Damn, how i hate those times, when you have written a detailed post and after hitting backspace, it initiates BACK action in browser instead of text editing.
So the short version:
Originally posted by vk4akp ...
4x 1.2V x 2.1Ah = 10.08Wh (What I currently use in the K200D)
4x 1.2V x 2.8Ah = 13.44Wh (2800mAh's easy meet your D-Li 90 @ 13.4Wh!)
6x 1.2V x 2.8Ah = 20.16Wh (You've just squashed your D-Li 90 @ 13.4Wh!)
6x 1.2V x 2Ah = 14.4Wh
...
It is not so straightforward.
AA NiMH's cant deliver their full charge at high current.
This is because of internal resistance, which increases noticeably long before the battery is discharged.
It can supply enough current for flashlight, remote or simple device.
DSLR "grabs" bursts of high current for processing, AF, SD writes, FLASH etc.
It is more pronounced for high capacity ones (>~2.5Ah).
LiIon's and LiPo's however, as well as professional NiMH's (like those you can find in drill battery packs, and cost like 15$ per 1.2V/3Ah cell) can deliver much larger currents, can be charged with much faster rate, and is capable to do so until almost fully discharged.
This is what is important for DSLR, not high capacity or voltage, but high usable capacity which can be fully delivered to the target at any moment.
Sometimes it seems so silly, that someone says k20d has faster AF because of higher voltage.
A real compromise would be design that allowed for both, proprietary RECHARGEABLE Li Ion and AA's.