Alkaline -- 1.5 volts, but the voltage can 'sag' when used with some bright LEDs.
NiMH -- 1.2 volts, but the voltage doesn't 'sag' as much or as fast with bright LEDs.
I used to think that a LED light that works with four 1.5 volt alkalines should also work the same with five 1.2 volt NiMH. But I've got one multi-LED dropin in a 4xD M@g, specified for alkalines, 6v.
I asked, and the fellow who made it told me not to swap the LED to a 5D M@g with NiMH.
I appreciate it as a warning -- eventually, in a 5D configuration, someone will put five alkalines in, deliver 7.5v, and blow it up.
But is there any difference, would five NiMH cells delivering 6v stay closer to six volts significantly longer, for example?
I don't know what the electronics are, it looks like it's basically direct drive. Might get an OK to post more details but it's not my product so I won't unless I get an OK; could be it's still being worked on, I did not buy it retail.