Steve wrote:
We have a solar powered alarm clock which automatically synchronises with
the terrestrial time signal. The clock is marked 'PRECISION', 'SOLAR
POWER' and 'RADIO CONTROLLED'.

The display became faint and eventually it stopped working. I took a look
inside. There was an unmarked AA size battery (leaking blue liquid which I
cleaned up). I measured the battery's voltage which was 1.6V, well above
that of a normal rechargeable (1.2V?).

How heavy is this?
NiCd and NiMH batteries are considerably heavier than lithium of any sort.
I would - as others have suggested suspect a 3V lithium non-rechargable
battery - simply as rechargable lithium are more expensive.