3.7V 400mAh LiPoly battery with an excellent 2C discharge rate for small projects.
Small robots need small power sources, and this little 3.7V, 400mAh LiPoly unit from Sparkfun is a nice package able to push up to a 800mA load. It is pre-wired with the standard 2-pin 2mm JST-PH connector, and has built-in protection against over voltage, over current, and minimum voltage.
Still, there is no substitute for good battery management, so try to treat all LiPoly cells gently, and best to recharge them slowly rather than at their maximum rate with a proper LiPoly charger.
Dimensions: 5 x 25 x 35mm