I have been using lead acid batteries for Azul and Li-ion 3.7 volt cells for the legs when not connected to a power supply. So went to an auto recyling yard and bought 3 battery packs over 2 days. Toyota Prius, Ford C-max, and Ford Fusion Hybrid. You can find these batteries sold by auto recyclers for between $100 to $500. For people looking for huge capacity in less weight, I think the cost per watt-hour of energy of a used hybrid car battery is very good. Li-ion battery packs offer a number of advantages over the nickel-metal-hydride (NiMH) batteries that power today's hybrid vehicles. In general, they are 25 to 30 percent smaller and 50 percent lighter, which makes them easier to package.