The extent of charge of lead accumulator is determined by
The extent of charge of lead accumulator is determined by
amount of $\mathrm{PbSO}_4$ in the battery
amount of $\mathrm{PbO}_2$ in the battery
specific gravity of $\mathrm{H}_2 \mathrm{SO}_4$ of the battery
amount of $\mathrm{Pb}$ in the battery
Solution
In a fully charged lead accumulator or lead storage battery, sulphuric acid has a specific gravity (i.e., density that varies from 1.260 to 1.285. But during discharge (i.e., when the battery is in use). $\mathrm{H}_2 \mathrm{SO}_4$ is used up.
$$
\begin{array}{r}
\mathrm{Pb}(s)+\mathrm{PbO}_2(s)+4 \mathrm{H}^{+}(a q)+2 \mathrm{SO}_4^{2-}(a q) \longrightarrow \\
2 \mathrm{PbSO}_4(l)+2 \mathrm{H}_2 \mathrm{O}
\end{array}
$$
As a result, the specific gravity of $\mathrm{H}_2 \mathrm{SO}_4$ falls when it falls below 1.230 the battery needs recharging.