Answer
Overall reaction that occurs in Lead-acid storage battery:
$Pb(s) + PbO_{2}(s) + 2H_{2}SO_{4}(aq) -> 2PbSO_{4}(s) + 2H_{2}O(l)$
Work Step by Step
First, write both balanced half reactions occur in lead-acid storage battery and add those equations to get the overall reaction.
Anode reaction:$ Pb(s) + SO_{4}^{2-} -> PbSO_{4} + 2e^{-}$
Cathode reaction: $PbO_{2} + 4H^{+} + SO_{4}^{2-} + 2e^{-} -> PbSO_{4} + 2H_{2}O$
No. of electrons are same in both the equations. So, we don't need to multiply with co-efficient before adding those equations.
After adding the equations, we get
$Pb(s) + PbO_{2}(s) + 2H_{2}SO_{4}(aq) -> 2PbSO_{4}(s) + 2H_{2}O(l)$
[Note: $2H^{+} + SO_{4}^{2-} = H_{2}SO_{4}$]
