I've previously reported here that I suffered an explosion
as well.  Mine was under the passenger seat and seemed only
split a little--no pieces broken off.  Internal short +
hydrogen gas, I assume.  Mine was labeled Bosch.  Since my
vanagon is early and has no teddy bear hide anywhere, just
vinyl seats and rubber floor, the noise was near deafening.
Happened in parking lot of my VW parts place--they loaned me
a wrench and sold me a new battery, no problem:  only lost
about 20 min. of errand running schedule that day.  I'd like
to install an Optima, but don't think it will fit.