A lead acid battery consists of two electrodes, one made from lead and the other from lead dioxide, immersed in a solution of sulfuric acid.
The chemical reaction that takes place at the lead electrode liberates electrons, so that's the negative terminal of the battery:
Pb + HSO4- ==> PbSO4 + H+ + 2e-
The electrons travel through the circuit to the positive terminal, where they are recycled in the reaction:
PbO2 + HSO4- + 3H+ + 2e- ==> PbSO4 + 2H2O
To maintain the reactions H+ ions must flow from the negative terminal to the positive terminal.