FWIW, I have used lithium AA's with no issues in both the MB-20 and MB-21 configurations; I have successfully used both 2000mAh and 2600mAh NiMH rechargeable AA's in the MB-21, but when I tried the MB-20, the camera did not function. This wasn't surprising, since most NiMH cells have a nominal voltage of 1.2v, compared with 1.5v for alkaline and 1.7v for lithium... or maybe I just didn't have a full charge on the NiMH's when I tested them? I normally shoot with the MB21 so I didn't worry too much about it YMMV