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 For example, in the first four hours of charging, about 70% of the ampere-hours previously withdrawn from the battery has been restored.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At this point, the number of ampere-hours returned to the battery is about the same as the number withdrawn, but the battery will still accept additional ampere-hours up to about 105% of the number withdrawn.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:

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).

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.

 

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

 

 

 

Table 1: Effect of Battery State-or-Charge on

Charger Efficiency in an 8-Hour Charge Period.

% Discharge

at start of

Charge

Total Line

Energy

(kWh)

Total Energy

to Battery

(kWh)

Charger

Efficiency

(%)

80

40

20

50

35

24

42

27

17

84

77

71

 

 

 

Table 2: Effect of Battery State-of-Charge on

Battery Efficiency in an 8-Hour Charge Period

%

Discharge

at Start of

Charge

Energy to Battery

Energy to Load

Efficiency

kWh

Ah

kWh

Ah

%

80

40

20

42

27

17

1030

652

424

32

17

8.6

960

480

120

76

63

50

 


 

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.

 

Table 3: Overall System Efficiency When Charging

for an 8-Hour Charge Period

% Discharge

at Start of

Charge

Charger

Efficiency

(%)

Charge/

Discharge

Efficiency

(%)

Overall

Efficiency

(%)

80

40

20

84

77

71

76

63

50

64

49

36

 

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)