Easily installs in all types of footwear. Transfers simply from one pair of footwear to another. Powered by high capacity, cold temperature operation, NiMH cells. Four (4) temperature-duration settings. Settings adjusted by easy to use, push-button controls. Easy to read Battery Pack LEDs indicate setting selected and power on / off. Battery Packs easily attach to secure area of footwear. Battery Packs m4 contain four (4) cells for extended duration.