Electrochemistry
Discharge

Fully discharged: two identical lead sulfate plates

In the discharged state both the positive and negative plates become lead(II) sulfate (PbSO
4), and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit.

Negative plate reaction:

Pb(s) + HSO−4(aq) → PbSO4(s) + H+(aq) + 2e−

Positive plate reaction:

PbO2(s) + HSO−4(aq) + 3H+(aq) + 2e− → PbSO4(s) + 2H2O(l)

The total reaction can be written as

Pb(s) + PbO2(s) + 2H 2SO 4(aq) → 2PbSO 4(s) + 2H 2O(l)
Charging

Overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, which is lost to the cell. Periodic maintenance of lead-acid batteries requires inspection of the electrolyte level and replacement of any water that has been lost.

Due to the freezing-point depression of the electrolyte, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze during winter weather when discharged.
Ion motion

During discharge, H+
produced at the negative plates moves into the electrolyte solution and then is consumed into the positive plates, while HSO−
4 is consumed at both plates. The reverse occurs during charge. This motion can be by electrically driven proton flow or Grotthuss mechanism, or by diffusion through the medium, or by flow of a liquid electrolyte medium. Since the density is greater when the sulfuric acid concentration is higher, the liquid will tend to circulate by convection. Therefore a liquid-medium cell tends to rapidly discharge and rapidly charge more efficiently than an otherwise similar gel cell.
Measuring the charge lev
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