Once I had a battery exploded in it's compartment as I wanted to start the car.  Must have had one of the cables sparking.  It made a mess and blew off that breathing cover behind the wheel on the outside.  It took a lot of cleaning and bi-carbonate soda to neutralise the acid.
The explosion was like when your car is booby trapped and as you start the car it explodes.

This was my story of battery explosion in my Westfalia camper
Zoltan