Hotronic FootWarmer S4 Power Set Boot Heaters 2018 This top of the line boot heater can be used with custom insoles. With four (4) cells in each Battery Pack, the Power Plus s4 is Hotronics most powerful, longest lasting FootWarmer! HOTRONIC's RECHARGER POWER PLUS e/m/s SERIES Advanced global charging technology with overcharge protection and input range of 100V-240V. Includes North American and European plug adapters Recharger LEDs indicate recharging mode and battery charge status. Compatible with both e Series and m Series Battery Packs. CONTENTS IN BOX 2 Battery Packs s4 with Integrated Heat Regulator.1 Recharger (global input 100-240VAC, 50/60Hz) with North American and European Plug Adapters.