From the Bog Dog team:
Fenix ARB-L18-3400U v3 is a Premium Quality High Capacity 18650, 3400mAh, USB-C rechargeable Lithium - ion battery specifically designed for high intensity flashlight use and is the perfect companion for all current Fenix 18650 powered lights.
ARB-L18-3400U v3 is a highly versatile battery that can be charged in a regular Li-ion charger, or for those customers who already have a charger with USB-C lead such as a mobile phone charger, the USB-C can be plugged directly into the battery, saving the expense of an additional charger.