Li Ion Charging of Very Old Cells

Having satisfied myself that leaving Li-on cells permanently on charge at a nominal 3.9-4.0V appears to do no harm to the cell I turned my attention to the pile of recovered cells I have. These were mostly from laptop battery packs but some came from old handheld radios. I have been using cells recovered from old Apple laptop batteries for many years now and never given it much thought. They just worked and allowed me to power many projects away from the bench. The newer cells I have from mobile phones and cameras are not the subject of this experiment.

I easily found a research paper that dealt with extreme discharging. However, I am not examining cells that have been forcibly discharged to the point of polarity reversal. Instead, I am looking at cells that self-discharged down to almost 0 volts.

It would be a mistake to believe a cell left to self discharge to this point is ruined. The self-proclaimed experts never present any research to back their assertions and most seem to be plagiarizing from each other. 

 One example of the utter nonsense on this subject: Do not boost lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer. Copper shunts may have formed inside the cells that can lead to a partial or total electrical short. 

Some people accept this as gospel but without ever questioning the basis for such a claim. What is the definition of partial electrical short? Where did "1 week" come from? Is 6 days 23 hours safe? Why wasn't the threshold 8 days 12 hours 52 minutes? I wonder how many believe the earth is flat?

So I am running a trial to at least see if a very old cell recovered from a battery pack can be useful. Some of the cells in this trial are close to 20 years old taken from old two-way battery packs. Those removed from old laptop battery packs were at least a decade old. Before charging the terminal voltage was less than 2.7V. Many were below 1V and some as low as 0.2V. 

I doubt any of these cells had seen a charger for at least 5 years, and some would be close to 10 years. I applied a constant 2mA until the cell reached around 2.7V. Then I used my CC/CV charging circuit to charge to 4V. I am leaving these to sit at 4V between discharge trials for several days to a week. That's to reflect how I intend to use these cells in real life. 

It's too early to post data on what has happened but the initial results suggest the cells are useful. Once parallel combination of two 18650 cells recorded just over 3000mAh on the first trial. Not bad for cells that had been left discharged for over 5 years then charged to 4V or about 80% of capacity.

I'll provide a full update in due course but for now it appears that a Li-Ion cell can be left sitting discharged for several years. To restore it to service charge at a few mA till the cell voltage rises to 2.7V. Then it can be charged with my CC/CV circuit mentioned earlier.

The only challenge I have faced to date is separating the cells. The pouch style cell has connecting tabs easily damaged and once broken you need a spot welder to attach a copper flake to the aluminum tab for soldering. Not something I can do so I reluctantly discard such cells.