Hello.
Im using a battery from a Nokia cellphone (battery #BL-6C) in a project. The battery have 3 pins, the positive (+) and negative (-) are marked, but what could be the 3rd pin (actually its in the center).
Btw the battery is Li-ion, 3.7V

 

It might be a connection for an integrated temperature sensor, like a thermistor.

Try measuring voltage to the + and - terminals using a DMM. If there isn't any, try measuring the resistance when it's at room temp; then charge it using the factory charger & phone for a couple hours and try measuring it again. Is it different?

 

The 3rd pin could be either of two:

- Thermistor (as have been already mentioned)
- 1-wire interface for a fuel gauge IC inside the battery.