An external temperature sensing resistor (NTC) measures the battery temperature and controls the maximum charging voltage level. When the battery temperature rises, the temperature compensation circuit drops the charging voltage level, and vice versa. The compensation coefficient for 12 V systems is −21.9 mV/°C.
When the battery maximum operating temperature limit is exceeded, BMU801 stops charging completely. For the operating temperature range of batteries, see the manufacturers' documentation.
The NTC resistor is connected between the battery and BMU801. If there are 2 batteries in your system, Vaisala recommends to have an NTC resistor for each battery.
BMU801 includes an internal temperature sensing resistor, too. It measures the BMU801 temperature which can be used as an estimate of the battery temperature when accurate battery temperature is missing.
If external NTC resistors are installed, BMU801 primarily uses them. If no external NTC resistors are installed, or they are damaged or disconnected, BMU801 automatically switches to use the internal temperature sensing resistor.