batteries WILL definitely explode, under the right conditions. i know of
two folks who barely escaped loosing their eyesight when this happened.
luckily, the worst damage was acid burns on their clothing and on the car
paint.

as i understand it, the most hydrogen gas is generated after you really
crank on it a while ... so BEFORE you attach the jumper cables, wait a
while and let the gas bleed off.  other folks i know always put a rag over
the top of the battery ... i've never quite understood the reasoning behind
this, but they always do it 'to prevent explosions'.  (all the hydrogen gas
that i ever knew would go right through a rag).

it is a tribute to the makers of the batteries in general that MORE of them
don't explode ... considering how batteries are treated by the 'mechanics'
and grease-monkies at auto-service businesses. and owner-drivers. :)

joel