Why do cell phone/ digital camera batteries have more than two terminals?

Question

I converted a Nokia battery into a multipurpose battery (now it can be used in Motorola and Sony Ericsson cell phones as well) and I came to know that third terminal (middle one) was just useless as it was circuited with nothing on tiny motherboard (or whatever you call it) of the battery.

Answer 1

The most common extra terminal is for a temperature sensor, so the charger can avoid overheating the battery.