Table 2-9: Typical maintenance-free alloys used in automotive battery positive grids

• Table 2-10: Positive grid alloy with enhanced conductivity of the grid/active material interface

The use of low-antimony alloys in maintenance-free automotive batteries was greater in Europe than in the United States. To reduce gassing to the lowest level possible consistent with good castability, an alloy was developed with antimony in the concentration range of 1.3%-1.5%. It was used in both positive and negative grids. A typical specification for the alloy is shown in Table 2-11. Selenium is added to the alloy as a grain refiner to improve castability.

• Table 2-11: Typical specification for very-low-antimony alloy used in automotive battery positive and negative grids

Calcium alloys were used in automotive battery negative grids before being used in positive grids, because grid corrosion is insignificant at the negative plate; thus, there is no problem with a passivating-corrosion-layer build-up between the grid and the active material. A common specification for a negative-grid alloy with rapid-hardening characteristics and with aluminum to prevent calcium loss is shown in Table 2-12.

• Table 2-12: Negative-grid alloy for rapid hardening for automotive batteries

Recently, automotive battery manufacturers have started to use elevated silver levels in positive-grid alloys to reduce corrosion and increase creep resistance at high temperatures. This practice is becoming widespread as under-hood temperatures continue to increase. An example of a calcium/tin/silver alloy is shown in Table 2-13.

• Table 2-13: Calcium/tin/silver alloy for high-temperature corrosion and creep resistance

Automotive battery plates are welded together in the cells using the cast-on-strap process. A good cast-on-strap alloy has a moderate amount of eutectic (27%-45%) to facilitate easy bonding to the lugs and terminals. The alloy has a large slushy region making it ideal for joining materials. It also contains moderate amounts of copper and selenium as nucleants. A cast-on-strap alloy is also often used for small parts casting. Three cast-on-strap alloys are shown in Table 2-14. The first is general purpose, the second has greater ductility to resist vibration and the third uses selenium in lieu of copper and sulfur as nucleants.

• Table 2-14: Examples of cast-on-strap alloys

Motive-power batteries use antimonial-alloy positive grids to achieve long cycle lives. The antimony concentration varies from 5%-6%. Table 2-15 shows examples. Note the use of selenium as a grain refiner in the 5% alloy.

• Table 2-15: Motive-power antimonial positive grids

In tubular plate design of motive-power batteries, positive-plate spines are cast from an alloy with a higher amount of antimony to improve flow characteristics into the tube mold. The eutectic composition of 10.5% antimony is often used. Table 2-16 shows the composition of a typical spine alloy.

• Table 2-16: 10.5% antimony alloy for tubular plate spines

Standby-power and valve-regulated batteries use lead-calcium alloys for both positive and negative grids. Batteries designed for cycling service have alloys with a significant amount of tin, while batteries designed for float service contain less tin. Examples are shown in Table 2-17.

• Table 2-17: Lead calcium grids for standby-power and valve-regulated batteries

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3.0 Electrode Paste Manufacturing

Grids are the support structure for the active material. In most lead-acid batteries, the active material is made from a paste of leady-oxide water, sulfuric acid with added fibers for greater strength and, for the negative plate, expanders. The paste formulas and properties are different for positive and negative pastes, each reaching its optimum performance at different densities.

The paste manufacturing process is very important as battery performance and life are determined by the properties of the paste. The way the paste is applied to the grid is also important to reduce the variability in both the weight of paste in the grid and the thickness of the pasted plate.

Before any grid can be pasted, it must be hard enough to withstand the stresses of the pasting operation. As noted above, both calcium and antimony alloys are soft after casting and require some time to harden. Hardening is achieved most commonly by stacking the cast grids on pallets for about three days at room temperature. Hardening can be accelerated with heat. Some battery manufacturers employ controlled-temperature heating chambers. At 150^°C (302^°F) the grids can be aged in less than 24 hours. The shorter aging time reduces manufacturer inventory of grids in process.

A battery paste is a complex chemical mixture. Its composition depends on the materials used and the nature of the mixing process. A freshly made paste is not at equilibrium and is still undergoing chemical and physical reactions. If not used immediately, it will become hard and unworkable. Thus, in a battery plant, plate mixing and pasting processes are usually coupled so that fresh paste is constantly delivered to the pasting machine and used right away.

3.1 The Chemistry of Paste

When water is added to lead oxide, it first fills the porous structure of the particles and then begins to coat them. The oxide continues to absorb water until all the particles are coated and water has displaced the air that was among the particles. At this point, the paste is a firm mass which will not flow unless force is applied to it. The addition of more water forces the oxide particles apart, causing bulking of the paste and an improvement of flow characteristics. Eventually, water fills all open space among the particles and the paste reaches its maximum plasticity. If water is added beyond this point, the paste becomes loose and solid/liquid surface energy and heat evolve. The pH of the paste at this point is between 9 and 10, indicating that a reaction is taking place between oxide and water. A probable result is the formation of lead hydroxide, releasing hydroxyl ions to the solution.

When acid is added to the mix, heat is evolved and the paste stiffens considerably. Work done by Ritchie et al (E.J. Ritchie et al, ILZRO Projects LE-82 and LE-84, final report, 12/31/71) indicates that the stiffening is due to an increase in the amount of water held in the colloid sheath around the particles. Since the stiffer paste requires more energy for mixing, more power is absorbed and its temperature increases. As more acid is added, the paste changes plasticity and texture and temperature continues to rise. This is due to the heat of reaction, the heat of dilution of the sulfuric acid and the mechanical work of mixing the paste.

When the acid first contacts the wet PbO, it probably reacts to form normal lead sulfate (PbSO₄), which reacts with PbO to form basic lead sulfates. At temperatures below 160^°F (71.1^°C), the stable basic sulfate is tribasic lead sulfate (TRB). At higher temperatures, tetrabasic lead sulfate is formed by loss of water and the reaction with PbO.

4PbO + H₂SO₄ → PbSO₄3PbOH₂O (below 160^°F)

PbSO₄3PbOH₂O + PbO → PbSO₄4PbO + H₂O (below 160^°F)

Ritchie et al speculate that other basic lead sulfates may be formed as intermediates in these reactions.

The chemical nature of the lead sulfate in the paste is important since it affects the facility with which the plate can be formed, its electrochemical performance and its durability under discharge/charge cycling. TRB is the preferred compound for engine-starting battery plates. It is readily converted to PbO₂ during formation and confers high initial performance.

TTB is considerably more difficult to form, particularly in sulfuric acid solutions of concentrations greater than 15%, but it contributes to the formation of a strong and stable morphology in the crystal structure of the plate. In battery types where TTB is preferred, it is more common to produce this during the curing process than during paste mixing.

Leady oxide made by the Barton process contains up to 10% orthorhombic (β) PbO. It has lower stability than tetragonal (α) modification and is partially converted to the tetragonal phase during mixing. The finished paste, at the point of completion of pasting, will contain the following compounds:

αPbO PbSO₄4PbO

βPbO (Barton oxide only) Pb(OH)₂

PbSO₄3PbOH₂O H₂O

Paste for Engine-Starting Batteries

Typical paste formulas for the positive and the negative plates are:

• *Plastic fibers are usually added to both positive and negative pastes to increase their dry strength. These are added at about 0.5-0.7 pounds per ton of oxide. Expanders are added to the negative paste at 0.5% to 1.00% of the oxide weight.

Most paste mixers are similar in design and operation. The procedure described below is usually followed. Oxide is dispensed into the mixing bowl. Other dry ingredients (fiber and expander) are added. As rapidly as possible, water is introduced. The water paste is mixed for 2-4 minutes to form a uniform mixture. Sulfuric acid is then added. To avoid hot spots, the acid is added slowly and distributed evenly over the paste, using a multi-nozzle dispenser. The sulfuric acid reacts with the lead oxide, causing the paste to get hot. For engine-starting batteries, the temperature of the paste should not be allowed to rise above 150^°F (66^°C). This prevents the formation of tetrabasic lead sulfate in the paste that would reduce battery performance. After adding the acid, the paste is cooled as rapidly as possible to prevent drying. It is cooled to 120^°F (49^°C) before being discharged into the pasting machine hopper.

The cooling is necessary to reduce water loss from the paste while it is consumed by the pasting machine. It takes about 20 minutes for the machine to use a ton of paste and considerable drying can occur if the paste is too hot. Excessive temperature can affect the pasting characteristics of the paste and its density, resulting in a variation of plate weights.

Paste consistency is usually measured by determining its density and plasticity. Density is measured by packing the paste into a cup with a volume of two cubic inches. The weight of the paste in the cup is converted to a density reading, which in the U.S. is curiously expressed in units of grams per cubic inch. Elsewhere, paste density is expressed as grams per cubic centimeter. A consensus on the correct density for positive and negative paste does not exist. However, paste density falls generally in ranges as shown below.

Plasticity of the paste is usually measured by a Globe #1 penetrometer. It is dropped from a height of six inches (152 mm) above the top of the density cup. Penetrometer readings for paste mixes are shown below.

Pastes for Industrial Batteries

Industrial batteries, in this book, include those designed for motive power, standby power and valve-regulated types. Such batteries are designed for long life under arduous operating conditions that can include repetitive deep cycling and long periods of float charging. Under these conditions, plate active material can soften and shed from the plate. Thus, the paste has a higher density to give it more strength. Different paste formulas are used for industrial batteries as shown below.

For industrial-battery pastes a temperature cap of 150^°F (66^°C) is not critical. It is desirable for industrial-battery positive plates to contain tetrabasic lead sulfate (TTB), which begins to form at temperatures above 160^°F (71^°C). The presence of TTB crystals in the plates can act as sites for the further growth of TTB during the curing process. Cooling the paste rapidly after all acid is added, to avoid drying out during pasting, is as important for industrial batteries as for engine-starting types.

Note that TTB is not formed in the negative-paste mix due to the inhibiting effect of the expander.

The penetrometer readings and densities for typical industrial pastes are:

Occasionally, due to variations in the oxides characteristics and the ambient conditions, the paste density and penetrometer readings may fall outside the specified range. If density is too high, it is acceptable to add water to the mix while it is still in the paste mixer. Water is never added after the paste mix has been discharged into the cone feeder or the pasting hopper. Water added at this stage does not mix properly into the paste, resulting in inconsistent pasting machine performance and variations in pasted plates. If the density is too low, oxide cannot be effectively added to the paste mix in either the mixer, the cone feeder or the hopper. The oxide never becomes properly incorporated into the paste. The paste formula is adjusted to add more initial water during paste preparation until the density is in the desired range.

3.2 Curing and Hydrosetting

Pasted plates are placed on pallets as they are removed from the pasting line. To prevent them from sticking together, they are flash dried in a tunnel dryer to a moisture content of 8%-9%. Afterwards, if they were made from leady oxide, they will contain about 12%-15% residual free lead. The free lead must be removed and the plates fully dried before they can be used in the assembly process. Curing is the process for accomplishing lead removal and full drying. Another function of curing is to provide the proper chemical structure in the plate for the battery application.

To prepare for curing, pasted plates are either stacked or racked. For stacking, they are placed on pallets in stacks of about ten-inches high separated by air spaces. Up to three layers of stacks can be placed on a pallet. When plates are racked, they are placed on serrated or notched rails separated by about an eighth of an inch. Racking allows better air circulation around the plates, but requires more space and more labor.

Curing can be accomplished in a number of ways. The most common method used in the late 1990s involved placing the pallets in specially constructed curing chambers. They were large ovens in which the temperature and relative humidity of the air could be controlled. Air inside the chamber was circulated to ensure temperature uniformity and to bring oxygen into contact with the stacks of plates.

Oxidation of free lead is embedded in the paste by:

• air being diffused into the plates through spaces between adjacent plates in the stack
• oxygen diffusing through the water coating the lead particles
• nitrogen in the air diffusing out of the stack
• water vapor diffusing out of the stack.

A curing chamber can hold over 20 pallets of plates at 8,000 plates per pallet, for a total of 160,000 plates. If each contains 60 g of paste containing 12% free lead, 1152 kg (2,537 lbs) of free lead is in the chamber. The amount of air required to oxidize the amount of lead can be calculated from:

Therefore, 1152 kg of lead require:

Since 32 kg of oxygen occupy a volume of 22.4 liters at standard temperature and pressure,

Since air is only 21% oxygen, the equivalent amount of air equals 296,500 liters (10,377 cubic feet) an amount greater than the volume of most curing chambers.

Based on the calculations, it appears unlikely that oxidation of free lead takes place entirely by diffusion of oxygen into the mass of plates. Additional oxidation can be explained as the result of electrochemical corrosion, as illustrated in Figure 3-1.

Figure 3.1: Electrochemical corrosion of lead particles during curing

The reaction depends on the development of anodic and cathodic areas on the lead particle and the presence of hydroxyl ions from water surrounding the particle. Lead ions dissolve from the particle and go into solution where they are precipitated as lead hydroxide. Therefore, the lead particle becomes negatively charged and attracts hydrogen ions onto its surface. Oxygen diffusing into the plate from the atmosphere is reduced by the hydrogen ions. Areas where oxygen concentration is highest maintain a cathodic state, while areas having less oxygen become anodic. At the anodic areas, lead goes into solution, combines with the hydroxyl ions and is precipitated as lead hydroxide. Thus, the lead particle is progressively converted to lead hydroxide.

This process explains why water is essential for the oxidation of free lead to occur. If the paste is too dry, the reaction stops due to the absence of water to provide the hydrogen and hydroxyl ions. If the paste is too wet, the reaction is inhibited due to the retardation of the diffusion of oxygen through the plate. Oxidation of free lead takes place fastest when water concentration in the paste is between 9% and 4%. Then, the paste contains enough water to supply the reaction, while at the same time allowing diffusion of the oxygen from the air into the plate.

Development of Crystal Structure in the paste occurs during the curing process. A paste in which tribasic lead sulfate predominates works best for engine-starting batteries. For industrial batteries, a paste with a considerable amount of tetrabasic lead sulfate is required. The optimum ratio of tribasic to tetrabasic lead sulfate in industrial-battery paste ranges around 1:1, with the tetrabasic lead sulfate content running from as low as 40% to as high as 60%. To produce a cured plate containing predominantly tribasic lead sulfate, the temperature during curing must not be allowed to rise above 150^°F (66^°C). In practice, battery manufacturers try to keep the temperature to a maximum of 130^°F (54^°C).

The conversion of tribasic lead sulfate to tetrabasic lead sulfate takes place at temperatures above 160^°F (71^°C).

PbSO₄3PbOH₂O + PbO → PbSO₄4PbOH₂O

Curing Processes for Automotive and Industrial Plates

Different curing conditions are suitable for automotive and industrial plates.

Automotive plates principally require the oxidation of free lead and the formation of tribasic lead sulfate. At the start of the process, the temperature of the curing chamber must not be above 100^°F (38^°C). The relative humidity in the chamber must be above 90% to minimize plate-water loss. Pallets of pasted plates are loaded into the curing chamber as soon as they are full. Chamber doors must be kept closed when not loading plates, as 20 pallets may be pasted during a typical shift. They must be placed in the chamber with adequate air circulation between them. Pallets are stacked in rows, three pallets high. Air circulation must be maintained during loading.

When the chamber is full, or at shifts end, the door is closed, temperature is maintained at below 130^°F (54^°C) and relative humidity is at a constant 90%. It takes about 16 hours to complete the process. As free lead is oxidized, heat is evolved and chamber temperature is increased. Since plate-stack temperature is higher than the surrounding air, moisture is lost from the stack to the atmosphere. This allows greater penetration of air into the stack, accelerating the oxidation process. After about 16 hours with free-lead oxidation completed, relative humidity is reduced to allow the plates to dry. Drying time may be several hours depending on the number of plates in the stack. A properly cured plate has a free-lead content of less than 2% and a moisture content of less than 1%.

Industrial plates are considerably thicker than automotive battery plates. This reduces the rate of diffusion of oxygen into the stack. Thus, industrial plates are placed on racks rather than in stacks. In addition to oxidizing residual free lead, the objective of curing industrial-battery positive plates is to produce tetrabasic lead sulfate. Curing processes for both types of batteries are the same, except for temperature. Upon completion of the loading of industrial plates into the chamber, the temperature is increased to 170^°F - 180^°F (77^°C - 82^°C). Relative humidity remains at 90%. Due to the greater thermal conductivity of the grid metal, it expands faster than the paste. This causes paste to loosen from the grid if the temperature is increased too quickly.

Therefore, the temperature is increased slowly to ensure that paste and gird remain at the same temperature. At completion of loading, chamber temperature is between 100^°F - 120^°F (38^°C - 49^°C). A 5^°F (3^°C)-per-hour increase is introduced until 170^°F - 180^°F (77^°C - 82^°C) is reached, maintaining 90% relative humidity throughout the process. At the high temperature, the conversion of tribasic lead sulfate to tetrabasic lead sulfate proceeds quickly. Maintaining the high temperature for two hours produces an acceptable amount of tetrabasic lead sulfate in the positive plates. Then, the temperature is reduced to 130^°F (54^°C) and relative humidity to 70%. This initiates the drying process, promoting oxygen permeation of the plates and oxidation of free lead. Under these circumstances, drying is achieved in eight hours. Cured positive plates contain less than 2% free lead, have a moisture content of less than 1% and have about equal amounts of tribasic and tetrabasic lead sulfates. Tetrabasic lead sulfate is not formed in the negative plate. Thus, they can be cured at the temperature level as are automotive battery plates. Figure 3-2 shows an example of a typical industrial late curing process.

Figure 3.2: Typical industrial battery plate curing profile

Figure 3-2:

Sometimes, after curing is completed, the plates are subjected to a second drying process to remove the last traces of moisture. The density of the paste is about 4.5 times that of water, meaning 1% of water by weight is equal to 4.5% by volume of paste. This water can inhibit formation by occupying space in the active mass.

3.3 Expanders

The term expander applies to various mixtures added to the negative paste to maintain performance of the plate during the life of the battery. Early in the development of lead-acid batteries, wood separators were widely employed. At the time, expanders were not used nor were they thought necessary. However, when wood separators began to be replaced by rubber separators, a significant loss of life was observed. Clearly, some component in the wood was contributing to the life of the battery. So wood flour was added to the negative plate to correct the problem. Subsequently, lignin, which had probably been sulfonated to some degree by the sulfuric acid electrolyte, was discovered to be the active species in the wood.

Today, most expander blends use lignosulfonates as a major component. There is also some use of synthetic compounds based on b-naphthalene/formaldehyde condensates. Barium sulfate and carbon black, too, are used in expander formulations. Thus, the modern expander usually is a mixture of barium sulfate, carbon black and an organic material. The proportions of these materials depend on the desired characteristics and the life desired from the battery.

The expander is both an anti-passivating agent and an anti-coalescing agent. It prevents the negative plate from being passivated by lead sulfate during discharge and it also prevents the lead particles from coalescing to form a hard, dense, low-porosity mass. Although the amount of expander added to negative plates is very small, it plays a major role in both the performance and the life of the battery. These materials are so effective that battery failure caused by the negative plate is virtually unknown.

Barium sulfate functions to provide sites for precipitation of lead sulfate during discharge. It is electrochemically inactive and has extremely low solubility in sulfuric acid. These properties ensure that it remains chemically unchanged in the negative plate even after prolonged cycling. The ability of barium sulfate to act as a site for lead sulfate precipitation is due to the similar crystal structure of the two materials. Strontium sulfate has also been shown to be an effective anti-passivating agent (A.K. Lorenz, Dissertation, Leningrad, Russia, 1953).

Barium, lead and strontium sulfates are isostructural (M. Miyake, H. Morikawa, I. Minato and S. Iwai, American Minerologist, Vol 63, (1978), 506-510). They belong to the orthorhombic space group and have similar R values and bond lengths as shown in Table 3-1.

• Table 3-1: Structural characteristics of barium, strontium and lead sulfates

By providing a large number of sites for the precipitation of lead sulfate, the barium sulfate prevents lead sulfate deposition as a thin, impermeable passivating film. The unit cell dimensions of lead sulfate and barium sulfate are so similar that less energy is expended by lead sulfate precipitating on barium sulfate crystals than if energy of nucleation was used to form a new lead sulfate crystal. In other words, lead sulfate deposits on sites where the least expenditure of energy is possible.

Two forms of barium sulfate are used in expanders; blanc fixe and barytes. Blanc fixe is a finely divided, chemically precipitated material, while barytes is ground, purified mineral ore. Typically, blanc fixe will have a median particle size of approximately 1 μm, while that of barytes will approximate 3.5 μm. Blanc fixe is a very effective anti-passivating agent; its small particle size provides millions of sites for nucleation to take place. Barytes is much less effective than blanc fixe and is considered virtually a filler. However, speculation has been proffered that barytes acts as a material for slow release of finely divided barium sulfate. Expanders are frequently made without barytes, but barytes is never used without blanc fixe.

Lignosulfonates and synthetic organic compounds are derived from wood as a byproduct of paper manufacturing. The pulping process for making paper involves either alkaline or acidic treatment of wood chips. In the alkaline pulping process, called the Kraft process, sodium hydroxide and sodium sulfide are used to treat the wood chips. This produces a liquid known as black liquor which contains kraft lignin, sugar acids and various inorganic compounds. This is subsequently treated with acid to precipitate the lignin which is then dissolved in alkali to obtain a purified kraft lignin. At this stage, the lignin is not sulfonated. If desired, the kraft lignin can subsequently be sulfonated to the required degree.

In the acidic process, the wood chips are treated with an alkali metal sulfite (usually Ca⁺⁺, NH₄⁺, Mg⁺⁺, or Na⁺), producing what is known as spent sulfite liquor. During this process, the lignin is partially sulfonated. The spent sulfite liquor contains lignosulfonates, polysaccharides and sugars. For the production of battery additives, the spent sulfite liquor is subjected to high-pressure and temperature catalytic oxidation to produce a partially desulfonated, oxidized lignosulfonate. The major differences between the two processes and the products derived from them are shown in Table3-2.

• Table 3-2: Comparison Between the Alkaline and Acidic Pulping Processes

Lignosulfonates are very strong rheological modifiers and are used in the production of ready-mixed concrete to reduce the amount of water needed to make a satisfactory consistency. This allows the concrete to dry quicker with less shrinkage and cracking. The same behavior takes place in the preparation of battery paste mixes. They are also strong anti-flocculents with a large hydrophobic organic part (R⁺) and a small hydrophilic inorganic fraction (SO_3-).

RSO₃Na = RSO_3- + Na⁺

The hydrophobic part of the RSO_3-anion is adsorbed on the surface of the lead particles, leaving the hydrophilic part facing out to the aqueous electrolyte phase. This results in an increase in the repulsion potential which prevents the particles from coalescing or sintering. This shows up in practice by a reduction in the particle size of the lead with a corresponding increase in the surface area. This improves the high-rate performance of the plates, particularly at low temperature. Examples of the effect of various lignosulfonates on the surface area of negative active material are shown in Table 3-3 (Boden et al Paper presented at the Battery Council International Convention, Nashville, TN, May 1999).

• Table 3-3: Effect of various lignosulfonates on the surface area (m²/g) of negative plates (concentrations in % of oxide weight)

The organic materials also inhibit passivation by modifying the surface coverage of active lead sites during high rates of discharge. Studies have shown that the organic becomes attached to the active lead surface, inhibiting precipitation of lead sulfate (E. Willihnganz. Transactions of the Electrochemical Society, 92, (1947), T.F. Sharpe, Electrochimica Acta, 1, 635, (1969)).

In addition to their effect on surface area, lignosulfonates also affect the electrode kinetics of lead sulfate reduction to lead. In cyclic voltammetry studies, a pronounced effect was observed on both the potential and shape of the reduction peak (Report on Advanced Lead-Acid Battery Consortium Project BE97-4085, 12/31/98).

The most pronounced effect of lignosulfonates on battery performance is the improvement in low temperature performance at high rates. Consequently, expanders for automotive batteries contain a high proportion of organic material. Many different lignosulfonates have been employed as expanders and these have widely different effects on the performance of lead-acid batteries.

Carbon is added to the expander to improve the conductivity of the active material. It assists in the initial formation and improves performance during deep discharges where the concentration of highly resistant lead sulfate is high. It is usually added to the expander formula in an amount equal to the lignosulfonate.

Expander compositions for various battery applications have specific formulations, including those for automotive, motive-power and standby-power batteries. These are characterized by different operating conditions, discharge rates and depths of discharge and require different proportions of materials as shown in Table 3-4.

• Table 3-4: Typical expander formulations for different battery applications

Occasionally, wood flour and soda ash are added in small amounts to motive-power expanders. Barytes is often a component of the total barium sulfate in industrial applications. Expander is added to automotive-battery negative plates at a rate of 0.5% - 1.0%, while 2.0% - 2.5% is generally recommended for industrial-battery applications.

The principle difference in the expanders used in automotive and industrial applications is the ratio of barium sulfate to carbon. In automotive batteries a high fraction of lignosulfonate is used, while in industrial batteries a small percentage of lignosulfonate is employed. The high percentage of lignosulfonate in automotive plates is necessary to produce the high cold-cranking rates required by these batteries. On the other hand, the large amount of barium sulfate in industrial batteries prevents passivation during deep cycling and gives excellent durability.

For telecommunications applications where uniformity of float voltages is important in long cell strings, lignosulfonate is often omitted from the expander due to the organic constituent having a strong effect on the hydrogen overpotential. This causes variations in voltage from cell to cell which can result in the string becoming unbalanced.

Both the type and amount of lignosulfonate and the amount of barium sulfate used in the plate have a marked effect on plate performance and life. Studies on automotive batteries (D.P. Boden, Journal of Power Sources, 1998) indicate that cold-cranking performance and cycle life increase as the amount of lignosulfonate is increased up to 0.5%. See Figures 3-3 and 3-4. Above this, the performance declines due to overexpansion of the plate with the consequent loss in conductivity.

Figure 3.3: Effect of lignosulphonate concentration on cold-cranking performance

Figure 3.4: Effect of lignosulphonate concentration on life cycle

The cold-cranking performance of automotive batteries is independent of the amount of barium sulfate in the plate as seen in Figure 3-5. This indicates that passivation is not a serious performance limiter at high rates of discharge. A more plausible explanation is that the performance is limited by mass transfer. Barium sulfate does, however, have a significant effect on cycle life as shown in Figure 3-6. The number of J240 cycles increases as the amount of barium sulfate is increased up to a concentration of 1.0%. Above this, the cycle life remains constant.

Figure 3.5: Effect of barium sulphate concentration on cold-cranking performance

Figure 3.6: Effect of barium sulphate concentration on life cycle

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4.0 Separators

A separator is a material that is inserted between the positive and negative plates of the cell to prevent short circuiting. The development of separators was essential to the growth of the lead-acid battery industry. They have gone through many changes during their history. The earliest designs of lead-acid battery employed materials such as porous pot, flannel and felt to separate the plates. In the 1880s, perforated hard rubber, India rubber, sponge and cork were used as separators. The first use of wood separators was in 1892 by E.P. Usher and by G.H. Roe and G. Sutro. Wood was the separator of choice for many years. It began to be phased out in favor of rubber, cellulose and PVC separators as recently as the 1940s and 1950s.

The separators used today in lead-acid batteries reveal a great deal of variation in materials, design and in how they are used in the battery assembly. They can be made from plastic, rubber, glass fiber, cellulose and sulfuric acid gel. Today, most separators for flooded types of automotive and industrial batteries are made from microporous polyethylene, while in valve-regulated batteries, absorptive glass mats or sulfuric acid gels are employed. While separators in flooded types of batteries act as a physical barrier between the plates, the separators in valve-regulated batteries perform an additional function acting as a medium for transport of oxygen from the positive to the negative plate.

Separators for flooded batteries have a standardized design a backweb on which ribs are formed. The backweb prevents the plates from touching, while the ribs form channels that contain the electrolyte and allow escape of gas from between the plates. A wide variety of backweb thicknesses and rib configurations are used by the battery industry, depending on the desired properties of the separator and the battery. A typical profile for an automotive battery separator is shown in Figure 4-1. Industrial battery separators have a considerably thicker backweb for longer life and taller ribs to increase the amount of electrolyte between the plates.

Figure 4.1: Typical profile of microporous polyethylene automotive battery separator

The most common separators for flooded batteries are produced from polyethylene, polyvinyl chloride, phenolic resins and natural rubber. Polyethylene separators are produced from a mixture of high-molecular-weight polyethylene, silica and oil which is extruded into a sheet at high temperature. Ribs are then added by passing the sheet through calender rollers. Some of the oil is then extracted with organic solvents, leaving about 10%-20% remaining in the separator to improve its flexibility and oxidation resistance. The flexibility of microporous polyethylene separators provides a very desirable mechanical advantage. It allows them to be formed into sleeves and pockets for improved isolation of the plates.

Polyvinyl chloride separators are manufactured from a mixture of powdered PVC, silica, water and a solvent. This is extruded at an elevated temperature and calendered to provide the required number and design of ribs. The solvent is extracted in hot water. After drying, a rigid, porous sheet results.

Rubber separators are produced from a blend of rubber, silica and water. These components are blended in a mixer, extruded into a sheet and calendered to form the ribs. The extruded sheet is then vulcanized to produce a hard, rigid sheet. Curing can also be done by cross-linking with an electron beam, producing a more flexible product.

Phenolic separators are produced by blending silica with phenolic resin and then forming this mixture into a sheet on a polyester scrim. Ribs are then extruded onto the resin sheet in a separate operation.

Although separators perform an essential function in ensuring long life, they reduce the capacity and high-rate performance of batteries. This is because they displace electrolyte and add electrical resistance. To reduce these losses, separator manufacturers are constantly working to increase the porosity and reduce the electrical resistance of their products without sacrificing mechanical strength.

The specific properties common to all separators are:

• Low acid solubility
• Good oxidation resistance
• Low electrical resistance
• Low acid displacement
• High tensile strength
• Good flexibility
• High porosity
• Good wettability
• Low level of leachable impurities.

4.1 Life

Low acid solubility and oxidation resistance are important to ensure that the separator is capable of functioning over the life of the battery. Battery separators are subjected to both physical and chemical degradation in service. Significant increases in temperature, electrolyte concentration, overcharge and service conditions can accelerate the degradation to some degree. The failure mode is very dependent on the material used to make the separator and the type of battery service. The rate at which the separator degrades in the battery depends on the thickness of the backweb and, in the case of microporous polyethylene separators, on the amount of residual oil. Increasing the amount of oil in the separator improves its oxidation resistance, but if the oil content is too high, it can leach out of the separator and cause problems, such as blocking of the vent caps. There is no industry standard test for physical and chemical degradation of separators, but the following tests are in use by battery and separator manufacturing companies:

• sulfuric acid/dichromate oxidation
• sulfuric acid/hydrogen peroxide oxidation
• overcharge oxidation.

4.2 Electrical Resistance

Electrical resistance of the separator is a measure of its ability to transfer ions from one electrode to the other. This has a direct effect on the performance of the battery by increasing the internal resistance of the cells. The major contributor to resistance is the backweb and, to a lesser extent, the resistance of the ribs. The resistance of the separator can be expressed as:

R_sep =R_el (t²/p-1)

Where: R_sep = separator resistance (Ω cm)

R_el = electrolyte resistance (Ω cm)

T = tortuosity = l/d

P = porosity

Figure 4-2 shows the relationship between backweb thickness and electrical resistance for a typical automotive battery separator. The graph shows that for every 0.001 inch increase in backweb thickness, the electrical resistance increases by approximately 7%. Clearly, separators for batteries designed to operate at high discharge rates must have a thin backweb. However, for industrial batteries which are discharged at much lower rates, a thicker backweb can be employed.

Figure 4.2: Electrical resistance of battery separators

4.3 Acid Displacement

Acid displacement results from separators taking up space that could be utilized for additional electrolyte. The amount of acid displaced is the volume of the solid fraction of the separator and can be calculated from:

D = (V_b + V_r) (1-P)

Where: V_b = volume of backweb

V_r = volume of ribs

P = porosity

For an automotive battery separator with a 0.008-inch backweb, the acid displacement will be around 95 ml/m². If the backweb is reduced to 0.006 inch, the acid displacement will be reduced to 80 ml/m². This provides additional electrolyte to participate in the cell reaction, which increases the capacity. Alternatively, the plates can be spaced closer together while still retaining the same electrolyte volume. This will improve the high-rate performance of the battery. Ideally, the backweb thickness should be as low as possible. However, consideration has to be given to the strength and durability of the separator. This results in a trade-off between performance, strength and life. As a general rule, separators for automotive batteries will have a backweb in the 0.008-0.010 inch range while a backweb of 0.030-0.040 inch is common in industrial batteries.

4.4 Porosity

Porosity is that fraction of the separator volume that is composed of voids that are capable of holding sulfuric acid. The greater the porosity, the greater is the amount of electrolyte that can be retained in the separator and, for a given thickness, the lower the electrical resistance. Not all the pores in a separator are filled with electrolyte. Some are totally enclosed by the separator material. Some are also dead ended, preventing ions from being transported between the electrodes. The pores can be thought of as storage areas holding the inventory of sulfuric acid that participates in the electrochemical reaction. Ideally, the higher the porosity, the better. However, once again, there is a trade-off between the ideal and serviceability.

4.5 Strength

Strength of separators encompasses a number of factors including stiffness, puncture resistance, rigidity, flexibility, brittleness and tensile strength. These factors become most important during assembly, when damage can lead to short circuits in the battery. The use of expanded metal grids with exposed sharp protrusions requires separators that have high puncture resistance, while flexibility is essential for enveloping or sleeving the plates. High tensile strength and flexibility are required for separators that are to be used with automatic wrapping/stacking machines. At the end of the 20^th century, automotive batteries were almost exclusively built with enveloped plates that require high flexibility and tensile strength. Therefore, microporous polyethylene separators are by far the most widely used.

4.6 Flexibility

Flexibility can be increased by reducing backweb thickness. However, this leads to reduced stiffness, which can result in problems during assembly. The loss in stiffness in the direction of the ribs is only minor, but is significantly more serious in the cross-rib direction. A reduction in backweb thickness from 0.008-0.006 inches results in a reduction in stiffness of only 2% in the rib direction, but a reduction of 60% in the cross-rib direction (W. Bohnstedt, Journal of Power Sources, 67 (1997) pp. 299-305). A loss of stiffness results in a loss in accuracy of cutting and folding during production of separator pockets. To reduce this problem, separator profiles have been developed with cross ribs that have a considerably lower profile to avoid hindering the release of gas from between the electrodes. With suitable design, cross ribs can compensate for loss in stiffness without significantly affecting the electrical resistance and acid displacement.

4.7 Puncture Resistance

Puncture resistance also depends on backweb thickness. This relationship is shown in Figure 4-3. The puncture resistance can be increased by increasing the ratio of polymer to filler in the material. This, however, increases the cost, reduces the porosity and increases the electrical resistance. The puncture resistance can also be increased by the use of polymers of higher molecular weight, but breakdown of the polymer chains during processing limits the benefits that can be obtained.

Figure 4.3: Puncture resistance of battery separators as a function of backweb thickness

Industrial battery separators are considerably thicker than those for automotive batteries and are available in a wider choice of materials. In addition, glass mats are often attached to the ribs to reinforce the active material and to prevent it from expanding into the space between the rib and backweb. On the following page in Table 4-1 is a summary of separator properties, as shown in G.H. Brilmyers Journal of Power Sources, 78 (1999) pp. 68-72.

• Table 4-1: Selected properties of lead-acid battery separators V = vertical; D = diagonal; S = serpentine

4.8 Glass Mat Separators

Glass mat separators are essentially used for valve-regulated lead-acid batteries. These batteries owe their existence to the discovery that if oxygen formed during charging of the positive electrode can be transported to the negative plate, it will be reduced and consequently depolarize the negative plate, thereby preventing hydrogen evolution. No electrolysis of water takes place and water consumption is eliminated. Although it had been known for many years that oxygen that diffused through the electrolyte in flooded cells was reduced at the negative plate, this took place at a very low rate. Milner (P.C. Milner, The Bell System Technical Journal, 49, 7, pp. 1321-1334 (1970)) calculated that the rate of oxygen reduction in a typical standby power cell was 20-35 A per Ampere-hour of cell capacity. In their groundbreaking patent, McClelland and Devitt showed that oxygen could be transferred from the positive to the negative plate through a glass mat that had been partially saturated with electrolyte so that unfilled channels were available for oxygen transport (D.H. McClelland and J.L. Devitt, U.S. Patent 3,862,861 (1992)). Today, microfiber glass mats are the most common separators used in valve-regulated lead-acid batteries. Like the separators used in flooded systems, they keep the plates separated, but also have the ability to allow oxygen transport from the positive to the negative plate.

The glass fiber mats used in valve-regulated batteries have a porosity greater than 90% and a surface area exceeding 1 m²/g. The large surface area is achieved by use of glass fibers having a very low diameter, around 1 micron. Glass fiber has a zero contact angle with sulfuric acid; therefore, the separators have high capillary forces which result in good wettability. They are compressible and conformable so that they provide support for the electrodes. They are usually made from blends of glass fibers of different diameter that are processed into mats on paper-making machines. Fibers below 1 om give a large surface area and a well dispersed structure of small channels for acid absorption and oxygen transport. But, due to their shortness, they give little tensile strength to the mat. Fibers having a larger diameter improve strength, but are also more brittle and increase the tendency to break when compressed. A typical composition will have a ratio of 20%-30% microfine fibers. This gives an acceptable balance between electrical properties, strength and cost. A simplified view of the glass-fiber-separator structure is shown in Figure 4-4 Typical properties of glass mat separators for valve-regulated batteries are shown in Table 4-2. (W. Bohnstedt, Journal of Power Sources, 78 (1999) pp. 35-40).

Figure 4.4: Simplified view of glass microfiber separator structure

• Table 4-2: Typical properties of glass fiber separators for valve-regulated lead-acid batteries

The strong capillary forces in glass mat separators result in good wicking characteristics. Figure 4-5 shows how the percentage of fibers (<1om) affects the rate of wicking.

Figure 4.5: Effect of fiber mix on wicking characteristics of glass mat separators

The time for the electrolyte to wick to a given height decreases as the percentage of fine fibers in the mat is increased. All voids do not fill uniformly. However, the smaller voids will fill preferentially and the larger voids more slowly. The result is a larger proportion of unfilled large voids in the upper part of the separator. This is the area where the greatest amount of oxygen transfer takes place.

Acid stratification can take place in valve-regulated batteries with glass mat separators just as it does in flooded cells. And for the same reason sulfuric acid formed during charging will diffuse downward and displace acid of lower concentration to the top of the cell. Stratification is difficult to avoid in glass mat separators since the electrolyte is never stirred by the bubbling action of oxygen and hydrogen when the battery is charged. Figure 4-6 shows how the height of the plates in a valve-regulated battery affects the stratification and the capacity (R.E. Nelson, private communication).

Figure 4.6: Stratification effects with glass mat separators

A viable strategy for reducing stratification in a valve-regulated battery is to turn the battery on its side, which reduces the wicking height. No leakage takes place because the cells contain no free electrolyte. Figure 4-7 shows how the orientation of the battery affects the capacity of valve-regulated cells during cycling. The graph shows that better retention of capacity is achieved when the cells are oriented on their side. The best result is obtained when the cells are positioned with the shortest side in the vertical orientation. For this reason, many valve-regulated cell installations position the cells on their sides.

Figure 4.7: The effect of orientation on the capacity of valve-regulated cells during cycling

Among the most important attributes of glass mat separators are compressibility and conformability. Conformability allows the separator to adapt to imperfections in the battery plate and maintain good plate-to-electrolyte contact. When the battery is discharged and charged, the plates will expand and contract. Conformability allows the separator to accommodate these changes without losing contact with the plate.

Recent studies have shown that high levels of compression can improve the cycle life of valve-regulated batteries. The glass fibers essentially act like springs between the battery plates and support the active materials. The support can be increased by using separators with higher density and by using a greater percentage of fine fibers in the glass blend. A two-gram sample of fine fiber (diameter 0.8 om) has approximately 5.6 billion fibers, while a coarse fiber (diameter 3.5 om) has only 28 million fibers.

The degree to which the separator is compressed is an important factor in the design of the battery. The optimum amount of compression has not been determined. It is affected by the type and thickness of the separator and the plate spacing. It is known, however, that too little compression will result in premature battery failure, while too much will compress the spongy lead in the negative plate. The compression is reduced when the cell is filled with electrolyte. Therefore, the compression remaining after filling is the critical value. For example, Zguris (Journal of Power Sources, 67 (1997) pp. 307-313) has reported that the force required to compress a stack of glass mat separators wetted with sulfuric acid of 1.23 specific gravity to a 46% compression was reduced to 60% of the value required to compress the dry material. The amount of compression will affect:

• porosity of the separator
• volume of electrolyte
• time to fill
• container wall strength
• shock and vibration requirements.

The ability of the separator to act as a spring is affected by the amount of compression. There is a permanent loss in thickness when the glass mat is compressed. Figure 4-8 shows the effect of different compressive loads on the thickness of the separator compared to when the load is removed. The data shows that a loss in thickness takes place that is proportional to the amount of compression applied.

Figure 4.8: The effect of compressive force on the thickness of glass-mat separator material

The issue of compression in glass mat separators still requires further work. Clearly, compression has an important effect on the performance and life of the battery. If the compression is too low, battery life will be compromised. If it is too high, compaction of the negative plate will take place, filling will be difficult and the amount of electrolyte will be reduced. In this case, the capacity of the battery will be reduced. The level of compression should take into account the design of the battery, the characteristics of the separator and the degree of saturation desired.

4.9 Gelled Electrolyte

Gelled electrolyte provides an alternative for the valve-regulated battery. As pointed out previously, there are still some unresolved problems with glass mat separators in valve-regulated batteries. Consequently, many valve-regulated batteries use an electrolyte that is immobilized by gelling. The most common gel is formed by the addition of between 5%-8% of silica to the electrolyte. These gels are thixotropic and can be fluidized by mechanical stirring. They can be added to the cell in liquid form and then allowed to gel after the required amount has been added. Initially, the gel provides an impermeable barrier to oxygen transfer, preventing any gas recombination. Overcharge results in water loss which causes the gel to shrink and develop cracks. This enables oxygen to migrate to the negative electrode and be reduced. Once the oxygen cycle has been established, no further water loss takes place. (O. Jache, German Patent 1,671,693 (1967.)) A conceptual view of the recombination process with gelled electrolyte is shown in Figure 4-9.

Figure 4.9: Conceptual view of the recombination process with gelled electrolyte

Batteries that use gelled electrolyte also use a conventional separator to prevent short circuits and to control the spacing between the electrodes. It is important that these have minimal acid displacement. The combination of the standard microporous separator and gel results in reduced electrolyte volume. Typical data for microporous separators suitable for use in gelled electrolyte valve-regulated batteries are shown in Table 4-3 (W. Bohnstedt, Journal of Power Sources, 78 (1999) pp. 35-40).

• Table 4-3: Typical properties of microporous separators for valve-regulated batteries with gelled electrolyte

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