Recharging Li/MnO2 Primary Batteries Safely?

[RFI-EMI-GUY]

So I have some primary disposable Li/MnO2 batteries that were manufactured well over 20 years ago for a Motorola Saber radio. Two of them, unused, seem to have a high level of charge remaining. Estimated 75 or 80 percent based on published self discharge rate and having over 3.1 volts per cell. These are sealed 9 volt packs consisting of 3 cells. One of the batteries was used but has about 2,8V per cell.
The literature says you cannot safely recharge these. However there are coin cells that are rechargeable at about a C/15 rate (15%).
The precautions are that the usual dealing with heat and sparking, but also with dendrites from the lithium reforming as a crystal.
I am considering to apply a constant voltage to the ~1800 mAh pack at a greatly reduced charge rate of 2% this would equate to about 36 mA * which is very little energy.
Could this be done safely and effectively? Has anyone done this successfully?

* Edit; They are 4200 mAh so 2% would be 84 mA which is still pretty low. I can go lower.

 

[dickie757]

Firstly, I wont say dont do it, but make sure they are completely away from stuff you cant lose.

Second, charge rate, C/n or Cn is not a percentage. C/15 would equate to 280mAh on the 4.2 pack. 15% is 630.

Lastly, it will take a long time, to what end? How many charges will it take before it ?? As a hobby experiment, fun, but to depend on it, no way.

 

[RFI-EMI-GUY]

Well ok, so ".02C" would be 84 mAh for a 4200 mAh . (I have seen charge rate poorly expressed C/n in manufaturer literature. )

Lets say I have for example a 4200 mAh battery depleted to 30% charge. That would leave 1260 mAh charge remaining. To restore the depleted 2940 mAh would require 35 hours at 84 mAh. Actually somewhat longer due to internal resistance, charge potential limitations etc. I could even use a much more conservative rate, because the goal is not to use the battery as a routine rechargeable, but simply restore its state as an back up battery. I simply want to nudge some ions back without creating a lot of stress or dendrite growth (if possible). The key would be to maintain the charging voltage to a safe point just above the open circuit rated voltage of the battery. This in itself will lower the current and slow the rate.

In my case there are three cells in series, in a sealed pack, so a very conservative approach is needed as there is no way to balance or observe the individual cell voltages. Again, just nudge ions over a long term.

 

[dickie757]

Go for it, but be prepared to lose all use of that pack.

Or, how big, what size, are the cells?

 

[RFI-EMI-GUY]

Go for it, but be prepared to lose all use of that pack.

Or, how big, what size, are the cells?

I would need to Xray it or cut it open to find out. If it fails without melting down I will cut it open. The plastic used, MAKROLITE is difficult to saw.