External Teperature Sensor
Temperature sensors are used both for some charge controllers and inverters with built-in AC chargers. Temperature sensors are needed when your battery bank will experience significant temperature changes. Charge controllers stop or start charging batteries bases upon their voltage. However, batteries have different "full" voltages at different temperatures.
What can happen if you don't have a temperature sensor? If the battery temperature is low then the full voltage point will be lower. However, the charge controller will unwittingly continue to try to raise the voltage level - causing your batteries to over charge, off-gas and eventually damage the batteries. If the battery temperature is relatively high, then the "full" voltage goes up. Without a temperature sensor the charge controller may think the battery bank is fully charged before it really is - leaving your batteries frequently undercharged.