Graeme wrote
Chris Green wrote

They were mostly non-rechargable button cells but I did have one that
had a rechargeable battery. Not usually too difficult to replace, just
take a look inside at the motherboard and it should be pretty obvious.

OK, I'm missing the obvious. The case is off, and I am looking for a
silver button battery, probably about the six of a sixpence. Could it be
entirely different?

See if you can see that Dallas chip. Characteristically it's much
thicker than a normal chip and has very square top corners.
https://www.dropbox.com/s/eblzvzymoi...ttery.jpg?dl=0

You did sometimes see some fit a normally battery carrier
with a couple of AA batterys in it to bypass those too.