Reduced size and weight with increased capacity and longevity; Battery Pack XLP 2S BT - High-performance, low-weight Li-ion batterie in a compact design; 4400 mAh capacity offers a category-best 18 hours of constant heating power; Bluetooth app controls the Battery Pack from any smart phone; Merino wool helps wick moisture away from feet and promote warmth; Surround Heating Element envelopes the toes; Safe, stable, and reliable; The "Welded" wires allows for a seamless sock for a better fit with more comfort; Material: 54% polyamide, 25% elastane, 21% wool (merino)