Comment by phyzome

Yeah, I really like that Fenix lights just take a Li-ion 21700 or 18650 (and they have a little caddy to adapt the latter for the former).

However, the manual is very specific about only using their specific batteries and not just arbitrary batteries of matching size and chemistry (generic 18650s). I'm currently trying to get some information out of the manufacturer about whether there is risk of damage or they're just trying to avoid people using shitty batteries and blaming them for a bad experience.

(The manufacturer is fenixlight.com, but the seller is fenixlighting.com. It's kind of confusing.)