The battery pack for Hotronic’s insole heaters the Hotronic S4+ Battery offers enhanced reliability and longevity. The NiMH batteries have an increased capacity for a combined 30 % in higher average temperatures and longer durations on a single full charge, for better performance when you need it most: in the cold! And no matter where you go HOTRONIC’s advanced global recharging technology senses when the batteries are fully recharged and protects them from overcharging damage by switching to trickle current recharging. Smart, right? Compatible with all S, E, and M Series Battery Packs, the Recharger’s LEDs indicate recharging mode and battery charge status. With three base level settings for comfort and warmth, and a fourth for an extra blast of heat when needed, selecting your heat level is through a simple press, hold, and release button on the battery pack. An easy to read LED light indicates what setting you’ve selected, and whether the power is on or off. Pick these up as a replacement for an aged battery, or, if you consistently run cold and prefer high heat settings, as a spare in your pocket. Don’t worry, these maintain up to 55 % of a full charge and 95 % of their initial capacity after four years of non-use.. so your spares will be ready to fill in for your original batteries at mid-afternoon no problem!