18650 batteries are used in many lights. Note! Despite its appearance, this is not the size of an AA battery but a larger batteryand it has 3.6 volts, so it can never be used as a replacement for a traditional AA battery. Fenix batteries are a safe choice. A battery this powerful could be a fire hazard, but Fenix batteries have overcharge protection, over-discharge protection, and short-circuit protection. This battery can be charged either with a separately available charger, or by plugging a USB-C cable directly to its own port. NOTE! The charging port has increased the battery's measurements enough that this battery is NOT compatible with the following light models: PD32 V2.0, PD35 V3.0, TK35UE V2.0, HM60R, HM61R V2.0, BC30 V2.0. - Lithium-ion battery - 14.4 watt-hour capacity: 3.6 V and 4000 mAh - Operating temperature: -20+60 °C - Output up to 7 A - Retains up to 70% of capacity even after 500 charging cycles - Multiple battery protections (overcharge, over-discharge, s