Well, I've just been putting Google through its paces once more and I've
found a few "Pull the battery to bits and do this" style tricks for the
Dallas Semiconductor RTCs. Have fun :-)
Personally, I'd use a craft knife and cut around the casing from the bottom
side, between the pins and the plastic cover. "Real Time", eh? More like
"Time Bomb". I *hate* those pesky Dalsemi RTCs...
Hmm... I might try and replace the battery in the 15xx I've got in my
junkbox at some point today...