I applied this fix on my BT-S2 bluetooth motorcycle intercom but you can use the same principal to recover other devices with similar issue.
ISSUE:
Device left on forever resulting to a totally flat battery unable to be recharged.
In my case this probably happened during moving from one house to another and my second helmet was cramped among other things so the power button of the intercom stayed pushed for who knows how long. Being my second helmet I didn't realise it till recently when I had to tour my guest around. I plugged it to the charger but I noticed that the led indicator light was not as bright as usual which looked suspicious so after a while I tried to turn it on to see if it works and it was dead. Tried to charge it for another several hours with no luck. I didn't left it overnight cause I don't like to leave devices charging unattended especially suspiciously faulty ones and I recommend this to you. You never know when Murphy's law will make you regret it.
So, time to open it up and see what is what. Here's the board:
The battery (located behind the board) looked fine to me so that gave me hope for a fix. First obvious move is to check its voltage. Since the battery's terminals are isolated with silicone from factory I used the exposed terminals on the board instead.
These ones:
I expected it to be flat but not THAT flat. It gave an absolute zero on my multimeter! Even my fingers provide more voltage 😁. Plugging it to the charger gave a poor red light on the indicator like before but an as poor voltage on the same terminals - only 2V while the battery is 3.7V and the charger is (supposedly at least) 5V.
This made me decide to override the charging circuit and apply voltage straight to the terminals to "wake up" the battery. I gave it 4.5V from another adjustable power supply I have for such attempts and after a short while I took it off to measure the battery and see if it managed to store any energy at all. A voltage of around 3V gave me a smile from ear to ear. I had lower expectations to be honest.
So, time to plug the original charger again and a full bright red led light this time gave me total satisfaction.
FIXED!
I wouldn't cry tears enough to fill a lake if I would have to recycle it since I bought it cheap enough but isn't it nice when a full recovery is only a matter of a few minutes of your time?
Hope this approach works for you in other cases as well, have a nice day.
