now obtain a 1/3AAA GP12AAAM cell (120mah) Nimh cell. you can get these for only $2.00 ea from:
http://www.batterystation.com/nicads.htm
prepare it by soldering some very small gauge red and black wires to it's + and - terminals, covering the ends with a small circular piece of masking tape and shrink wrapping it to provide insulation. then use a tiny amount of hot glue and attach it to the rear of the PCB.