The Rechargeable Li-Ion Battery Pack for RC40 from Fenix can be used as a replacement for the battery pack that came with your RC40, or as a spare. It features built-in protections against over-charge, over-discharge and over-heating, and charges in the light. The unique design has the positive contact in the center and the negative on an outer ring on both ends. This means that there is no wrong-way to install the pack, so you don't have to worry about reversing the polarity and damaging the battery or light. The specifically designed battery pack is the same one that comes with the RC40 light, and will not work with any other light.