Anybody knows what the middle pin on this battery is for? I charged it applying a voltage to + and -. It charged up to 3.8V (limit indicated is 4.2V).

The cellphone however still shows it is completely depleted. I put a 20R resistor from + to - and it stays at 3.75V, so it is indeed charged.

This is a new battery because the old one was also completely empty and the phone wouldn't charge it.

I measure the same voltage from + to the middle pin and from + to -. I measure 0V from middle pin to -. The middle pin and - are not directly connected though.

 

Looks like it is for communication
http://batteryuniversity.com/learn/article/the_smart_battery

 

I hope you are wrong.

I had the old battery taken apart (don't know where it is now). There were indeed 1 or 2 ICs inside, nothing fancy, it didn't look like a uC, but I may be wrong.

 

I hope you are wrong.

I had the old battery taken apart (don't know where it is now). There were indeed 1 or 2 ICs inside, nothing fancy, it didn't look like a uC, but I may be wrong.

Some places said it may me a thermistor for temperature monitoring, but that wouldn't really corroborate what you're experiencing....

 

Ok, after the first time charging the battery via the + / - pins I placed it back in the phone. It immediately accused depleted battery and turned off.

I took the battery out and applied voltage to the + and middle pin. There was no current. I was doing some other tests but can't remember the order.

After placing it back into the cellphone it showed that it was half charged....

So maybe really something like communication... It seems the phone can charge a slightly discharged battery but not a completely empty one...