Lithium cell phone battery: Why 3 pins?

[SamTexas]

Most small single cell phone battery has 3 pins (instead of 2). What is the middle pin for?

Here are the voltages of a fully charged battery: 4.10V between the two outermost pins, and 4.07V between the other two pins. Same difference when cell is empty: 3.60V and 3.57V.

Thanks.

 

[bigmoose]

The third one may be a thermistor to ground to keep an eye on temperature for catastrophic failure protection. To test it measure the voltage across a 10 K resistor. Place this resistor across + to - and you should still read battery voltage. If there is a 10K thermistor in there it should read around half the battery voltage when you bridge it correctly.

That said, the battery may have a smart chip in it that identifies it digitally. That third wire may be the digital I/O and doing the above test may damage your battery.

 

[grindz145]

Very often a cell phone battery with have SMBus communication. but that would require power ground and 2 data pins (clock and data).

 

[SamTexas]

The third one may be a thermistor to ground to keep an eye on temperature for catastrophic failure protection.

That's what I thought at first but it does not appear to be so.

(Click on the pic to get a larger version)
The UNLOADED voltage between the Negative and Positive pins is 4.10V, the voltage between the middle and the Positive is 4.07V, 0.03V difference.


When I put 2 small LEDs (in series, forward voltage is 2.04V each) across those pins, here's what I see:
LOADED voltage between Neg and Pos is 4.03V and the LEDs is brightly lit.
LOADED voltage between the middle and Pos pins is 3.72V and neither LED is lit.
So I use only 1 LED across the middle and Pos. Loaded voltage is 1.72V and the LED is DIMLY lit.

Do these data points tell you anything? Thanks.

 

[parabellum]

Some of them may be for battery identification. I got few different motorolla phones ones with same physical battery shape but different model numbers and every battery worked in its own phone but were not interchangeable. In some cases phones detected other batteries but as wrong battery with limited usability (like no charge status and not chargeable in wrong phone). If detected in any way, LVC was taken from main lead voltage.

 

[whatever]

i was assuming third wire is something to do with cut off charger but have never researched it