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). H2SO4 is used up. Pb(s)+PbO2(s)+4H+(aq)+2SO42−(aq)→2PbSO4(I)+2H2O As a result, the specific gravity of H2SO4 falls when it falls below 1.230 the battery needs recharging.