I recently had to revive a (as it at first seemed) dead Li-Ion battery. It was the battery of a newly bought R.O.GNT external speaker which refused to work or even charge. The device was DOA (dead on arrival) but it was so cheap that sending it back would have cost more than I’ve paid for it.
The speaker has an internal Lithium-Ion battery to allow mobile usage. My guess was that this battery slowly discharged while waiting for a buyer and at some point the undervoltage protection kicked in. Normally this protection should prevent a defective cell from being charged. In my case I hoped the cell would still be okay and survive a jump-start. It was successfully done before in other cases.
Caution: The protection circuit is there for a reason! Jump-starting and charging defective Li-Ion batteries may work but can also cause flash fires or even explosions.
I opened the case of the speaker and carefully removed the shrink tubing from the battery to get access to the protection circuit. The first thing I noticed was how ugly it was soldered. There were tiny drops of solder everywhere:
Besides that the battery was down to about 0.9 V (compared to about 3.7 V when fully charged). I connected the USB connector resulting in 4.2 V coming from the speakers circuit. There are two pairs of pads on the protection circuit marked as VCC and GND. One pair connected to the speakers board via cables and one pair soldered to the battery itself. To my surprise it was enough to shorten the ground pads for about 20 seconds to initialize the charging process. I expected to bridge both pairs, but okay… Besides that the battery was charged within two hours. I was all the time having a eye on the connected voltmeter – just in case.
So far the speaker works again as expected. 🙂