It is used to measure the battery temperature.
 Also known as
- Battery Temperature Indicator
 What is BTemp?
The battery block contains NTC (and usually BSI) resistors for temperature measurement. Temperature and capacity information is needed for charge control. These resistors are connected to BSI and BTEMP pins of the battery connector.
The phone stops the charging, if the battery is starting to overheating.
In case of NTC thermistor, if the temperature increasing, the resistance decreasing. The temperature of the battery make the resistance beetween the GND and the BTemp change.
There are batteries which don't have BTemp connector. In this case, the phone has the temperature meter which is in the mobile phone, close to the battery.