The Hotronic Power Plus S4 Battery Pack is stronger than ever before. This is a battery for with ski boot heaters from Hotronic. This super powerful battery contains 4 NiMH cells that work even in the coldest weather. Fully rechargeable, the S4 battery has 4 temperatures settings ranging between 84 and 173 degrees Fahrenheit. At its most efficient, the S4 lasts up to 21 hours. Use the first 3 temperatures settings for keeping feet warm, and reserve the fourth heat blast setting for when your cold toes need an extra surge of warmth. Upgrade your current Hotronic battery to the S4 for longer lasting, more intense foot warming capabilities.