We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Jim S saying
something like:

The next question seems to be which type of battery do I replace with?
The current (no pun intended) ones are nickel metal hydride.

I had to do exactly that with my cordless batteries - simply bunged in
some AAAs of 900mAH instead of the original weedy 600mAH ones which had
lasted several years. The cradle charger in the phone base seems to cope
perfectly well with the extra capacity - at least, there have been no
problems with the replacements at all and they've been in two years or
so.

NiMH will be fine, if that's what was in them.