The biggest and baddest Fenix battery just got upgraded! Please welcome the Fenix Fenix ARB-L18-3500U USB rechargeable li-ion battery! The ARB-L18-3500U battery can be charged by its own USB port and by the 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!!