Here is how to make a working Green Lantern power battery
(Hal Jordan's version) for around US$20. My son (5) has decided that the Green Lantern (especially Hal Jordan) is the coolest superhero ever, so for Christmas 2009 he will be getting Hal's Green Lantern power battery (the flower pot edition). The Green Lantern (Hal) comics seem to have a variety of different looking power batteries depending on year and artist, so this is an attempt for the middle ground.
If you would like a more modern power battery, see related instructable - How to Make a Green Lantern Power Battery (2011)