I have a Bluetooth headset that is 7-8 years old. I used it occasionally in the car. It just quit completely and could not be charged. I never threw out a piece of electronics without opening and studying it. So I peeled the cover off and took a look inside. It is designed around Philips BGB204 Bluetooth 1.2 system-in-a-package radio chip that includes the Bluetooth core and an ARM7 processor.
The first thing to check was the battery. I was surprised to find the battery voltage was at dead zero. I was expecting it had at least some voltage albeit low, but it should not be at exact zero volt. I took the battery off the circuit board and powered the board with external supply at 3.7V. Everything seemed to work fine: it connected to the PC and transmitted sound through Bluetooth. So the culprit had to be the battery.
I carefully peeled the tapes off the package and exposed the battery contacts and the protection circuit. I measured the voltage across the battery contacts and there was some voltage as I expected. So it was the protection circuit that completely cut off the power. Probably the voltage had dropped too low to even power the protection circuit. Also the pouch that contained the battery cells looked a little puffy. It appeared that the battery cells might have released gas. I was tempted to pop the pouch to vent the gas, but I resisted, thinking that it could possibly cause fire because of lithium built up. I started to charge the battery directly at 100mA to 4.2V; it took about one hour. And at about the half way, I switched the charging through the protection circuit and it charged fine the rest of the time. Then I tested the battery by connecting it back to the headset. It worked and seemed to hold charge.
So it is curious what happened to the battery. And since the battery pouch is a little puffy, it is also a little worrisome. I have to be careful to continue using it.
After some research online, it seems that the battery puffing is caused by excess oxygen which is released from the organic solvents in the electrolyte likely because of overcharging.
