How is that a dangerous feature?
Here’s how it’s usually done:
If between 2,0-2,5V, then charge at 100mA until the cell reaches 3,2V.
If between 1,0-2,0V, charge at 50mA until it reaches 3,2V.
If below 1,0V, charge at 25mA until it reaches 3,2V.
Most thermal runaway scenarios in relation to overdischarge happen because at such a low internal voltage, a lithium-ion cell has massive internal resistance, and the low voltage potential means parasitic reactions take place, robbing the cell of capacity.
However, when charging at normal current levels, the cell bounces back to normal voltages, but not before internal damage is sustained, and parasitic *chemical *reactions are pushed at a massive rate, resulting in elemental lithium plating, and a thermal runaway can occur.
In most cases, this doesn’t happen fortunately. What instead happens is that the massive voltage spike robs the cell of a lot of capacity.
That’s what I noticed back in late 2017: by charging cells back up at very low currents, my 18650/20700/21700 recovery cell yield had gone up tremendously. I did some research, my own testing, my own research, and came to the conclusion.
It not only improved the yield of used cells massively, especially powertool cells, but the percentage of higher capacity cells being recovered got up by a nice margin.
It’s also a very safe method of recovery. It does need an additional step in software to make the charger stop charging if the voltage stops rising after a predetermined period of time.
Some more stuff from other members: Processing Low V Cells | Second Life Storage & Solar