Up to 4 hours continuous use! The next generation of hand/body warmer, with built in rechargeable lithium-ion battery. Super-slim (¼”/6mm thick), super-lightweight design (1.6 oz./45g weight). Automatic thermostat controller maintains comfortable temperatures of approximately 104 F/40 C. On/Off switch for warming as required. Includes USB cord for charging through any USB port (2 hour charging time) - Up to 4 hours of continuous operation, LED lamps indicate status of warming, low battery power, and recharging, Includes knitted storage sock.