The philips sonicare toothbrush has a NiCd battery inside. Of course after a couple of years this will develop memory effect and die.
The battery can be replaced easily, but the case is hard to cut through. I used this as a basis for my repair:
I used a dremel with a grinding wheel, but did not have measurements or directions on where to grind, so I chose the seam on the side, and ground until I could see a bulkhead.
I replaced the old NiCd with a 2 AA Nimh rechargeable battery pack from ebay (meant for a portable phone). Nimh has almost the same charge curve as NiCd.
I also ordered white shrink tube (40mm) from ebay. and used it along with some silicone to seal the device when finished.
The shrinktube gets in the way of the charger, so I cut a notch in the bottom of it as you can see in the photo.