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4.1.1.1 Discharge Types

In talking about battery capacity and discharge performance, it is sometimes useful to compare a battery to a jar containing molasses. Extracting power from a battery is like turning the jar upside down—you can get a lot out very quickly. But in both cases, there is a significant residue that does not come out in the first rush. To totally deplete either they must be allowed to trickle discharge for many hours. With a battery, it is important to remember that the performance differences between a quick discharge and a long, slow, total discharge may be quite significant.

There are three general classes of discharges for which sealed-lead batteries are typically applied. Each one of them has its own design considerations and each serves substantially different forms of applications. The differentiating parameter is the rate of discharge—whether it is high, medium, or low. Some considerations regarding each category will be presented below.

4.1.1.2 Design for Discharge Performance

Any battery is the result of a multitude of design compromises, many of which may affect its behavior during discharge. To understand how discharge performance varies with changing loads or changes in the surrounding environment, it helps to understand a little about discharge mechanisms. In particular, the discharge process has two components: an early phase and a long-term phase.

The early phase is dominated by surface reactions. There is not time for transport mechanisms to have much impact, so all the activity is concentrated at the interface between the plate active material and the electrolyte. To maximize short-term response, i.e. that needed for high-rate performance, the plate surface area per unit volume should be a maximum. Advanced sealed-lead batteries use thin plates increasing the surface area available for reaction within a given volume. The result is enhanced high-rate performance.

For long-term response, i.e. the response to a deep, slow discharge, surface effects are less important. Here there is time for transport mechanisms to come fully into play. This means all of the active materials, not just the surface layer, may be involved in the reactions. The key parameter in determining deep-discharge performance is the weight of active material per unit volume of battery. Starved-electrolyte sealed-lead batteries obtain superior performance in deep discharge through elimination of excess electrolyte which increases the proportion of the battery’s weight devoted to other active materials. The result is energy densities which give good performance in deep cycle applications.

4.1.2.1 Capacity Stabilization

One consideration in evaluating the discharge performance of some sealed-lead cells and batteries is the early rise in capacity. Part of the manufacturing process for all lead cells or batteries is the conversion of the pastes on the electrodes into the active materials needed for successful operation of the cell. This final step in the manufacturing process, called formation, does not normally proceed to completion; some of the paste remains unconverted. The early use of the battery completes the conversion process as the battery is charged in service. The result is growth in battery capacity until the battery stabilizes at a level that may actually be greater than 100 per cent of the nominal capacity. Unless noted, all results presented in Section 4 and Appendix B refer to stabilized values.

4.1.3.1 Battery Capacity Definitions and Ratings

Battery or cell capacity simply means an integral of current over a defined period of time.

This equation applies to either charge or discharge, i.e. capacity added or capacity removed from a battery or cell. Although the basic definition is simple, many different forms of capacity are used in the battery industry. The distinctions between them reflect differences in the conditions under which the capacity is measured. Commonly used capacity terms are introduced in Table 4-1 and summarized below.

Standard capacity measures the total capacity that a relatively new, but stabilized production cell or battery can store and discharge under a defined standard set of application conditions. It assumes that the cell or battery is fully formed, that it is charged at standard temperature at the specification rate, and that it is discharged at the same standard temperature at a specified standard discharge rate to a standard end-of-discharge voltage (EODV). The standard end-of-discharge voltage is itself subject to variation depending on discharge rate as discussed in Section 4.1.7.

Since cells (or batteries) coming from production may have slight variations in capacity, the value of standard capacity may lie anywhere within the statistical distribution of capacity as manufactured. Figure 4-2 illustrates a typical capacity distribution. Unless otherwise stated, this Handbook uses standard capacities.

When any of the application conditions differ from standard, the capacity of the cell or battery may change. A new term, actual capacity, is used for all nonstandard conditions that alter the amount of capacity which the fully charged new cell or battery is capable of delivering when fully discharged to a standard EODV. Examples of such situations might include subjecting the cell or battery to a cold discharge or a high-rate discharge.

That portion of actual capacity which can be delivered by the fully charged new cell or battery to some nonstandard end-of-discharge voltage is called available capacity. Thus, if the standard EODV is 1.6 volts per cell, the available capacity to an end-of-discharge voltage of 1.8 volts per cell would be less than the actual capacity.

Cells and batteries are rated at standard specified values of discharge rate and other application conditions. Rated capacity (C) for each cell or battery is defined as the minimum standard capacity to be expected from any example of that type when new but fully formed and stabilized. The rated value must also be accompanied by the hour-rate of discharge upon which the rating is based (e.g. 1 hr, 5 hr, 10 hr, 20 hr, etc). The rated capacity for each sealed-lead cell and battery type produced by Gates is indicated in Appendix B.

Rated capacity is always a single specific designated value for each cell or battery model (type, size and design), as contrasted with the statistically distributed values for all other defined capacities. Thus a group of D cells with a rated capacity of 2.5 amp-hours might have standard capacities ranging from 2.5 to 3.0 amp-hrs with an average of 2.65. The Gates process for the manufacture of sealed-lead cells (and batteries) produces a comparatively tight spread in the overall distribution of standard capacity as shown in Figure 4-2.

Figure 4-2 refers to single-cell capacity and NOT multi-cell battery capacity. In any multi-cell battery, the lowest capacity cell in the battery determines its capacity. The distribution of battery capacity, therefore, has the same minimum value as in Figure 4-2 (rated capacity), but its maximum capacity may be somewhat reduced. This reduction depends on the number of cells in the battery and the width (statistical variance) of the capacity distribution of the particular population of cells from which the batteries are actually constructed. If only identical capacity cells were used within each battery, the distribution of battery capacity would be the same as the distribution of cell capacity.

If a battery is stored for a period of time following a full charge, some of its charge will dissipate. The capacity which remains that can be discharged is called retained capacity. Section 4.3 discusses storage and its effect on capacity.

4.1.3.2 Measurement of Fully Charged Capacity

The capacity of a cell, or battery, is normally measured by completely discharging it while integrating the current over the period of the discharge. Variations in the capacity measurement procedure can result in data inconsistencies.

The most common method of measuring capacity is to discharge the battery with a constant-current load. The load circuit adjusts to maintain a constant discharge current as the battery voltage declines. Recording battery voltage versus time results in a discharge curve similar to Figure 4-1. Calculation of discharged battery capacity is thus only a multiplication of the time needed to reach the specified end-of-discharge voltage (EODV) times the current. An added refinement is the simultaneous use of a current integrator or current shunt to ensure that the load is stable and accurate in maintaining the constant current. A variety of packaged loads designed specifically for constant-current discharges are available for capacity measurement.

An older, less common, and less accurate method of measuring capacity is to place a fixed resistance load across the battery terminals and monitor the voltage as a function of time as the battery discharges. With a fixed resistance, the current decreases as the battery voltage declines. A recorder is used to record the voltage drop across the resistor. The discharge recording of resistor voltage drop is translated to current and then manually integrated over time to calculate the discharged capacity. Use of a current integrator in the circuit can speed the capacity measurement. Unfortunately, the discharge current, which influences actual battery capacity, is variable in this procedure. Thus, relating the results to other application conditions can be quite difficult.

Measured battery capacity depends also on the end-of-discharge voltage used in the measurement. For most accurate results in measuring total battery capacity, the voltage used to terminate the discharge should be below the knee of the discharge curve. This simply means that the end of the discharge should occur after the battery has left the flat plateau of the discharge curve and the voltage is falling rapidly.

Higher values of EODV, when used in measurement procedures, may decrease the accuracy of the results. For EODV’s on the voltage plateau, for example, voltage is dropping slowly with time, so small errors in measured voltage may result in significant errors in the time (capacity) to the end of the discharge. Once the battery is off the plateau, the voltage falls very rapidly and the remaining effective capacity is slight so there is little capacity difference between different EODV’s.

4.1.3.3 Capacity as a Function of Discharge Rate

The rate at which current is drawn from a battery affects the amount of energy which can be obtained. At low discharge rates the actual capacity of a battery is greater than at high discharge rates. This relationship is shown in Figure 4-3. See Section 4.1.3.7 for a more detailed discussion of capacity ratings.

The information from Figure 4-3 can be used to create a valuable curve of run time versus discharge rate such as shown in Figure 4-4. This shows the amount of time that a certain size of sealed-lead cell or battery will support a given discharge current at room temperature. The data presented in this chart should be regarded as nominal performance for a fully charged battery that has been stabilized at full capacity. Differing conditions, either relating to the battery or the environment, can affect these nominal values.

4.1.3.4 Capacity as a Function of Battery Temperature

Starved-electrolyte sealed-lead batteries may be discharged over a wide range of temperatures. They maintain adequate performance in cold environments and may produce actual capacities higher than their standard capacity when used in hot environments. Note that the discharge temperature of concern is that experienced by the active materials within the battery. The time required for a battery to come to thermal equilibrium with its environment may be significant.

Figure 4-5 indicates the relationship between capacity and cell temperature. Actual capacity is expressed as a percentage of rated capacity as measured at 23°C. Quantitative derating curves for the effects of non-standard discharge temperature are presented in Appendix B.

4.1.3.5 Capacity During Battery Life

The initial actual capacity of sealed-lead batteries is almost always lower than the battery’s rated or standard capacity. However, during the battery’s early life, the actual capacity increases until it reaches a stabilized value which is usually above the rated capacity. The number of charge-discharge cycles or length of time on float charge required to develop a battery’s capacity depends on the specific regime employed. Alternatively if the battery is on charge at 0.1C, it is usually stabilized after receiving 300 per cent (of rated capacity) overcharge. The process may be accelerated by charging and discharging at low rates.

Under normal operating conditions the battery’s capacity will remain at or near its stabilized value for most of its useful life. Batteries will then begin to suffer some capacity degradation due to their age and the duty to which they have been subjected. This permanent loss usually increases slowly with age until the capacity drops below 80 per cent of its rated capacity, which is often defined as the end of useful battery life. Figure 4-6 shows a typical representation of the capacity variation with cycle life that can be expected from sealed-lead batteries.

Section 4.4 discusses in more depth the amount of time or number of cycles that can be expected prior to end of useful life.

4.1.3.6 Effect of Pulse Discharge on Capacity

In some applications, the battery is not called upon to deliver a current continuously. Rather, energy is drawn from the battery in pulses. By allowing the battery to “rest” between these pulses, the total capacity available from the battery is increased. Figure 4-7 presents typical curves representing the voltage delivered as a function of discharged capacity for pulsed and constant discharges at the same rate. For the pulsed curve, the upper row of dots represents the open-circuit voltage and the lower sawtooth represents the voltages during the periods when the load is connected. The use of discharged capacity as the abscissa eliminates the rest periods and shows only the periods of useful discharge. Because each application is unique, individual testing should be performed to evaluate the relative capacity gain of pulse discharge compared with continuous discharge.

The significant difference between total discharge capacity values for pulsed and for steady current discharge is caused by a phenomenon known as concentration polarization. When current is delivered by the battery, the active material in the plate interacts with the electrolyte to reduce the concentration of the acid in the immediate vicinity of the plate. Since the amount of electrolyte available in the plate pores is less than that required for complete discharge, the delivered capacity at continuous high rate will be limited. However, when time is allowed for the acid to diffuse from the separator back into the pores of the plate, such as during the rest period when pulse discharging, the overall capability to deliver energy is increased.

4.1.3.7 Battery Capacity Ratings vs. Discharge Rate

Battery performance may be rated differently depending on battery type and application. This can be confusing for the designer trying to find a suitable battery for a specific application. Most of the confusion centers around the discharge rates used to specify the capacity of a cell or battery and relates to the fact that the deliverable capacity varies inversely with the discharge rate.

Some batteries are rated at the one-hour rate, some at the five-hour rate, some at the 10-hour rate while many specify capacity at the 20-hour discharge rate.

Table 4-2 shows the nominal capacity of sealed-lead batteries at a variety of different rates. Notice that the batteries at the 20-hour rate have about 8 per cent more capacity than at the 10-hour rate.

If all batteries were rated on the same basis, it would be easier to compare one type against another by just considering the data on the battery label. But even then, the relative performance differences between two battery types at one set of conditions may well be different from their relative performance at another set of conditions. Thus, even though battery ratings are a convenient shorthand, the only reliable way to select a battery is to examine actual performance data for candidate batteries at the desired application conditions.

4.1.4.1 Effective No-Load Cell Voltage, E₀

The effective no-load voltage (E_o) of a sealed lead cell is a function of the average specific gravity and temperature of the sulfuric acid electrolyte in the cell. When the cell is fully charged, the specific gravity will be at its peak and, correspondingly, the E_o voltage will be at its highest. As the cell discharges, the specific gravity of the electrolyte declines as the acid is gradually converted to water. The no-load voltage of the cell correspondingly decreases as shown in Figure 4-9.

The no-load voltage, E_o, discussed in this section differs from the open-circuit voltage of a cell. The effective no-load voltage discussed here is the Thévenin circuit equivalent voltage which is determined by plotting discharge voltage, at a specific state of charge, against discharge rate and extrapolating to zero rate.

4.1.4.2 Effective Internal Resistance, R_e

The effective internal resistance (Re) is a gross value comprised of a number of smaller contributors which appear in the equivalent circuit analysis as resistive elements. These include the resistivity of the plate grids, the lead posts, and the terminals, and the interface contact resistance between these parts. But, the classic resistive elements represent only a portion of the total R_e. Another portion comes from the electrochemical system of the cell including resistance to ionic conduction within the electrolyte, the interface of the electrolyte with the active materials of the plates, and the resistivity of the active materials and their interface with the plate grids. All of these contributors, which when added together make up the total R_e of the cell, will vary independently as a function of changing conditions. The electrochemical components, for example, are affected dramatically by the specific gravity changes of the electrolyte in the cell. The various parameters affecting the gross R_e of the cell are discussed in the following paragraphs. Techniques for measurement of R_e are discussed in Section 4.6.

4.1.5.1 Mid-point Voltage

A common way of evaluating the discharge characteristics of a cell is to use midpoint voltages. Mid-point voltage, by definition, is the voltage of the cell when it has delivered 50 per cent of its capacity at the given discharge rate. In other words, it is the half-way point for any given discharge rate. The voltage characteristic for many sealed-lead batteries (Figure 4-14) is such that the mid-point voltage is also the approximate average voltage for the plateau of the discharge curve. This makes it a convenient point to estimate average performance in terms of voltage delivery to the load. The mid-point voltage concept will be used extensively throughout this section.

4.1.5.2 Battery Discharge Voltage as a Function of Discharge Rate

The effects of increased discharge rate on the battery voltage are manifested in three ways: depression of the voltage plateau, an increase in the slope of the plateau, and shortening of the length of the plateau. Figure 4-15 shows a family of discharge curves for three different discharge rates as a function of time. As can be seen from those plots, low to medium-rate discharges behave similarly. Although there is some voltage depression with the increase in rate, the primary effect is shortening the discharge time. However, the high-rate (10C) discharge behaves quite differently. For this reason, high-rate discharges are discussed separately in Section 4.1.6.

4.1.5.3 Battery Discharge Voltage as a Function of Battery Temperature

As the temperature of the sealed lead cell changes, both the E_o and the R_e of the cell vary, impacting the discharge voltage at the terminals of the cell.

The cell’s E_o varies slightly with temperature due to minor changes in the interface between the active materials and the electrolyte as well as solution activity effects at differing temperatures. At higher temperatures the E₀ is slightly higher and at low temperatures the E_o is slightly lower.

The impact on cell voltage from changes in R_e with temperature is normally greater than that from E_o. R_e, as discussed earlier, is a combination of many elements in the cell-resistive and electrochemical. The electrochemical parts of the total R_e vary quite broadly, particularly at low temperatures. The combined temperature effect on R_e was shown in Figure 4-11 to be dramatic. When the temperature effects upon E_o and R_e are taken into consideration, the overall effect can be displayed as a family of curves at different discharge rates as shown in Figure 4-16. Note that this characterizes the discharge profile by its mid-point voltage. An alternative approach to visualizing the effect of temperature upon discharge voltage is shown in Figure 4-17.

4.1.7.1 Cell and Battery Discharge Limits

The discharge voltage at which 100 per cent of the usable capacity of the cell has been removed is a function of the discharge rate and is shown in Figure 4-21.

Most battery applications require more than 2 volts, however. This means that cells must be connected in series to make up the required battery voltage. In a series string of cells the battery performance is determined by the behavior of the individual cells. Fortunately, the cells are quite uniform, one to another, in voltage and available capacity. It should be recognized, though, that some variations do exist.

The voltages of the different cells in a given battery are normally very close to each other as the battery is being discharged. The largest impact upon battery voltage, other than an absolute failure, comes from the capacity of the individual cells as the battery is deeply discharged. Figure 4-22 shows how a battery might look as it is deeply discharged. (The battery voltage scale on the left is purposely plotted differently from three times the cell voltage scale on the right to show clearly the deep discharge effect on the battery.)

The battery voltage near the end of useful discharge is determined by the lowest capacity cell in the battery. The knee of the discharge characteristic is sharper than that of the individual cells and once the lowest cell is totally expended, the battery voltage drops rapidly.

4.1.7.2 Disconnect Circuits

Leaving the battery connected to a load after discharge should be avoided to enable the battery to provide its full cycle life and charge capabilities. Some form of battery disconnect or kickout circuit is often supplied to remove the battery from the load once the battery capacity is exhausted. After discharge and removal of the load from the battery, the cell voltages will normally increase and stabilize at the open circuit voltage as sketched in Figure 4-13. Because of this phenomenon, some hysteresis must be designed into the battery disconnect circuitry so that the load is not reapplied to the battery under this condition. The disconnect circuit should also be designed so that it does not itself impose a load on the disconnected battery, i.e. the battery is truly open-circuited.

4.2.2.1 Cell Pressure, Temperature, Voltage, and Current Interrelationships During Charging

The cell’s voltage, current, pressure and temperature are the principal parameters that vary during charging. When those variations are understood, one can then design a charger to effectively and safely charge the cell. The pressure, temperature and voltage profiles are shown in Figure 4-25 for a discharged sealed-lead cell that is charged with a medium charge rate, such as 0.1C constant-current in a 25°C environment.

In Section 2.5.1 the charging chemical reactions were presented. When the cell is at a low state of charge all of the electrical energy input to the cell is converted to chemical energy-producing PbO₂ at the positive plate and sponge lead at the negative. During this efficient conversion of the active materials the cell pressure remains low and there is little temperature rise. The voltage rises slowly as the electrolyte is gradually converted from a weak solution to a higher acid concentration. As more electrical energy is pumped into the cell, a gradual change takes place; the positive and negative electrodes can no longer convert all the electrical energy into chemical energy. An increasing amount of charge goes to generate oxygen gas at the positive plate and hydrogen at the negative. This phenomenon is often referred to as gassing. Note that gassing is internal to the cell; if the gas escapes the cell, the process is called venting.

Cell pressure remains low until the cell approaches 80 per cent state of charge and the positive plate begins to generate oxygen. At the same point or shortly thereafter, the negative plate also begins to gas hydrogen. As the cell gradually transitions from relatively low gassing levels to a condition where the majority of the charge current is going into gas generation, venting may begin depending upon the overcharge rate. As time progresses the gassing rate and subsequent venting will approach a steady rate that is characteristic of the particular overcharge conditions employed.

The cell temperature profile is similar to the pressure profile, but lags it in time. The cell temperature increases as oxygen recombines at the negative plate releasing heat. The shape of the internal cell temperature curve and the temperature level achieved is again a function of battery design, ambient conditions, and the overcharge regime.

The relatively abrupt increase in cell voltage seen in Figure 4-25 is due to the negative plate going into overcharge with an attendant increase in the rate of hydrogen generation at the negative. The voltage reaches a peak level and then drops off due to oxygen recombination “pulling down” the potential of the negative plate. The shape of this part of the curve is variable, again depending upon a complex set of design and performance parameters.

When a cell is charged by a constant-voltage charger the temperature and pressure relationships are quite different from the constant-current charging situation. Figure 4-26 shows typical parameters for constant-voltage charging. The big difference inside the cell between constant-current charging and constant-voltage charging is the pressure and temperature in overcharge. The charging current drops off significantly in constant-voltage charging as the cell becomes fully charged. The reduction in charging current means that there is less driving force to generate gas and hence less venting. Since there is less oxygen to reduce at the negative there is less heat generated, resulting in only a very small rise in temperature, or possibly a rise and decline as shown, depending on the charge voltage.

4.2.3.1 Effect of State of Charge on Charge Acceptance

The state of charge of the cell will dictate to some extent the efficiency with which the cell will accept charge. When the cell is fully discharged, the charge acceptance is immediately quite low. As the cell becomes only slightly charged it accepts current more readily and the charge acceptance jumps quickly, approaching 98 per cent in some situations. The charge acceptance stays at a high level until the cell approaches full charge.

As mentioned earlier when the cell becomes more fully charged, some of the electrical energy goes into generating gas which represents a loss in charge acceptance. When the cell is fully charged, essentially all the charging energy goes to generate gas except for the very small current that makes up for the internal losses which otherwise would be manifested as self-discharge. A generalized curve representing these phenomena is shown in Figure 4-27.

4.2.3.2 Effect of Temperature on Charge Acceptance

As with most chemical reactions, temperature does have a positive effect upon the charging reactions in the sealed-lead cell. Charging at higher temperatures is more efficient than it is at lower temperatures, all other parameters being equal, as shown in Figure 4-28.

The cell temperature impact on charge acceptance is overlaid upon the generalized curve of Figure 4-27 to illustrate the compound effect of cell state of charge and cell temperature. For this representation the other important parameter, charge current, is held constant. Figure 4-28 shows the charge acceptance is still very high at lower temperatures.

4.2.3.3 Effect of Charging Rate on Charge Acceptance

The starved-electrolyte sealed-lead cell charges very efficiently at most charging rates. The cell can accept charge at accelerated rates (up to the C rate) as long as the state of charge is not so high that excessive gassing occurs. And the cell can be charged at low rates with excellent charge acceptance.

Figure 4-29 shows the generalized curve of charge acceptance now further defined by charging rates. When examining these curves, one can see that at high states of charge, low charge rates provide better charge acceptance.

4.2.3.4 Other Factors Affecting Charge Acceptance

When a sealed-lead cell ages, the ability of the cell to accept charge decreases and the capacity available from each charge is reduced. This is partly due to the reduction in the ability of the positive plate to conduct electrons to the external circuit because of the gradual oxidation of the grid. But it is also due to a lack of continuity in the active material of the plates of the cell, particularly the positive plate, which impacts charge acceptance. For this reason, an aged cell will begin gassing sooner in the charge cycle.

In multi-cell batteries another factor may be involved. The charge acceptance of any given cell in the series string may seem reduced if the battery pack is charged with a constant-voltage charger set at a low voltage. Under this condition, one cell in a battery may have a slightly lower charge acceptance than the other cells. When the battery is discharged, this cell will recover its capacity more slowly than the other cells in the pack. It is quite possible that this one cell will not obtain a full charge if the charge time is short before the next discharge. Repetitive rapid cycling of the pack without sufficient recharge will generally cause the low-charge-acceptance cell to become progressively lower in capacity. This problem is often associated with series-parallel applications. In those cases, interconnection between cells as discussed in Section 4.1.8 may reduce the likelihood of cycle-down.

4.2.4.1 Oxygen Recombination Reaction

As described in Section 2.5.2, when the cell reaches full charge, virtually all of the active material in the positive electrode is charged while the negative electrode, by design, still contains some uncharged active material. This balance of materials results in oxygen being generated at the positive electrode during overcharge prior to generation of hydrogen at the negative. The oxygen that migrates to the negative plate promptly recombines, converting the sponge lead to lead oxide. Since the oxygen recombination reaction is exothermic, heat is liberated as part of the process. The bisulfate form of HSO₄ in the sulfuric acid electrolyte then reacts quickly with the lead oxide to form lead sulfate with water as a byproduct. This lead sulfate at the negative plate is then reconverted back to sponge lead and sulfuric acid by the overcharge current. Thus, although oxygen is being generated at the positive plate during overcharge by the breakdown of water, it is being converted, or recombined back to water by the chemical reactions occurring at the negative plate.

Although the recombination process theoretically keeps the negative plate from going into overcharge by continuously forming lead sulfate by reaction of oxygen with the sponge lead, under many operating conditions some hydrogen is generated at the negative in overcharge. In principle this hydrogen can migrate to the positive electrode and chemically recombine back to water by a series of reactions similar to those described for oxygen recombination, but this is not known to occur readily. Instead small amounts of hydrogen escape from the cell under some operating conditions, depending on the level of overcharge.

These reactions continue as long as overcharge current flows. However, little electrolyte is lost because venting is minimal under most circumstances due to the recombination process. The vast majority of the electrical energy in this equilibrium overcharge process is converted to heat energy raising the temperature of the cell. The oxygen recombination capability of the starved-electrolyte cell permits true maintenance-free operation. While the recombination process is highly efficient under most normal charging conditions, small amounts of hydrogen and/or oxygen may be vented from the cell. However, the amounts of these gases are much lower than those discharged by other types of lead batteries being charged under equivalent conditions. There are, of course, situations of high overcharge currents and/or abnormal temperatures that may result in excessive levels of gas venting from the cell. These conditions, if allowed to persist, may permanently damage the cell.

4.2.4.2 Oxidation Effects of Overcharge

One other aspect of overcharge may also be detrimental to the life of the sealed-lead cell. Even though no electrolyte may be lost in the overcharge reactions described above, and even though the inorganic separator is not degraded by the temperature and oxygen created by the overcharge reactions, it is still best to limit overcharge to minimize oxidation of the positive grid.

The positive plate, made up of a metallic lead grid onto which the active material is pressed, undergoes a gradual change during overcharge. The grid metal, when exposed to sulfuric acid and elevated temperature in overcharge, oxidizes forming lead dioxide. This reaction takes place quite slowly because the grid is completely surrounded by active material (PbO₂) which tends to isolate or shield the grid from the electrolyte. As the grid is gradually oxidized, the lead cross-section is reduced and the current-carrying capability of the grid is correspondingly reduced because lead dioxide is a relatively poor conductor. Eventually the oxidation may become so severe that the grid ceases to adequately perform the function of conducting electrons to the external circuit. Oxidation of the grid also creates other problems. Lead dioxide occupies a greater volume than the lead it replaces. As the positive grid oxidizes, the positive electrode thus expands. The resulting plate “growth” can result in electrical shorts or physical damage, such as buckling, to the positive plate. The cell also becomes increasingly fragile as the structurally weaker lead dioxide replaces the metallic lead in the grid. The high-purity lead grids found in advanced sealed-lead batteries minimize the oxidation rate thus delaying the onset of these problems.

The oxidation of the lead grid can be delayed, thus extending the life of the cell, by minimizing the amount of overcharge. Both the magnitude and the duration of the overcharge have a direct influence upon the rate and degree of oxidation of the grid.

Again, it should be stressed that the majority of application problems with sealed-lead batteries are attributable to undercharging. Many power supply designers may be willing to accept a slight decrease in cell life as the price for increased assurance of meeting the application requirements.

4.2.4.3 Overcharge Characteristic - The Tafel Curves

When a cell is in overcharge in a state of equilibrium, the voltage/current relationship is quite predictable. The cell voltage versus overcharge current characteristics, called the Tafel curve, can be used by the charger designer to select the proper constarrt voltage setting for a specific charger/battery application. Typical Tafel curves for sealed-lead cells are shown qualitatively in Figure 4-30 and repeated with the appropriate scales in Appendix B. The overcharge voltage for any given current is quite dependent upon temperature, but as can be seen, it is not a linear relationship. A typical sealed-lead cell will have an overcharge voltage that is inversely related to temperature.

4.2.6.1 Uses of Constant-Voltage Charging

Constant-voltage chargers are most often used in two very different modes: as a fast charger to restore a high percentage of charge in a short time or as a float charger to minimize the effects of overcharge on batteries having infrequent discharges, i.e. in most standby power applications. In between these two extremes is an area of cycling-type duty where the constant-voltage charger is not as well-suited. These rather contradictory statements can be explained by remembering that supplying electrons is the key to charging batteries. Because sealed-lead batteries have very low internal resistances, they will accept very high currents when fully discharged if not limited by the current-supply capabilities of the voltage source. Initial charge currents as high as 4C are common on large power supplies. Although the current on a constant-voltage charger starts out extremely high, as illustrated in Figure 4-31, it then decays nearly exponentially.

The important parameter is the area under the curve, the integrated product of current supplied over time. Thus, in the early going, with large current flows, a constant-voltage charger may return as much as 70 per cent of the previous discharge in the first 30 minutes. This proves useful in many applications involving multiple discharge scenarios. As the battery charges, its voltage increases quickly, reducing the potential difference that has been driving the current, with a corresponding rapid decay in the charge current. As a result, even though the battery quickly reaches partial charge, obtaining a full charge requires prolonged charging. The length of the charge period may be significantly affected by the choice of charge voltage.

Given this behavior, constant-voltage chargers are frequently found in applications that normally allow extended charging periods to attain full charge. Here the fast charging to a significant fraction of the previous discharge is a significant advantage in accommodating multiple power-loss situations. Exploiting the constant-voltage charger’s fast charging features in a cycling application is difficult, since repeated discharges without the intervening time to asymptotically reach 100 per cent capacity, will result in cycling down of the battery.

4.2.6.2 Approaches to Constant-Voltage Charging

Described below are typical ways of applying constant-voltage charging to sealed-lead batteries.

4.2.6.3 Charging Parameters

The choice of charging parameters (charge voltage and current limit) requires weighing economics, battery life, and operational considerations. Increased battery temperature, a higher current limit on the charger, or elevated charger voltage setting all serve to decrease battery charge time.

4.2.6.4 Temperature Compensation of Constant-Voltage Charging

A large improvement in the constant-voltage charger can be made through compensating the voltage setting for changes in temperature. In Section 4.2.3.2 the voltage dependence upon temperature in a sealed-lead cell was discussed. The range of variation in voltage with temperature is a few millivolts per degree C depending upon cell temperature and overcharge rate. Designing the charger to vary the float voltage based on the actual battery temperature will produce a more reliable and long-life charger/battery system. This is obviously most important for batteries routinely operated at other than standard room temperature conditions. It is especially consequential for batteries that spend much of their life in electronics enclosures where temperatures can climb substantially above ambient conditions.

The recommended amount of compensation is provided in Appendix B as a function of charge rate and temperature.

4.2.6.5 Tradeoffs in Constant-Voltage Charger Design

In many circumstances, the product designer must balance the trade-offs of faster charge time with higher float voltage settings on one hand versus longer total battery life with lower float voltage settings on the other hand. Some general recommended values are provided in Appendix B. For specific suggestions on the proper charging voltage for a given application, contact the battery manufacturer.

4.2.7.1 Single-Rate Constant-Current Charging

Most constant-current chargers are simple single-rate chargers. They are readily available from charger manufacturers or they can easily be designed into the end product. Single-rate constant-current chargers are frequently chosen for portable cyclic applications such as garden tools, flashlights and portable appliances. The prime advantage of constant-current chargers is their low cost. Single-rate constant-current chargers require no switching or sensing circuits and generally consist of a simple transformer and diode rectifiers.

4.2.7.2 Split-Rate Constant-Current Charging

The concept of split-rate constant-current charging is accepted as one of the best methods of charging sealed-lead batteries. The idea is very simple. For the first part of the charging period, while the battery is in a low state of charge, the charge current will be a medium to high charge rate. At a certain point, usually when the battery is approaching full charge, the charging current is switched back to a very low trickle charge rate. The charger then may be left connected to the battery indefinitely.

4.2.9.1 High-Voltage Batteries

Charging of high-voltage (over 40 volts) batteries is a concern for two major reasons: personnel safety and charge balancing. Although both items represent areas for concern, they can both be accommodated by the use of appropriate design measures and applications precautions. Batteries with voltages in the 40 to 60 volt range have been used quite successfully in a variety of applications. Batteries with higher voltages have been constructed and used successfully, but both safety issues and application methods require much more attention.

4.2.9.2 Series/Parallel Charging

While series strings (batteries) are routinely used to increase voltage, parallel strings to increase capacity are much less common. However, some common uses for sealed-lead batteries such as many telecommunications applications utilize parallel strings to provide the requisite discharge capacity. Charging batteries in parallel requires use of constant-potential charging since constant-current charging may result in uneven distribution of charging current among the batteries. Charging series-parallel batteries requires particular attention. The precautions suggested in Section 4.1.8 regarding interconnection of paralleled strings should be employed.

4.2.9.3 Low-Temperature Charging

In general, charging batteries at moderately low temperatures (−5°C) is relatively straightforward. Either constant-voltage or constant-current may be used although the preference is for a properly temperature-compensated CP charger. Charging at lower temperatures (−20°C) is feasible, but highly inefficient. Again, either constant-voltage or constant-current methods may be used. In either case, high voltages (2.6 volts per cell and up) will be required and venting may occur since more gas may be generated than can be recombined. Often, the result of charging attempts at low temperatures is enough heating within the cell to warm it to a level where charge acceptance is improved.

Charging any battery in ultra-low temperatures is a problematic process. The starved-electrolyte battery, because of its design, survives the cold as well or better than other batteries. It accepts charge in many situations where other batteries will be damaged by charging.

4.2.9.4 Elevated Temperature Charging

Charging at high temperatures may be done using either a constant-current or constant-voltage charger. Charging rates and voltages should be adjusted to minimize overcharge using the Tafel curves from Appendix B. Note that self-discharge rates are increased because of the temperatures and trickle charge currents may have to be increased proportionately to accommodate this.

4.2.10.1 DC Power Sources

If DC power, provided at the right voltage, is already available, a dedicated charging system is not required. However, great care must be taken to ensure that the voltage available is truly useful for charging. DC systems relying on batteries to replace the primary power often operate at a system voltage closer to the voltage expected from the battery on discharge load than to the voltage necessary to charge the battery.

4.2.10.2 AC Power Sources

Most sealed-lead batteries are charged from some form of AC power source. The exact source specification may vary drastically in voltage (110, 220, etc.), frequency (60, 50, 400 Hz), and quality of regulation. Basically, the battery is concerned about only two things, the degree of voltage variation and the amount of ripple passed to the battery. Depending on the AC electrical grid, the amount of voltage variation seen may range from negligible to quite substantial. The effects of differing line voltages can have dramatic effects on chargers, especially taper chargers where the line voltage variation may produce unsatisfactory charge voltages.

In addition to the gross voltage variations attributable to the line voltage, most batteries are exposed to some degree of voltage ripple due to imperfect rectification of the AC current. The worst case is obviously a half-wave rectifier where the battery sees the maximum amount of ripple. While the starved-electrolyte battery can tolerate this, it does impact the battery life. At the other extreme, highly filtered sources may essentially reduce the ripple to zero.

The choice of the degree of filtering is usually an economic one, trading filter cost against reduction in battery life.

4.2.10.3 Photovoltaic Sources

In many remote applications, the combination of a battery and photovoltaic charging system are becoming increasingly popular as the source of electrical energy, especially for such items as radio repeaters, weather instrumentation, and navigational aids. The starved-electrolyte battery is particularly well-suited to use with photovoltaic panels because of its high tolerance for overcharge currents. In some cases, the direct output of the solar panel can be used to charge the battery; however, maximum performance and life will be obtained with the use of voltage regulation circuitry.

Photovoltaic cells act basically as constant-voltage sources with highly variable currents that depend on the time of day, weather, etc. With a typical silicon cell capable of delivering about 0.45 volts, cells will have to be connected in series to supply a charge voltage appropriate to charge the battery. The cell area then determines the charge current. Sizing of the collector array is a complex series of tradeoffs depending on location, weather, use scenario, array cost, and required battery life. A blocking diode should be used between the battery and the cell array to prevent the battery from discharging through the cells when the light intensity is low.

4.3.4.1 Effect of Temperature on Charge Retention

Since the self-discharge reactions are temperature-dependent, the temperature at which a battery is stored will greatly affect the time it can be stored and remain viable. The variation of storage life for a fully charged sealed-lead battery is illustrated in Figure 4-45. At room temperature a fully formed and fully charged battery may last nearly three years before it needs recharging. However, the allowable storage life decreases about 50 per cent for every 10 degrees Celsius that the temperature increases. Of course the reverse effect will benefit batteries stored below room temperature.

4.3.5.1 Storage Limits

Since storage temperature greatly affects the speed of the self-discharge reactions, it is the primary determinant of the time a battery may be stored. As indicated earlier, if the goal is to retain some functional capacity, Figure 4-43 indicates the rate at which capacity decays at various temperatures. If, however, the intent is just to avoid permanent damage to the product in storage, Figure 4-45 shows that storage times at all typical temperatures are relatively lengthy.

If cells or batteries are going into long-term storage, an initial charge prior to storage is recommended. A schedule for periodic charges should be developed based on that initial charge date.

4.3.5.2 Recovery After Storage

In most normal manufacturing situations, the batteries will go into use well before their OCV drops to 1.98 volts per cell (the zero capacity point).

If cells or batteries have been stored for extended periods which have resulted in their OCV’s being significantly below 1.98 volts per cell, although greater than 1.8 volts per cell, the special conditioning described below is recommended prior to use. As cells sit in storage and self-discharge, the active materials convert to lead sulfate (PbSO₄) just as they do in other discharges. But, in self-discharge, the lead sulfate forms as larger crystals that have the effect of insulating the particles of the active material, either from each other or from the grid. This result, which is known as sulfation, is normally reversible, but may require a special conditioning procedure prior to use of the battery.

Since sulfation increases the resistance of the cell, the most effective approach to reversing it is to use a low-rate (typically 0.05C) constant-current charge with a supply capable of high charging voltages (5 volts per cell). Even though the charger may not be able to force the set-point current through the battery, it normally will be able to force a small current through. As that current works on the lead sulfate crystals, they gradually convert back to active material and the impedance of the cell goes down. Ultimately, the sulfate crystals are broken down and the battery impedance drops to normal levels. Continued charging then returns the cell to its original fully charged state. Figure 4-46 shows typical behavior for a sulfated D-cell. The initial conditioning charge may take as long as 48 hours to break down the sulfation. It is often followed by a standard discharge (0.1C) and a charge to ensure the process is completed.

4.4.1.1 End-of-Life Definition

In general, sealed-lead cells and batteries behave as indicated in Figure 4-47. At the time the cells or batteries are shipped, they are at 80 per cent state of charge or better. In use, either cycling or on float charge, they improve in capacity. After a few cycles, they have reached their rated capacity. With continued use, their capacities normally continue to gradually improve until they stabilize at or somewhat above 100 per cent of their rated capacity. The batteries then begin a very gradual decline in capacity. This decline usually continues through the end-of-life point (defined as 80 per cent of rated capacity) and on downward.

The choice of the appropriate end-of-life criterion is a subjective judgment. Some designers choose to define the end of battery life when the capacity is 50 per cent of its rated value. While this does give a longer life projection, it does so with some adverse effects on many battery applications.

For most applications, the parameter that really counts is the end-of-life capacity. Selection of the appropriate battery size must be done based on supplying the load at the end of the battery’s life. If the end of battery life is defined as 50 per cent of rated capacity, the battery size required will be double that indicated by use of the rated capacity. If the end of life is specified as 80 per cent of rated capacity, the over-sizing necessary will only be 25 per cent. If the rate of capacity loss per cycle were to be constant between 80 per cent and 50 per cent of rated capacity, the extra battery size would be compensated by an equal increase in battery lifetime. But, since the rate of capacity loss increases between 80 per cent and 50 per cent, the extra capacity does not buy the user an equivalent increase in battery lifetime. Also by designing to a 50 per cent end-of-life criteria, the product is spending a substantial fraction of its life depending on an older battery that is vulnerable to increased failures from mechanical problems.

4.4.2.1 Plate Morphology

As a battery is used, a fundamental change in the structure of the active material on the positive plate occurs. Initially, the PbO₂ is in its β crystalline form which is highly active electrochemically. As the cell is charged and discharged many times, there is a gradual loss of surface area and some of the βPbO₂ is converted to an amorphous structure which is electrochemically less active. The progress of this conversion accounts for the bulk of the capacity degradation noticed as the plate ages through cycling.

4.4.2.2 Grid Oxidation

The other major culprit in cell aging is the oxidation of the lead metal in the positive grid to PbO₂. This process is primarily a function of the degree of overcharge (amount of current and length of time) experienced by the cell. The oxidation mechanism was discussed in conjunction with overcharging in Section 4.2. The effects on cell life are either mechanical (as discussed below) or electrical. The electrical effect is attributable to the substantially lower electrical conductivity of PbO₂ when compared with the metallic lead in the grid. As the conductive cross-section of the grid is reduced, the cell resistance increases, diminishing both the current delivery on discharge and the cell’s ability to accept charge on a constant-voltage charge. This gradual loss of capacity must be weighed against the catastrophic failure caused by inadequate or improper charging.

4.4.2.3 Mechanical Deterioration

Mechanical deterioration of the cell is largely confined to the changes in the positive plate due to oxidation of the grid. Since PbO₂ occupies a volume about 20 per cent greater than metallic lead, as the positive plate oxidizes, it “grows.”

This growth degrades the interface between the plate active material and the current-carrying grid. In conventional lead-acid batteries, the result is shedding of the positive active material where the active material falls off the plate and collects in the bottom of the cell. With the starved-electrolyte cell, the intimate contact between the plate and the separator lessens, but does not eliminate, degradation of the interface with grid oxidation.

Just as the grid grows out, it also grows up vertically. If clearances are too tight, this may result in the positive plate growing into contact with the negative plate terminations resulting in an internal short. Or, if the positive plate is restrained too tightly, it may actually buckle which normally also results in a short.

Lead dioxide (PbO₂) has little structural integrity. The ruggedness of many starved-electrolyte batteries comes from the design of the positive plate that uses the separator to compress the PbO₂ active material against the grid which supplies the mechanical strength. As the grid is itself converted to PbO₂, the plate support becomes increasingly fragile. Thus, as the battery ages, it becomes more vulnerable to failure due to mechanical damage. Typically this manifests itself as loss of electrical continuity for part or all of the plate as the grid fractures due to mechanical shock or vibration.

Mechanical failures of sealed-lead batteries other than those attributable to grid oxidation are exceptionally rare when the batteries are used in normal environments. Fatigue failures of the current-conducting structures may occur in prolonged and/or intense vibration exposures. Contact the battery manufacturer for additional information on applications in nonstandard environments.

4.4.3.1 Life Definition

In float applications, the primary concern regarding life is calendar time, specifically the amount of time spent on overcharge. Most standby power systems are rarely exercised to the point of one deep discharge as often as once a month. Over a ten-year life, this amounts to 120 discharges. As we will see in the cycle-life section, this is well within most batteries’ normal cycle life. As a result, concerns over plate morphological changes are usually subordinated to minimizing the rate of grid oxidation while maintaining adequate charging. The different parameters that affect grid oxidation and their effect on float life are discussed below.

4.4.3.2 Factors Affecting Float Life

Although there are a variety of factors that may affect the life of the battery in a float application, they all focus on minimizing the overcharge current which in turn minimizes the grid oxidation.

4.4.4.1 Life Definition

In a cyclic application, the important variable is not usually calendar time, but number of cycles at a given depth of discharge. While the concerns about overcharge and resulting grid corrosion may still be pertinent, there is an additional factor—changes in the positive plate structure—that must also be considered.

4.4.4.2 Factors Affecting Life

The variables affecting life in a cyclic environment are not nearly as simple as those occurring under float conditions. With a float application, the dominant concern was overcharge current control. Here that is just one of a variety of concerns. In a cyclic application, performance during discharge and charge affects life as much or more than overcharge.

4.4.5.1 Reliability vs. Life

In many applications it is less important to get absolutely the last drop of life out of a battery than it is to be assured that the battery will perform correctly when needed. This assurance may be gained in a variety of ways. The battery may be retired early, before grid oxidation has proceeded to the point where the battery may fail catastrophically. The battery can be oversized to reduce the depth of discharge. Charge voltages can be set higher than optimum for long life so that proper charge may be ensured thus avoiding failure by cycle down.

4.4.5.2 Charger Cost vs. Life-Cycle Savings

It is possible to trade expenditures for chargers against savings in extended battery life. For instance, two-step constant-current chargers can do much to extend the useful life of both cyclic and float applications. Whether the incremental cost of a two-step charger is justified can be determined only from the details of the specific application.

4.4.5.3 Product Life vs. Battery Life

Battery and charger designers sometimes get so involved in attempting to develop an optimum solution for long battery life, that the life cycle of the product is overlooked. There is little point in designing a battery to last for eight years if the product is only expected to last three.

4.5.1.1 Economic Features of Cyclic Applications

Because starved-electrolyte sealed-lead batteries can be charged and discharged many times, they are frequently selected for cyclic duty applications.

It is tempting to think of the cost of a battery in terms of its procurement cost only. The procurement cost is often evaluated in terms of cost per watt-hour by dividing the purchase price by the watt-hour rating. Different batteries can be evaluated in terms of dollars or cents per watt-hour. But this simple ratio neglects other important factors including two rather important considerations: actual deliverable energy and battery life. It often will result in selection of a battery that has a higher life-cycle cost.

The actual deliverable energy depends largely upon discharge rate and battery temperature and may differ significantly from the rated conditions. (Sections 4.1 discusses these conditions in greater detail). If the drain rate is quite high or if the battery temperature is quite low in the specific application, the actual deliverable capacity may be dramatically reduced from the rated amp-hours. Starved-electrolyte batteries deliver a high percentage of their rated capacity over a wide range of conditions. The derating needed by starved-electrolyte batteries in low-temperature use is less than other batteries, providing a significant economic advantage for these applications. Hence, the cost per watt-hour for the starved-electrolyte battery (or any other battery) should be evaluated on the basis of actual deliverable energy in the specific application rather than simply using the battery’s nominal rating.

The operating life in terms of number of discharge/charge cycles is also an important element to consider. Rechargeable batteries of the same rated capacity when cycled on the same load may provide widely different numbers of acceptable discharge/charge cycles. This makes the cost per watt-hour less relevant. Battery cost for cyclic applications is more properly evaluated as the cost per actual cycle.

To be exact, the costs used for comparison should include much more than the initial costs of the batteries. They should also factor in the costs of the charging system needed by each battery and the costs associated with battery failure including the cost of replacing the battery and any cost to the system (product lost, recreating records, mission failure, etc.) when the battery fails. Since many of these costs occur later in time, standard economic approaches such as discounted cash flow may be used to make all costs comparable.

To summarize, for cyclic duty applications, it is important to compare the costs of different batteries in the actual application. In addition to the initial cost, this comparison should consider the actual deliverable capacities of the batteries at the discharge rate and battery temperature of the specific application; use the projected life of the battery under these conditions to develop a cost per cycle; and ensure that the cost per cycle includes all relevant costs. In many cyclic applications, especially those where reliability is important, starved-electrolyte sealed-lead batteries are an economical choice even though they may not have the lowest purchase price.

4.5.1.2 Standby Power Applications

In standby power applications the battery is called upon to discharge only during abnormal situations. When a power outage does occur, the battery must dependably respond to provide the needed power during the emergency. Hence, the reliability of the battery is a very important consideration. As in the discussion for cyclic duty applications, a battery can be evaluated in terms of dollars and cents per watt-hour. Again, it is important to understand and evaluate the battery upon its characteristics in the specific application. The actual deliverable energy under application conditions, not the rated performance, is the critical consideration. It is also important that all evaluations consider the battery performance at the end of life as well as when new. As batteries age, their performance degrades even though the demands on them do not. Therefore all batteries have to be oversized based on initial capacity to ensure that they meet the end-of-life performance requirements. As the degree of performance degradation with age varies widely with batteries, the amount of oversizing required and, thus, the initial battery cost also may vary greatly.

Most standby applications do not see many discharges, so calendar time rather than number of cycles is the important measure of battery life. Operating life in terms of years of service in a float condition impacts the economics of the battery selection. Standby battery costs are normally evaluated in terms of dollars and cents per year of service rather than per cycle as was the case with cyclic applications.

In addition to the initial cost of the batteries, there are a range of other costs that should be factored into the cost comparison as well. The cost of the charger, if required, should be included. The costs of battery failure need to be carefully considered. There are the obvious expenses of the service call and cost of the new battery to replace a failed or expended battery. In addition, there is the cost impact on the overall system of having the battery fail to perform when needed. This is especially important in standby applications where the battery is often backing up a critical piece of equipment. Evaluating the cost of battery failure may be tricky, but it often overshadows other cost components. Again, many of these costs occur at different times so they should be adjusted to reflect their timing using discounted cash flow or the equivalent.

Because the costs of failure are often much greater in a standby application, battery reliability is even more important than in many cyclic applications. Because of their better reliability performance when compared to traditional lead-acid batteries, starved-electrolyte cells and batteries have consistently proven themselves to be the low-cost performer in many situations.

4.5.3.1 Battery Packaging

The designer has a choice of a variety of ways of packaging the single-cell battery to meet the requirements of the application. Some of the considerations in determining the proper packaging are:

The following discussion provides only a brief introduction to battery packaging. Additional information on packaging options is also available from battery manufacturers.

4.5.3.2 Mounting the Battery

When determining where and how to mount a sealed-lead battery, the application designer can take advantage of its battery’s ruggedness and sealed, no-maintenance construction. Mounting constraints, such as keeping the battery upright, away from valuable equipment, and accessible for routine maintenance or replacement, are largely eliminated.

The mounting flexibility of the sealed-lead battery is illustrated by two examples:

The following sections will describe how best to utilize the flexibility of the sealed-lead battery.

4.5.4.1 Operating Temperatures

Batteries operate best in moderate temperature conditions. Extreme operating temperatures are generally detrimental to the sealed-lead battery system, causing either degradation of performance (cold temperatures) or reduction of operating life (high temperatures). While, in certain applications, sufficient performance may be obtained outside these limits, a maximum operating range of −40°C to +60°C is typically recommended.

Operation of these battery systems at high temperature accelerates the nonreversible degradation of performance. As discussed previously, proper temperature compensation of chargers will minimize this degradation. High temperatures also increase self-discharge rates thereby decreasing the time the battery can be stored before a charge is necessary.

Operation toward the low end of the recommended temperature range results in a predictable loss of available capacity, but there is no danger of physical damage to charged sealed-lead batteries from low temperatures. Thus fully charged sealed-lead batteries may be stored without problem at temperatures down to −65°C.

4.5.4.2 Relative Humidity

Sealed-lead batteries may be operated in all humidity ranges normally suitable for electrical and electronic components. When operated continuously at 95 per cent relative humidity or higher, the terminals, interconnecting straps and steel cases (as applicable) may show modest cosmetic rusting. The sealed-lead cells and batteries have successfully passed the humidity and salt-spray tests of M1L-STD-810.

4.5.4.3 Vacuum or Pressure

Being sealed, starved-electrolyte batteries may be used in either a vacuum or positive-pressure environment. The resealable safety vent operates on differential pressure so that it adjusts to whatever external pressure exists.

4.5.4.4 Corrosive Atmosphere

Sealed-lead batteries are constructed so that they will resist corrosion in most environments found in commercial applications. In very severe corrosive environments it is possible for the metal parts to corrode, but in most commercial applications this is not generally a problem. Contact the battery manufacturer for assistance if the ambient environment is expected to be extremely corrosive.

4.5.4.5 Shock and Vibration

While sealed-lead batteries offer excellent overall vibration resistance, exposure to prolonged and/or intense vibration may cause premature failure. Most standard product lines are designed to withstand the shock and vibration encountered in routine shipments via common carriers as well as that encountered in most commercial applications. The survival of sealed-lead batteries in walk-behind lawn mowers demonstrates a vibration resistance not found in other lead-acid batteries. There are a few design measures that can be used to reduce the vulnerability of sealed-lead cells and batteries to vibration damage. Two types of vibration-induced failures are seen most often: shearing of the plate tabs and current collectors within the cell or failure of the cell terminals external to the cell.

The internal failure mode is most pronounced when the battery is vibrated in the “vertical” direction. In such situations, the pack containing the plates and separator may start moving relative to the cell container. This movement, if it persists, normally results in fatigue failure of the parts that carry the current from the positive plate to the positive terminal. If the battery can be oriented so it experiences vibration in the “horizontal” rather than “vertical” direction, its vibration resistance is normally increased.

External failure of the terminals is predominately seen with batteries of single cells although batteries of multiple monoblocs may also experience the same difficulty. The problem arises when two cells or monoblocs that are rigidly interconnected begin to move relative to each other under the influence of the vibration. This will normally cause fatigue failure of the terminal or the interconnection. Two methods may be used to reduce or eliminate these failures. One approach is to eliminate cell-to-cell movement within a battery case by either gluing the cells to the case or using an interference fit to hold the cells in the case. A second approach is to provide a flexible interconnect between cells which allows movement between cells without fatigue failure of the electrical linkage. If soldered stranded-wire intercell connections are used in a vibration application, they must be sufficiently long to allow adequate flexibility after the wire has been tinned.

• Standby battery power

• Engine starting

• Alternate power source

• Portable power

4.5.6.1 Standby Battery Power

One excellent application for starved-electrolyte sealed-lead batteries is in systems which require backup or standby power. As electrical and electronic systems continue to replace human and mechanical systems, everyone is increasingly dependent on the continuity of electrical power. The increased importance of electrical service during power outages and momentary interruptions is leading many equipment designers to provide backup power. This backup may be built into the device or it may be an external unit such as an uninterruptible power supply. In addition, some devices are designed to operate only during the period of a power outage, to provide safety for people in situations which might cause alarm or danger as a result of the power interruption. These situations also require a reliable battery, ready to serve upon instantaneous call.

There are three ways a sealed-lead battery can be used in standby power applications:

The major use of batteries in standby applications is to maintain vital functions despite the loss of power from the grid. An excellent example is the telephone system which uses an array of batteries and engine-generator sets to provide phone service even when the grid power is unavailable. Sealed-lead batteries are used in a variety of telephone installations for backup power. Their size, reliability, ruggedness, and no-maintenance features make them especially well adapted for distributed applications such as switching systems and line concentrators. Sealed-lead batteries are also used in a multitude of other products such as security alarms, computers, and medical equipment that must continue to work during an interruption in grid power. Sealed-lead batteries are especially well-suited for backup that is integral to the product. Because they are clean and maintenance-free, they can be embedded in the unit rather than having to be separated in their own area.

Even though integral backup is becoming more common, many applications choose to use a separate external power system instead. These uninterruptible or switched power systems are increasingly choosing starved-electrolyte sealed-lead batteries because of their reliability and long life.

A good example of the case where the battery initiates a function once the power interruption occurs is battery-powered emergency lighting. Normally the light is off and the charger is keeping the battery fully charged. Once the line power fails, the battery is used to power the light.

In other situations, the basic equipment function is not sufficiently vital to be kept going when the power goes off, but there is a shutdown procedure that should be accomplished to minimize problems with the system. A classic example is seen with many computer systems where records need to be transferred from volatile memory to nonvolatile memory before the system shuts down and the information is lost.

As microprocessors become integral elements of things as simple as toys and household appliances, the importance of limited battery backup increases. Many of these devices use volatile memory to retain instructions or data. They need backup power to retain their memory even when the power is shut off.

Starved-electrolyte sealed-lead batteries are the choice for many designers of equipment requiring standby power because of their very long life, reliability, and simple charging in float applications. The flexibility in mounting these batteries is a significant feature where space is at a premium.

4.5.6.2 Engine Starting

Starved-electrolyte sealed-lead batteries have very low internal resistance which permits very large peaks of power for short periods. One application for short, but very high, power delivery is in gasoline engine starting. One application in particular, electric-start walk-behind lawn mowers, is considered by many to be one of the roughest tests of a battery that exists today. Practically everything is wrong. The battery is often mounted where it is exposed to high temperatures from the engine and high vibration from the blade. The charging system is rudimentary. Duty cycles may combine long periods of disuse with periods of many discharges in a brief amount of time. Starved-electrolyte sealed-lead batteries have been acclaimed as the best batteries available for this application.

4.5.6.3 Portable Power

Sealed-lead batteries are often used as the primary power source for portable consumer appliances, tools, lights, and toys.

Duty cycles for many of these items are close to float service. Consumers may use their portable vacuum or rechargeable light once a week and leave it on charge the rest of the time. This type of service is well-suited to constant-voltage float charging using the sealed-lead battery. Strength in charging combined with high current deliveries and acceptable power densities have made sealed-lead cells and batteries economical choices for many portable power applications.

4.5.6.4 Alternate Power Sources

In addition to the portable power applications discussed above, sealed-lead batteries are frequently used as an alternate power source to the normal AC line power. These applications are also portable but are normally powered from the AC line through a built-in DC power supply. When the portable feature is desired, the AC line cord is unplugged and the unit is immediately portable. The battery is normally charged from the built-in DC power supply when the unit is plugged in and is usually fully charged, ready to go, when the line cord is unplugged. The sealed lead battery is ideal for these applications because it can be charged continuously using a properly designed constant-voltage charger with little or no reduction in overall life. And because the charge retention of these batteries is excellent, the battery holds its power during long periods of “off” time when the device is not being operated.

A good example of an alternate battery power source application is the portable television set. The battery equipped portable television set is normally operated from AC line power. Whenever the set is plugged into the line, it operates from line power while the battery is charged and maintained at full charge. If someone wishes to operate the TV away from the convenient wall outlet, the battery is ready to do the job. After the use, the battery is automatically recharged once the set is plugged back in the AC power receptacle and is quickly ready for its next outing.

A second example of an alternate battery power source application is in portable instrumentation, such as voltmeters and oscilloscopes. Again, the instrument normally runs from the AC line, but is ready to be removed to a remote location in a fully charged condition.

• discharge performance — can the battery meet the electrical requirements for the specified length of time at the lowest temperatures expected for the application?

• charge acceptance — will the battery recover quickly enough to meet the required use profile? How will that recovery be affected by a continuing series of charge/discharge cycles?

• cycle life — how many charge/discharge cycles can the battery reasonably be expected to survive?

• deep discharge — can the unit gracefully survive being completely discharged before recharge?

• Storage life — how long can the product be in the distribution chain and still retain functionality?

• R_etesting — determination of the battery’s effective internal resistance allows prediction of the battery’s voltage under various current loads.

• mechanical/environmental behavior — will the battery operate properly in the location and orientation proposed for it? Will it survive the mechanical abuse that the unit might experience?

• discharge performance — can the battery carry the load for the specified length of time at the lowest temperatures specified?

• charge acceptance — will the battery recover quickly enough to meet the required multiple outage scenario, especially at low temperatures? Is the charging adequate to maintain battery balance? What are the overcharge currents, especially in any elevated temperature scenarios?

• float life — how long will the battery survive in this application under the proposed overcharge currents?

• R_e testing — determination of the battery’s effective internal resistance allows prediction of the battery’s voltage under various current loads.

• mechanical/environmental behavior — will the battery operate properly in the location and orientation proposed for it?

4.6.1.1 Preparation for Testing

No matter what characterization testing will be performed, the first step is to develop a properly prepared and conditioned sample of batteries. For various reasons, batteries may arrive for test at different states of charge. Therefore it is important that batteries be prepared for testing by cycling them until their actual capacities are stabilized at a value equal to or greater than the rated capacity.

4.6.1.2 Discharge and Charge Acceptance Tests

Often the first set of tests to be performed are those that confirm the successful operation of the battery under nominal conditions, i.e. with the battery new, but stabilized; charged in the optimum manner; and at room temperature. If the battery can not meet the load requirements under favorable conditions, there is no point in continuing. Assuming those tests are favorable, battery operation should be confirmed at the temperature extremes for which the product is expected to operate.

Once the basic ability of the battery to accommodate the load has been determined, then the charge acceptance of the battery can be tested. It is essential that the ability of the proposed charging system to charge the battery within the expected application profile be closely examined. This is where careful testing and feedback to those developing the application requirements will pay substantial benefits to the designer. The result can be substantial cost savings and/or performance improvements through a better understanding of battery behavior.

4.6.1.3 Life Tests

As might be expected, the type of life test employed depends on the type of duty the battery will see.

4.6.1.4 Storage Life

Tests for capacity retention in storage can be conducted using elevated temperature tests similar to those used to estimate float life. Since the pertinent self-discharge reactions in the battery follow the Arrenhius equation, storage time as a function of temperature is of the form shown in Figure 4-53. Thus, by testing batteries for self-discharge at 65°C, three years storage at room temperature can be duplicated in about two months.

4.6.1.5 Overdischarge Recovery

For those applications where economics preclude using a disconnect circuit, the battery may be vulnerable to overdischarge by inadvertently being left connected to the load. In this situation, tests that indicate battery performance under deep discharge conditions may be useful. These tests typically involve leaving the battery to discharge through a resistor for an extended period and then requiring the battery to meet a certain discharge standard after one or two charge/discharge cycles. While such tests are instructive, the designer should understand that deep discharge of this magnitude is an abusive condition for many common batteries. He or she should not expect the battery to survive repeated or prolonged deep discharges of this type without adverse effects.

4.6.1.6 Effective Internal Resistance, R_e, Testing

Numerous measurement methods have been used for determining effective internal resistance of a cell. The value measured on a given cell depends on the method chosen. It is therefore important that, in any comparison of R_e values or in any communication about R_e values, the measurement method be fully defined.

Methods presently in use include AC milliohm meters (1000 Hz), mid-point voltage comparisons at various constant current discharge rates, and the two-current method. The second and third methods yield approximately equal measured values, while the first method results in a very low measured value. The two-current method is endorsed by many manufacturers and battery users because it results in what is felt to be the most useful value and because the measurement can be made very rapidly. The R_e value is used to predict the expected voltage delivery as a function of a constant discharge current. See Section 4.1.4.

The method recommended by many manufacturers is similar to that given in Paragraph 9.4 of ANSI Standard C18.2-1984: Specifications for Sealed Rechargeable Nickel-Cadmium Cylindrical Bare Cells. It was selected because it provides a realistic measured value while promoting standardization within the industry.

The test is conducted by discharging the battery at a high rate and then switching to a substantially lower rate. The voltage at the battery terminals and the current is measured immediately before switching and again after the voltage has stabilized after switching. The effective internal resistance is then calculated as:

R_e = –ΔV/ΔI = (V_L – V_H)/(I_H – I_L)

Designers interested in conducting R_e measurements should either refer to the ANS1 publication or contact the battery manufacturer for specific recommendations on the testing protocol.

4.6.1.7 Mechanical and Environmental Tests

Batteries used in most commercial applications typically have few formal environmental or mechanical requirements imposed. Those requirements that are imposed are often tests to ensure that the batteries can survive normal shipping and customer use. Sealed-lead batteries, both single-cell and monobloc, are rugged enough that these tests are rarely a problem.

4.7.2.1 Handling Cells and Batteries

Since the terminals of the sealed-lead battery are often upright and exposed, short circuits are best prevented by use of good housekeeping practices as summarized below:

4.7.2.2 Shipping and Storage

Millions of cells and batteries, properly packed, have traveled millions of miles without incident. However, improperly packed cells or batteries when exposed to the vibration of long-distance truck transportation can short and possibly even ignite the packing materials with unpleasant consequences. The keys to proper shipment are simple:

If shipping quantities of cells or batteries, consulting in advance with the battery manufacturer on packaging design and shipping procedures can help ensure that the product is received intact.

• Dispose of these batteries after first discharging them or insulating the terminals to prevent accidental shorting.

• Do not incinerate or expose to fire or high heat, as the cell may burst and spray acid over a large area.

• Disposal of sealed-lead batteries should always conform to applicable regulations.