If you own the Fenix LR40R flashlight, a spare battery will definitely come in handy so you always have one ready in case your light runs out of power. The Li-Ion battery designated ARB-L37-12000 boasts a solid capacity of 12,000 mAh, which corresponds roughly to three to four fully charged smartphone batteries.
The battery life is at least 500 charging cycles. At the same time, it does not suffer from memory effect, so you can recharge it even after shorter use. It features protection against overcharging, complete discharge, and short-circuit.