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A lead acid battery consists of a negative electrode made of spongy or porous lead. The lead is porous to facilitate the formation and dissolution of lead. The positive electrode consists of lead oxide. Both electrodes are immersed in a electrolytic solution of sulfuric acid and water. In case the electrodes come into contact with each other through physical movement of the battery or through changes in thickness of the electrodes, an electrically insulating, but chemically permeable membrane separates the two electrodes. This membrane also prevents electrical shorting through the electrolyte. Lead acid batteries store energy by the reversible chemical reaction shown below.

The overall chemical reaction is:

$Pb{O}_{2}+Pb+2{H}_{2}S{O}_{4}\underset{charge}{\overset{discharge}{⇔}}2PbS{O}_{4}+2{H}_{2}O$

At the negative terminal the charge and discharge reactions are:

$Pb+S{{O}_{4}}^{2-}\underset{charge}{\overset{discharge}{⇔}}PbS{O}_{4}+2{e}^{-}$

At the positive terminal the charge and discharge reactions are:

$Pb{O}_{2}+S{{O}_{4}}^{2-}+4{H}^{+}+2{e}^{-}\underset{charge}{\overset{discharge}{⇔}}PbS{O}_{4}+2{H}_{2}O$