what about old video camera batteries. like the huge vhs cameras of the 80's and early 90's. i bet they had to eat tons of power. I haven't reasearched this idea, so I don't really know if it is feasible, but its just an idea. those batteries are heavy and large, but maybe more weight efficent than D's. search for the power usage calculators, they could give you rough estimates on most devices. good luck.