Can I charge them in an old NiCad charger? More than once? Thanks.

Should be fine. You do have to be more careful about overcharging with NiMH as they damage easier, so don’t leave them on too long if it’s a slow charger – though plenty of chargers sold as suitable for NiMH are slow chargers which are much the same as NiCad ones. With smart fast chargers in theory the cutoff should be a bit earlier for NiMH, but it won’t make a significant difference.

Strangely, I was reading up on this last week. It seems that it depends on a few things …

Hmm, mostly right, but not convinced by the bit about fast smart chargers. I have a homemade fast charger which uses a microncontroller I’ve programmed – I’m using a cutoff algorithm in that which is supposedly suitable for NiCad (-ve delta V), but it works fine with NiMH. Supposedly with NiCad the voltage drops once they are fully charged, whilst with NiMH the voltage stays constant, so you detect the fully charged state differently. In my experience (I’ve done quite a bit of testing of this) the voltage of NiMH also decreases when fully charged, and this is a more reliable way to detect end of charge than detecting constant voltage, hence why I’m still using that. You get a slight overcharge, which will slightly reduce the life compared to an earlier cutoff, but it’s not a significant difference.