I've got one of those super-bright flashlights, 1000 lumen, that uses a single li-ion 18650 rechargeable battery. I was in a pinch and instead used a CR123A lithium primary photo battery (3.2V), made a spacer from a small piece of 1/2" copper pipe with two copper end caps so that the length of the battery + spacer = length of the 18650, wrapped the spacer in plastic electrical tape to insulate it, and the flashlight worked well and almost as bright as the li-ion. Just an idea that others might find helpful.
