I recently did a post on bike lights mainly the Cree XM-L T6 1300Lm as i had purchased one on eBay. When they turned up they didn’t have any Batteries so i though no more of it and headed straight for an AA battery thinking it would fit, but they don’t fit as you need 18650 Batteries.
On the net i couldn’t see any pictures comparing a standard AA battery against a 18650 battery.
As you can see there’s quite a bit of size difference. I bought 2 UltraFire 18650 3.7v and a charger for £12 delivered from eBay