Hotronic’s Foot Warmer S4 offers more than ever before. With enhanced reliability and longevity, the Foot Warmer S4 batteries maintain up to 55% of a full charge and 95% of their initial capacity after four years of non-use!  Plus, an increase in capacity offers up to a combined 30% in higher average temperatures and longer durations on a single full charge, all when needed most, in the cold!!!