I have been using rechargeable NiMH AAs for pretty much everything for a long time. Since my camera and strobes take them, it made sence to use them for my lights as well. I got tired of coming home from a trip with multiple sets of partly used Cs. I don't have a use for them at home and don't like the idea of taking partly used batteries on a trip. The discharge curve on NiMH batterises is different than standard batteries, which is better is a matter of debate. Standard batteries start at a higher voltage (1.5) which gives off a brighter light but that only last for a few minutes as the max voltage drops off farily quickly then drops off on a almost linear curve until it's dead- the light gets dimmer and dimmer. NiMHs on the other hand start off slightly dimmer since their full voltage is 1.25 as opposed to 1.5 however, their discharge curve is almost flat until they get near discharged where they abruptly drop low. The light starts a little lower but stays constant until the end where it quickly fades. Cavers prefer the long predictable fade of standard batteries- the light just keeps getting dimmer. I prefer the longer higher light of the rechargables, the fact that their somewhat unpredictable and quick drop in light does not bother me since I never do overheads and carry a backup at night.
The other good thing about rechargables is ....well they are rechargable. Unlike regular batteries, the capacity of AAs, Cs and D NiMHs is pretty much the same so you can just buy AAs, one type charger and C or D adapters to use the AAs in. You will get longer usable light out of high capaciry NiMH than you will from standard AA but not as long as good non-rechargable Cs or Ds. While the time is shorter, it's still more than 2 dives and since recharging them between dives is possible I don't see that as a limitation. It does require me to carry more batteries, but since the AAs are so much smaller than Cs or Ds the total room and weight is less. I carry 3 full sets for the camera and strobes and 2 for the lights plus enough charger capacity to charge 1 full set (12 batteries in my case) at a time. When buying a charger get at least one that is capable of discharge testing the batteries. Sooner or later they start to drop in capacity and you need a way to determine the health of the batteries. I do this before each trip. I test all my batteries, match the capacity of the good batteries and discard the ones that are getting weak.
