When a lead storage battery is charged, the chemical reactions are reversed from when it is discharged. During charging, lead sulfate $\left(\mathrm{PbSO}_4\right)$ is deposited on the lead electrode. Sulfuric acid $\left(\mathrm{H}_2 \mathrm{SO}_4\right)$ is regenerated, not decreased.
Here's a more detailed explanation of the chemical reactions that take place when a lead storage battery is charged:
- Positive electrode (anode): $\mathrm{PbSO}_4$ (lead sulfate) $+2 \mathrm{e}^{-} \rightarrow \mathrm{Pb}$ (lead) $+\mathrm{SO}_4{ }^{2-}$ (sulfate ion)
- Negative electrode (cathode): $\mathrm{PbSO}_4$ (lead sulfate) $+4 \mathrm{H}^{+}$(hydrogen ion) $+2 \mathrm{e}^{-}$ $\rightarrow \mathrm{PbO}_2$ (lead dioxide) $+2 \mathrm{H}_2 \mathrm{O}$ (water)
As you can see, the lead sulfate on the electrodes is converted back into lead (at the anode) and lead dioxide (at the cathode) during charging. The sulfuric acid concentration also increases because sulfate ions $\left(\mathrm{SO}_4{ }^2\right)$ are not used up in the reaction, but instead recombine with hydrogen ions $\left(\mathrm{H}^{+}\right)$to form sulfuric acid $\left(\mathrm{H}_2 \mathrm{SO}_4\right)$.