Faulty Macwheel MX2 electric scooter fault finding and fix (similar to swagtron swagger) battery fix

I got this scooter for dirt cheap on fb marketplace. It wouldn't turn on, or charge.
I was able to fault find enough to find a slightly lower voltage 18650 lithium cell. I was then able to charge it up on it own using a tp4056. This then solved the issue.
The question remains what caused the cell to drop its voltage too much? (probably repeated trying to use the scooter when its already very low battery, it can cause the lowest impedance cell to discharge first. Or even a cell that is self discharging. Which is when a tiny short circuit forms through dendrites between the cathode and anode of the cell, and slowly wastes energy in the cell in the form of heat from the small short circuit, and this slowly discharges the cell. To test this you can charge the cell to full (4.2v), let it sit for 2 weeks and check the voltage again. It will drop slightly, thats OK but if it drops below 4v in under 2 weeks there is a problem and the cell is junk at that point.

The weird output voltage coming from the mosfet output even tho the BMS was in protect mode and off was confusing to me as I had never seen that before. Some residual voltage can come through the mosfets despite being off. But they have almost 0 current. So your multi-meter might say there is XX volts, but if you put any load on the outputs, the voltage would instantly drop to 0. So in reality, the are off, but just look like they are on. Talk about a red herring!

Since this video I've not had any problems with the cell going too low again. So I consider it fixed.