The Mega Lithium BackPack is an Open Source Hardware battery shield for the Arduino Mega that snaps to the back of the board, and provides around 15-27 hours of battery power to circuits built with the Arduino Mega (depending on the circuit). It gives a 3.3 volt, 5 volt, ground, and battery capacity testing signal that can be plugged into the Analog input port to test how much battery power is left.

The BackPack sells for $48.

Introducing the Mega BackPack, a battery for the Arduino Mega