OK.
Um... I'd recommend checking everything out real carefully anyway. If the battery was sitting in a felt case for the last day charged...and you just put it in the PV...
Something set it off. Just to be sure...make sure the button isn't shorted ....double check. Either the button was pushed, there's an internal short, or ???? just the motion of installing it set off the battery ????. IDK. If the button was never pushed, the battery case is intact, there's no internal short...why did it get hot? Could be internal failure/overcharge, but it sat for a day just fine.... so double check the button and connector to be safe.