Hello Yukon Jack,
The basic principle works the same for both chemistries.
That unit (as far as I can tell) gives a pulse of AC on some sort of time frame. That pulse of AC fires up your "overnight" constant current charger that is plugged into it. The cells, instead of seeing a constant current, see a pulsed current. The idea is that the "time on" is just enough to overcome the self discharge of the cells, and not enough to "cook" them.
Since the self discharge of NiMh cells is more than NiCd cells, you might still have a bit of self discharge, but this would go a long way to keeping them topped off.
I believe you can do the same thing with a timer. Just have it come on for a short time every day. I just thought this device was a bit more sophisticated.
Tom
