The Xtar Over 4 Slim battery charger is an intelligent, fast charger with impressive temperature control. To safely charge the battery and maximize the life of the battery the charger has been enhanced with two internal protections. 1. Internal Resistance Protection and a Temperature protection. The Internal Resistance Protection first detects the resistance of the battery. If this becomes too high to be charged with 4.1A the charger will automatically switch to 2A. You can still manually switch to 4.1A but that could damage the life of the battery. The Temperature Protection: the charger guards the temperature while charging. If the temperature is too high the charger will automatically lower the charging current to protect the battery. The LCD display accurately gives the charging status per battery. The percentage charged and the temperature are also displayed.