Hi everyone -
the 3W works anywhere in the 3v - 9v range.
I run it on a 3.7v rechargable 17650 Li-Ion battery in a G2 and a Brinkmann Maxfire. Add a clickie to the G2 and it's a great all-around work light.
The 5W as I understand it is ONLY for 6v use. I haven't read anything to the contrary.