'ThePuck' homemade mod uses 3-4aaa NiMh batteries, variants use 3-4aa batts (I use a 4aa variant)... NiMh batts being the key, alkaline batts can be used in a pinch, but won't last long b/c you're over stressing them...
A new chemistry just coming out (my local Fry's carries the batts, but not...