Discharge and Charging Reactions
The lead acid battery is a truly unique device - an assembly of the active materials of a lead dioxide (Pb02) positive plate, sulfuric acid (H2SO4) electrolyte and a sponge (porous) lead (Pb) negative plate which, when a load is connected between the positive and negative terminals, an electrochemical reaction occurs within the cell which will produce electrical energy (current) through the load as these active materials are converted to lead sulfate (H2SO4) and water (H20). When the load is removed and replaced by an appropriate DC current source, electrical energy (charging current) will flow through the battery in the opposite direction converting the active materials to their original states of lead dioxide, sulfuric acid and lead. This "recharging" of the battery restores the potential energy, making it again available to produce the electrical current during a subsequent discharge. This reversible electrochemical process is illustrated in Equations 1, 2 and 3.
1. Pb02 +2H2S04+ Pb- = PbSO4+ H2O + PbSO4
2. Reaction at the Positive Plate Pb02 + 3 H++ H 2SO4+ 2e- = 2H20 + PbSO4
3 Reaction at the Negative Plate Pb + HSO4- = PbSO4 + H+ + 2e-
In theory, this discharge and recharge process could continue indefinitely were it not for the corrosion of the grids onto which the lead dioxide (Pb02) and lead (Pb) active materials are pasted, deterioration of the lead dioxide and sponge lead active materials of the positive and negative plates, and in the case of VRLA batteries, drying of the electrolyte. While internal local action and deep discharge do play a roll in grid corrosion and active material deterioration, and elevated operating temperatures do further aggravate the situation, it is most often that improper charging techniques are primarily responsible for premature battery failures.
Overcharging
It only requires between 107% and 115% of the ampere hours energy removed from a lead acid battery to be restored to achieve a fully charged system capable of delivering 100% of its rated capacity. For example, if 10 ampere hours of energy had been removed from a battery during discharge, then 10.7 ampere hours of energy would have to be replaced through the charging activity to restore 100% of capacity. Charging at too high a rate or forcing more than the 107% required into the battery constitutes overcharging and results in additional grid corrosion , gassing and consumption of the water in the electrolyte. This overcharging is a common cause of premature battery failure.
Vented Lead Acid Cells: Overcharging and Gassing
Once the plates of the battery are fully converted to their original lead dioxide (Pb02) in the positive plate and sponge lead (Pb) in the negative plate, most of the additional ampere-hours or charging current are consumed in the electrolysis of the water in the electrolyte. In the vented (flooded) cell, this occurs at the positive and negative plates as shown in Equations 4, 5 and 6.
4. Positive Plate 2H2O=O2+4e-
5. Negative Plate 4 H++ 4 e- = 2H2
6. Net Reaction ?2H2O=2H2+O2
FIGURE 2: Lead Acid Battery Recharge
As shown in Equation 4, the water (H20) in the electrolyte at the positive plate is broken down into oxygen gas (02), free hydrogen ions ( 4 H+ ) and free electrons (4e-). The free electrons are "pulled" from the positive plate by the connected charger and "pumped" to the negative plate as noted in Equation 5. As the free hydrogen ions (4H+) migrate through the electrolyte and contact the negative plate, where there is an excess of electrons, then hydrogen ions take on an electron, and hydrogen gas (2H2) is formed. Being a vented cell with liquid electrolyte, the oxygen gas (02) generated at the positive plate and the hydrogen gas (2H2) generated at the negative plate will percolate up through the electrolyte and into the surrounding atmosphere as the electrolyte level declines. Since the water that is gassed off can be replaced, this consequence of overcharging with has no impact on the life of the vented cell.
Summary of Charging Methods for Valve Regulated Lead Acid Batteries
The following summarizes the previous discussion concerning charging methods. The summary is, in most cases, divided into the method employed during the specific phase (bulk, absorption or float) of the charging regime. The actual charging regime selected will be a combination of the following individual methods. For example, methods 7 and 8 and 9 are the preferred method.
Criterion for Charging VRLA Batteries in Float (Standby) Service:
1. Do not exceed 2.40 volts per cell for constant voltage equalize/freshening charge.
2. Do not exceed 2.30 volts per cell @ 25°C (77°F) for the final constant float voltage if this voltage level is also relied upon to drive the bulk and absorption phases of the charging operation. 3. Do not use below 2.25 volts per cell @ 25°C (77°F) for the final constant float voltage if this voltage level is also relied upon to drive the bulk and absorption phases of the charging operation.
4. If the final float voltage is not used to drive the bulk and absorption phase of the charging operation, once the battery is fully charged, a final float voltage as low as 2.2 V/C may be used to maintain the battery.
5. Do not exceed the recommended initial bulk charging current recommendation.
6. Utilize temperature compensation of the charging voltage where wide temperature variations and extremes are anticipated.
7. If constant current is to be employed for the final "trickle" charge, it should not exceed 1 milli-ampere per ampere-hour of-capacity for the gelled battery or 2 milli-ampere ampere-hour of capacity for the AGM battery.
8. Approximately 107% to 115% of the ampere-hours removed during the discharge must be restored to reach 100% state of charge.
