Akom's Tech Ruminations

I've had this toothbrush for over a decade, and the batteries finally died. I suppose that's a long time for NiCd cells to last. For the last year or so the toothbrush would not complete the 2 minute cycle but would instead stop sometime earlier. The batteries are clearly still alive but simply don't have enough capacity.

The toothbrush is not designed to be opened (it is waterproof after all). That should not stop us. The safest way of opening the casing that I could think of was cutting it with a PVC rope saw. This makes a relatively quick cut and mostly does not affect anything besides soft plastic. The circuit board was spared. The location of the cuts is important. I had no idea where the batteries would be so I took a guess and cut it in half. Pulling the two parts apart caused fine wires to get ripped out of the charge coil, something that I could have avoided had I just cut around the batteries. The ideal cut would be just tracing around the batteries on the back side, creating a "battery door" shape. The batteries are soldered directly to the circuit board, so they need to be carefully cut away so that extension wires can be soldered on to allow for some wiggle room.

Opening the casing the wrong way

As I mentioned, I ripped the coil wires apart so I had to repair that first.

Wires ripped off the board. Don't do this!

Hacky repairs to coil wires

Since I wasn't sure that any of this would work at all, I first soldered in some alkaline cells for a test, but this photo shows the layout.

Testing with alkalines

Finally all that was left to do was glue it back together. I used some marine epoxy and packaging tape to shape it into something half-way decent:

End Result