Step 1: Set-up. The cell is written as \(Pb \mid H_2SO_4 \mid PbO_2\). The negative plate is spongy lead, the positive plate is lead dioxide, and the two are immersed in sulphuric acid.
Step 2: What happens on discharge. The battery delivers electricity by a redox reaction. Lead is oxidised (loses electrons) and lead(IV) in \(PbO_2\) is reduced; both end up as lead(II) sulphate.
Negative plate: \(Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^-\)
Positive plate: \(PbO_2 + 4H^+ + SO_4^{2-} + 2e^- \rightarrow PbSO_4 + 2H_2O\)
Net: \(Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O\)
Because acid is consumed and water is produced, the specific gravity of the electrolyte drops, which is how a garage checks whether a battery is run down.
Step 3: What happens on charging. An outside DC source forces electrons the other way, so every step above runs backwards. The \(PbSO_4\) on both plates is reconverted:
Net: \(2PbSO_4 + 2H_2O \rightarrow Pb + PbO_2 + 2H_2SO_4\)
Sulphuric acid comes back and its density rises, so the battery is ready to use again.
Step 4: Summary. Discharge and charge are the same reaction read in opposite directions, which is why the lead-acid cell is rechargeable (a secondary cell).