Lots of new batteries (for mobile devices, MP3 players, etc) have connectors with 3 pins. I would like to know what is the purpose of this and how should I use these three pins?
They are usually marked as (+) plus, (-) minus, and T.
The third pin is usually for an internal temperature sensor, to ensure safety during charging. Cheap knock-off batteries sometimes have a dummy sensor that returns a "temp OK" value regardless of actual temperature.
Some higher-end batteries have internal intelligence for charge control and status monitoring, in which case the third pin is for communications.
In mobile phones, some Li+ battery packs have 3 terminals. Two possibilities:
If you want to explore what’s inside single-cell Li+ battery packs, look-up bq27000 gas gauge IC and associated application notes. Could be a good starting point.
Some packs have 4 terminals: positive, negative, SDA, SCL. The latter 2 lines are I2C or SMBus. Look up the bq27200 gas gauge IC (shares datasheet with bq27000).
EDIT: This was written as an answer to a duplicate question, which got merged with this one.
For Nokia batteries, one of the pins may be a BSI (Battery Size Indicator) pin, which contains a fixed resistor to ground, enabling the handset to identify which battery is connected. Examples of BSI resistor values include:
- BMC-2 3k3 NiMH 640mAh
- BMC-3 5k6 NiMH 900mAh
- BLD-3 22k Li-Ion 780mAh
- BL-4B 68k Li-Ion 700mAh
- BL-5B 75k Li-Ion 820mAh
- BL-4U 82k Li-Ion 1000mAh
- BL-5C 82k Li-Ion 1050mAh
- BL-4J 100k Li-Ion 1200mAh
- BL-5J 110k Li-Ion 1450mAh
See also: BSI - cpkb.org