The Therm-ic C-Pack 1700B (1 Piece) has the highest battery capacity and is able to keep your feet extra warm for up to 17 hours.  Fitted with Bluetooth technology, the C-Pack 1700B allows you to remotely control your heat settings via your smartphone using the Thermic Heat Control App. The app provides increased adjustability, personalization, and battery life through accessing 10 different heat levels, motion control, and eco mode. C-Pack batteries are suitable for use with Therm-ic Insoles Heat Flat, Heat 3D and Heat Kit range only.