Back in 2011, I've bought a portable speaker (Compex X-Mini v 1.1) and it has been working great for a long time. But I left it for more than a year lust laying on the shelf and once I needed to use it again, it wouldn't turn on. So I plugged in the charger, yet no charging light appeared. Only blue, showing that it's powered on. It would work on external power, but wouldn't charge. I searched online, and on their website they have:
"Sometimes after long periods of non-usage, you will find that the speaker cannot be turned on. Try charging the speaker with the power switch in the "ON" position for a few minutes.
Next try turning it off and on again and it should start working. If the LED light is on and there is sound, then you have successfully jumpstarted your X- mini Capsule Speaker. After which, continue to charge it for 2 hours."
This wasn't helpful at all. So it was obvious that the battery had drained itself too much due to it's self-discharge over this long period of time. But is it possible to save it? I decided to find out.
Step 1: Disassemble Your Device
Take your time while taking it apart. You don't want to rush this, as we don't want to destroy what might still be a working speaker. I found a few little screws hidden from the sides, and slowly took the bottom off it. Be careful, there are often wires connecting the two parts. You don't want to tear them.
Once I opened the speaker, I quickly found where the battery was soldered onto the board. Just a quick desoldering and I was ready to save it.
Step 2: Taking Care of the Battery
The voltage did drop below 2.5 volts, which is very bad for a Li-ion battery, but there can still be capacity left. I used iMax b6 charger from ebay (which is most likely a clone, but it works fine) and set it to Li-Ion charge, and the smallest charging current possible, as you don't want too much current rushing into the small battery that was almost dead. Charge at 1/10C (c=capacity, for 230mAh battery its 230mA=1c) at most and watch the voltage slowly rise (best done with lab bench power supply with precision current limit). I also had the battery on a metal plate, just in case it decided to vent or ignite. Li-ions are not the safest battery chemistry out there. I got the battery to 4.2 volts eventually, yet the mAh to get it there were suspiciously low. I tried discharge mode to see how much energy it held, and it did a really bad job. After that, I ran the charge/discharge cycle all at lowest current setting several times, and the total capacity eventually got to about 125mAh (from original 230mAh, that's way better than what I've expected) which is still enough for up to 3 hours of loud playback. (Down from original 6 hours.)
Step 3: Putting It Back Together
After all that we've done, the battery should be good again for some usage. It's not new one (which you can always buy on internet and just solder it in), yet having a 50% capacity from 7+ year old battery is pretty good.
Just solder it back in where you've desoldered it, assemble it back and it should work. Now the battery management circuitry should be able to charge it again, as long as the battery stays within the voltage of 3-4.2v.
This should work for most of portable electronics that refuse to charge Li-ion or Li-pol batteries that were over-discharged, yet their capacity is now much lower than before.