The toothbrush in question was the top of the line one - I believe later versions are a bit fancier now, but it was pretty expensive, and after a few years use, the battery life had dropped from 4-5 days or so down to about 1-2 days between charges.
The battery is a NiMH tabbed cell, soldered in. The choice appeared to be between approx. 55 quid for a refurbished brush, or £8.30 inc. P&P for a new cell. I decided to take the latter option... despite Braun's warnings that doing it yourself will knacker the water seals. Frankly, for less than a tenner, you're not going to lose much even if the brush does suffer from water ingress at a later point.
Getting the thing open wasn't too hard - there are tutorials on Youtube, but the internal plastic retaining clips were a little more fiddly than anticipated... thankfully didn't snap the induction coil wire when it gave way!
Under the LCD screen, there was lurking an MSP430, which was slightly surprising, as I'd have thought they'd be using a more cut-down controller for something like this. Certainly in the lower cost brushes, I seem to recall seeing an unknown uC.
In any case, even an MSP430 cannot run off 1.2V so there must be a boost converter on there... probably that little SOT package just below, as there's a chunky-ish inductor just above it. I presume that the motor runs directly off the battery via. a MOSFET under control of the MSP430, as the boost converter is unlikely to be that efficient at this low a voltage.
The charging circuit looks pretty simple. I haven't traced it out, but noticed that it's a pretty slow charge, so probably nothing exotic.
Here's the old battery in place, with the retaining spring removed between the motor and the battery. Note that there's actually quite a lot of spare length in the chassis - the original battery is 42mm long, but you could probably fit another 10mm or so in there. Such a shame that it wasn't ever so slightly longer to take a regular AA.
They've also decided that a 0.1uF cap across the motor terminals was necessary... wonder if this is for RFI or whether it was adversely affecting the microcontroller in some way.
New battery in place. The cell is rated 2000mAh, and is allegedly a Sanyo, so of decent quality. The terminal strips were a little wider than the original but are easily cut down with scissors as the metal is so thin.
And then, put it all back together again... still charging, but seems to be fine so far... :)
