My father’s electric shaver wasn’t holding charge; he asked if I could have a look at it and see if the internal battery could be replaced. Here’s the exploded view:

The green battery is a NiMH, not NiCad as I’d expected. I suppose it’s not that old. Which is as well, as I couldn’t find any NiCads in the drawer when hunting around before, and was planning on gutting it and fitting a LiPo battery and tiny charging module.

I wasn’t sure what the battery would turn out to be, but it’s actually just a single 1.~5V cell, with a little ‘battery manager’ through-hole IC which implements a current-regulated SMPSU directly from the mains feed in the base. The waveform was not very pretty:

But then it might well be sampling the cell voltage in the troughs (sampling in the peaks is a good deal less reliable, particularly if charging at anything approaching C).

The question was: would a standard AA fit? That cell looks AA sized:

Hmm. Probably. And whilst we’re at it, would it work?

Yes, it charged and held charge this time. Still, that cell’s quite old and I don’t know how well it will last.

Then to cut down a battery pack, as I had no single AA holders and it needs to be small:

and mount it to the board with little bits of wire, and cut away most of the battery-supporting superstructure in the case to allow enough space:

And then try to get it back together. It was a bit of a squeeze, and I think I might need to bend the front switch contacts back a bit to decrease resistance when it next needs opening, but it works, and holds charge for a good while longer than before. (Update: a few months down the line it doesn’t really hold charge any more. Oh well. There are plenty more NiMHs in the cupboard: one is bound to be good. And at least it’s a standard battery now.)