Thermalution Far-infrared Power Heated Lumbar Belt (also known as the Red Belt) is made of neoprene and state of the art non-metallic heating fiber wire that is safe, comfortable, durable, and flexible. It emit safe Far Infrared Ray that provides soothing warmth to penetrate your skin and gently elevate your body heat, improve blood circulation, promote tissue and cell regeneration to promote healing. It comes with two compact Thermalution Smart Li-ion Rechargeable Batteries that last between 7~12 hours and heats up to 104 degrees Fahrenheit in 5 minutes to keep your lumbar warm and support your back to help relief lower back pain. Each battery measures 2.76″ L x 1.85″ W x 0.87″ H and weights 4.13 oz., 7.4V/2.6Ah, 19.24Wh. Power charger charges two batteries at the same time and takes up to 4 hours to charge. Power control provides 4 heating levels to choose from: low, medium, high, and boost. Batteries will automatically shut down after 180 minutes of continuous use for safety. This product, including batteries, are not waterproof and cannot resist water pressure. Please do NOT use it for any water or underwater sports.