I'm just away to replace the battery inside my tedious old alesis sr16 with a mind to flogging it on ebay. I opened it up and the battery is like a watch type disc one except its soldered to the main board by a few pins. Gonna remove it then get another 3v similar battery, solder some leads to the battery then solder the leads to the board, cover the battery with some insulation tape and whack it inside the case, sorted.. Then again it may follow the path of various other things i've tried to fix, and after getting more and more enraged at how fiddly it is eventually break something terminally and then throw it round the room before finishing it off by stamping on it repeatedly.