Hotronic Extension Cords is designed to be stored near your chest or (120cm), allowing you to run the cords under your gear to your insoles. For use with Battery Packs S4, S3, e4,e3,m4,m3, and earlier. Keeping the battery in a warm space during use such as near your core will ensure lasting foot comfort all day. We also carry the entire line of Hotronic Products! For use with Battery Packs S4, S3, e4,e3,m4,m3, and earlier.