The BP4872B li-ion 7.4v external battery pack is used to double the runtime of the Night Eyes HL50 and HL60F headlamps (not compatible with the 3.7v HL50-Q dual color headlamp or any other 3.7v Night Eyes headlamps). Included with the battery pack is a 31-inch extension cable so it can be conveniently attached to a belt or placed in a pocket. The battery pack is charged using the 7.4v charge adapter that comes with the HL50 Headlamp Kit.