The Hotronic Footwarmer S4 Custom Ski Boot Heater offers extra warmth in the cold. It installs in all types of footwear and can be transferred from your ski boots to your après footwear. Use while skiing, snowboarding, bicycling, horseback riding, and during other general cold-temperature activities. Offers up to a combined 30% increase in higher average temperatures and longer durations on a single full charge. Maintains up to 55% of a full charge and 95% of the initial capacity after four years of non-use! Provides power when needed most, in the cold!