Lead acid batteries produce extremley explosive pure hyrdogen gas when charging. A spark near the battery (like jump starting) can ignite the gas and boom! This is why you should ground the negative cable to the block or frame and not the battery being charged. I've seen it happen once and the guy had battery acid all over his face and eyes. Ruined his day.

The batteries in our trucks are constantly being drained and charged which produces hydrogen gas, and any spark could light them off. This is another reason why (IMHO) I have gone to the gel cell technology (like new aircraft use) of the Optima Red Tops. They are sealed, can't spill or leak, and are 44 pounds lighter than standard batteries. No leaks = no corrosion.

Gel cells are the battery of the future, and Optimas are a good gel cell battery.