A Lead Storage Battery acts as a galvanic cell during discharge.

• Reaction: $Pb(s)+Pb{O}_2(s)+2H_{2}S{O}_4(aq)\to 2PbS{O}_4(s)+2H_{2}O(l)$
• Analysis: Notice that Sulfuric Acid ($H_{2}S{O}_4$) is being consumed from the electrolyte while Water is being produced.
• Because the dense acid is replaced by less dense water, the density (specific gravity) of the electrolyte solution decreases. This is why measuring the density of the acid is used to check the state of charge in these batteries.