I have an old cygolight. After the original 6v lead acid battery died, I used to run it on 6 x AA for a nominal voltage of 7.2v which gave an appreciable improvement in brightness.
Flogged the AA's to death though as the current draw was a bit high. I looked at going to C or D size NiMH, but the cost was that high I started looking at alternatives, which led me to here which led me to LED.
I still have the cygolight as an emergency back-up - should the three LED's I now have all fail. I simply wired in a magishine plug & can run it off the 7.4v battery packs. Running the globe at higher voltages gives you more heat, more light & reduces globe life, but not so much that I would worry about it.
It's a pretty small investment to buy a lion pack & charger & gives him the option to upgrade to an LED if he wants to later on