Why do batteries specifically vent *hydrogen* in the event of abuse?
Lead storage batteries use an electrolyte of sulfuric acid and water. On overcharging you are electrolysing the water with emission of $\ce{H2}$ in one electrode and $\ce{O2}$ in the other.

Other batteries use molecules with H and Li. The idea is to use light elements so you can accumulate more charge for the weight.