I've had a lot of unfortunate and unsual things happen to me over the years with various rides, and also seen a number of things with friends rides. While I know it's possible, I have never personally had a battery explode on me. I was swapping out my winter ride for the summer ride today after work and when I went to start the car in garage - BOOM!! Loud like a gun shot or a tire expoding. My ears had a ring in them for a couple of minutes following....
I quickly got out of the car and looked around the car to see if a tire had in fact expoded and everything was fine. Confused, I then got back in the car and tried to start again. This time no power to crank over. Then it donned on me I had a battery problem. I opened the trunk and a plume of smoke billowed out along with that swell smell of battery acid.... Yes. how to make ones day in a matter of seconds....
Pieces of battery casing strewn about on three sides where the casing had ruptured and obivious acid spray. Unhooked the leads and removed the hulk as best possible without spillling more fluid. The photos don't well depict the carnage, but the trunk carpet took a good chunck of things. The remainder was spray on back of seats and along the side panel and across the rear bulkhead. The clean-up took an hour and consumed a number of litres of water along with three full packages of baking soda. I finally got to the point where I had no bubbling and a spare wheel well full of water.
Battery was 5 years old and had never had any problems. I keep my cars on a 0.5A battery tender over the months when they aren't out and about and strive to start them up for a half hour run every could of months. This same battery has started the car a number of times this spring after I did all the engine and drive train work without problem.
I entirely understand why and how a battery explodes, and guess the level of acid was low on one of the cells permitting the plates to warp and touch to create a spark that ignited the hydrogen gas. What confuses me is that these batteries are vented and hook up to a vent tube to ensure they don't build up that gas to support an exposion.