Lead Storage Battery

The lead storage battery — familiar as the standard automobile (car) battery — is a secondary (rechargeable) cell whose construction is:

• Anode: spongy lead (Pb).
• Cathode: a lead plate coated with lead dioxide (PbO₂).
• Electrolyte: roughly 38% (by mass) sulphuric acid, with a density of about 1.2 g/mL.

Discharge. At the anode, lead is oxidised: $\text{Pb(s)} \rightarrow \text{Pb}^{2+}\text{(aq)} + 2e^{-}$, and the resulting Pb²⁺ ions immediately combine with sulphate ions from the acid to precipitate as lead sulphate, $\text{Pb}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{PbSO}_4\text{(s)}$. At the cathode, PbO₂ is reduced: $\text{PbO}_2\text{(s)} + 4\text{H}^{+}\text{(aq)} + 2e^{-} \rightarrow \text{Pb}^{2+}\text{(aq)} + 2\text{H}_2\text{O(l)}$, and these Pb²⁺ ions likewise precipitate as lead sulphate. Summing all four steps gives the overall discharge reaction: $\text{Pb(s)} + \text{PbO}_2\text{(s)} + 4\text{H}^{+}\text{(aq)} + 2\text{SO}_4^{2-}\text{(aq)} \rightarrow 2\text{PbSO}_4\text{(s)} + 2\text{H}_2\text{O(l)}$. A single cell delivers an emf of about 2 V, so six identical cells are combined in series in a typical car battery to produce the familiar 12 V output. Because the discharge reaction consumes both H⁺ and SO₄²⁻, the concentration of sulphuric acid steadily falls as the battery discharges, and once the cell potential has dropped to around 1.8 V the battery needs recharging. …