Discharge Chemistry
In the discharged state, both the positive and negative plates become lead (II) sulfate (PbSO4). 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) + HSO4-(aq) → PbSO4(s) + H+(aq) + 2e-
Positive plate reaction: PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e- → PbSO4(s) + 2H2O(l)
Combining these two reactions, one can determine the overall reaction:
Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4-(aq) → 2PbSO4(s) + 2H2O(l)
Charge Chemistry
This type of battery can be recharged. In the charged state, each cell contains negative plates of elemental lead (Pb) and positive plates of lead (IV) oxide (PbO2) in an electrolyte of approximately 33.5% v/v (4.2 Molar) sulfuric acid (H2SO4-). The charging process is driven by the forcible removal of electrons from the positive plate and the forcible introduction of them to the negative plate by the charging source.
Negative plate reaction: PbSO4(s) + H+(aq) + 2e- → Pb(s) + HSO4-(aq)
Positive plate reaction: PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e-
Combining these two reactions, the overall reaction is the reverse of the discharge reaction:
2PbSO4(s) + 2H2O(l) → Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4-(aq)
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.
Therefore, the overall equation for the cell is:
Another example of a dry-cell battery is the alkaline battery. Alkaline batteries are almost the same as zinc-carbon batteries, except that the electrolyte used is potassium hydroxide (KOH) rather than ammonium chloride. In some more modern types of so-called "high-power" batteries (with much lower capacity than standard alkaline batteries), the ammonium chloride is replaced by zinc chloride.