Mobile phone battery requires only two pins for charging, then why there is a third pin? The question is definitely a curiosity generating one. The third pin called BSI pin performs very important functions and act as an interface between the Mobile phone and the Battery. When the battery is removed, without switching off the Mobile phone, the BSI circuit is first interrupted which convey the information to the processor to shut down all communications to prevent damage. To communicate with the BSI, the battery pack also has a BSI pin. BSI stands for Battery Status Indicator or Battery System Indicator.