The Heat Sock Set XLP 2P BT Surround Comfort is the pinnacle in warmth, performance, and longevity.Combining a pair of HOTRONIC XLP 2P BT Battery Packs with the Soft Compression and Surround HeatingElement of the Surround Comfort sock, and then allowing control via a Bluetooth App, HOTRONIC raisesthe bar with convenient Constant Heating Power when needed most - in the cold! FEATURES: Reduced size and weight with increased capacity andlongevity 4400 mAh capacity offers a category-best 18 hours ofconstant heating power Bluetooth app controls the Battery Pack from anysmart phone Merino wool helps wick moisture away from feet andpromote warmth Surround Heating Element envelopes the toes Safe, stable, and reliable. Just what one would expectfrom HOTRONIC Compression: soft - PFI 50, 10 - 15 mmHg Material: 74 % nylon, 20 % lycra / spandex, 6 % merino