A battery, usually an old one, sometimes gets into a deteriating condition that my Dad refers to as "sulphating." He was in the electric fork truck business for many years, thus has a lot of experience with batteries.

In this condition the battery is giving off explosive gas. The more common scenario that causes an explosion is the spark from jumper cables.

The warning; whenever using jumper cables, wave your hand over the battery to disperse any concentration of gas before connecting the last cable. Better yet, hook the positive jumper cable to the battery being jumped and make the negative connection last and somewhere on the vehicle away from the battery.

A battery that requires jumping is a very possible candidate for being in the "sulphating" condition, thus could be proned to explosion.

Have a great day,