I am going to try one of these power bank unit for the ALTAIR EZ-1 they look like a perfect power source for this class of robot.
Can supply 200 - 400 amps on a jump start.
Weighs only 0.9 pounds (408 grams)
Size 6.5 x 2.9 x 1.2 inches
6 hours charge time.
Supplied with charger
Cost £60 (UK) and $79 (US)