I had a sonicare elite toothbrush which stopped working. It would only
run few a few seconds and then run out of power. I figured out how to
replace the battery. You remove the gray rubber bumper on the top of
the unit, pry out the clip (be sure not to lose it). Then remove the
brush from the twist on part and put it back on the brush. Twist hard
to the right, and it will loosen. Then you should be able to pull
straight out to remove the insides. You will see a battery at the
bottom. Desolder the battery from the PC board. I used desoldering
wick to remove the solder from the bottom of the unit, then pried the
bottom out with coathanger wire, then unsoldered the top. I then
removed the tabs from the battery by ripping them off with needle nose
pliers and restraighted them. I then resoldered these back into the PC
board. You can then take a rechargable AAA type battery and slide it
in as the replacement. You may want to wrap the AAA so that it is
about the same size as the old battery. Put the unit back together and
it is as good as new. There is a rubber seal, so I don't think you
have to worry much about water getting inside. They say the
disassemply process is not reversable, but from what I can see, it is
very reversable and it is terrible that they just don't sell a
replacement battery.

fhusonicare