Can I use any 18650 battery in my Fenix flashlight?
NOT all 18650 battery brands are the same size (or quality). Fenix lights require 18650 batteries that are 3.6/3.7 volts. That's very important. If you are shopping other brands, then you need to get one that is the same size (length and diameter) as our Fenix 18650 batteries.
We recommend specifically to avoid using Nuon brand and UltraFire brand. Nuon batteries frequently do not work in Fenix lights and we have had lots of reports from customers of UltraFire batteries being unsafe. If you're interested in learning more, check out our blog about 18650 batteries.