The Ferei BP4836 is a high quality, high capacity li-ion battery pack compatible with Ferei headlamps and select Ferei bicycle lamps. The BP4836 provides quadruple the runtime of Ferei's standard headlamp battery (18650) but is still relatively lightweight at 240 grams. In addition to being compact and lightweight, the BP4836 is also weatherproof and ultra-durable. When using the BP4836 with the Ferei HL08 or HL20 headlamp, an EC80 extension cable is highly recommended.