Roger Long wrote:

Has anyone heard or experienced this?

It's not uncommon for batteries to explode during charging if they've been deeply
discharged. It's a good idea to remove the caps on batteries that have them when
charging one.

Your mechanic is correct. What probably happened is that the battery was deeply
discharged starting the plane on an occasion on which it was hard to start. The
gasses were produced as the alternator charged the battery and something set it
off.

George Patterson
Great discoveries are not announced with "Eureka!". What's usually said is
"Hummmmm... That's interesting...."