The 18650 USB Rechargeable Lithium-Ion Battery from Streamlight has a capacity of 2600mAh and an output power of 3.7V. The battery charges through the built-in Micro-USB port, but can charge in a conventional charger or with the compatible 18650 Charge Cradle. At the top of the battery, a green light turns on to indicate when the battery is fully charged, which takes about 5 hours. The on-board safety control circuit protects the battery from overcharging or from losing power completely. Also, the battery has a pressure vent and a PTC surge protector.