Battery explosion?

Has anyone heard or experienced this?

On changing the battery in our plane the other day, the A&P said, "You had a
battery explosion." On looking closely, I could see that the box was bowed
out and rounded as if someone had tried to inflate it with air. I'd
noticed that it wasn't exactly square and the cover didn't fit very well but
thought it was just poor workmanship.

He said it probably happened during starting from hydrogen build up that the
propwash hadn't had a chance to blow away yet and the pilot thought it was a
backfire. No harm done except we'll need a new box because of corrosion.
It could have happened years ago.

--
Roger Long