Charging a battery also generates hydrogen. And because heat drives up hydrogen output, you can expect more trouble generally in hot weather. The danger is that hydrogen will explode if a spark occurs nearby. One source of sparks can be the battery itself.
What can happen if you get battery acid on your skin?
Sulfuric acid is a highly corrosive chemical. If you get this on your skin, it can cause severe skin burns. If you get some battery acid on your skin, immediately flush with water and wash with a weak base (most soaps are weak bases) to neutralize any remaining acid.