The biggest and baddest Fenix battery just got upgraded! Please welcome the Fenix Fenix ARB-L18-3500U USB rechargeable lithium-ion battery! The ARB-L18-3500U battery can be charged by its built-in USB port or by an external battery charger as well. A protective circuit is built into the anode and covered with a riveted steel hat. The battery is fully covered with the steel shell; thus, the potential risk of short circuiting caused by collision or impact is hugely eliminated!