A genuine Ledlenser 21700 lithium-ion rechargeable battery with a capacity of 4,800 mAh (Ledlenser article 502262). The 21700 is a larger, higher-capacity cell than the 18650 and powers several current Ledlenser lights, including the H7R headlamps, which ship with this cell. A spare lets you swap in a charged battery and keep working or exploring. The cell measures about 22 x 71 mm, weighs 71 g and has a steel housing with a PVC wrap. It is designed for lights that take a single 21700 Li-Ion cell; check your torch or headlamp manual to confirm the battery type before buying, and charge it in the light or a suitable Li-Ion charger.