if there AA or AAA then you can just use any rechargeable, the higher the mAh the longer the batteries will last and the more expensive they are, but you can use any mAh, just match the battery size.
Alkaline batteries are 1.5v purely because that's what voltage that chemical reaction give out, same for the 1.2v for NiHM its not about having a lower voltage for power spikes.
if your lenser has a battery pack that's 3 AAA batteries in a line you could always opt for a lithium battery pack replacement, lithium will last a lot longer and unlike NiCAD or NiHM the voltage will stay at a constant till the end of the battery so you will always have a bright light rather than a dull light at 50% capacity.
PM me with pics and I'll see what I can find if you want.