I use my self some 18650 Li-Ions. These are found in laptop batteries.
Build a solid power control unit with 5V regulators some 20V zener diodes and PCBs from radio shack.
I can post details on my setup if anyone is interested.
All the stuff adds up but you get solid hours out of it. My projects normally runs Dev boards with the Pineapple. As the HackBerry A10, RPI, Ordroid U2. So Ive built powerful cells.