Charging and batteries for HP 34C

[Hsilop]

Hi!

I am looking for an old HP 34C or similar calculator, and was wondering about how to go about charging it, what (replacement) battery packs to use, etc.

I am guessing that rechargeable batteries in those days were quite different from those today, and would require careful maintenance in order to perform their best.

Should I charge them as seldom as possible, letting them run down completely and then charge them as fully as possible - or should I charge them as often as possible and try to maintain full charge as much of the time as I can?

Could I damage the batteries or the calculator while charging, e.g. by overcharging?

Most grateful for all help - especially from people who remember how things worked in the good old days ... :-)