Can I use another brand's battery at the same time as a Fenix battery?
We do not recommend that you mix battery brands, even if both batteries are listed at the same milliamp level. Even a Fenix 2600 mAh 18650 and a Tenergy 2600 mAh 18650 could have a small difference in the actual usable capacity of the battery, which would cause undue strain on the one that has the slightly higher capacity. It is always the best idea to use the same brand and same capacity battery when powering multi-battery flashlights and other electronic devices.
Learn more about 18650 li-ion batteries.