Clear cut thing to do when 18650 Li Ion Cell Has Been Overdischarged

Question

I think close to everyone knows over-discharge of Lithium ion cells is bad news, creating fire and chemical burn hazards.

These are my cells

I accidentally over-discharged two Lithium Ion cells.
What I would like to know is whether that should be the end of their recycled lives, or whether there is a margin of safety I can rely upon to keep.

I had a 2-cell (in series) unprotected 18650 lithium ion 2000 mAh capacity pack hooked up to some led lights running 2 A last night. I forgot about the need to avoid over-discharge. So I guess the batteries discharged in about an hour and have been sitting with the lights out but batteries connected for 15 hours.

After disconnecting the batteries from the lights, the voltage in the cells measured with a multi meter is 0.65 V these batteries are supposed to be kept at least 2.7 volts.

Is recycling my only good option?
Any hope in restoring these cells?

Answer 1

I have deliberately discharged lithium pouch type cells all the way down to zero volts (with several different cell designs) on several occasions for test purposes. They all recovered most of their original capacity after pre-charge followed by normal charge. Note that this is in the context of cells which would be charged and discharged at low rates (C/10 discharge, C/3 charge). Note that these were protected cells, but I bypassed the protection circuit to achieve the deep discharge. Pre-charge and re-charge were done with protection circuit in place.

In my experience, if you recharge an over-discharged pouch cell immediately after discharge, and using the proper pre-charge procedure (C/10 or so) then it will recover much of its previous capacity.

I would like to make a note about high power type cells, though. I have accidentally deep discharged high power cells (Samsung or LG) and found that they could not be recovered by pre-charge. I don't know for sure what the difference is.

So it may depend on the specific type of cell you have. In any event, it won't be safe to charge or discharge the cell rapidly after a deep discharge. But if it recovers, it may be safe for low-rate applications (say C/3 or less). Ultimately, it is up to you to decide on safety. Don't blame me if your battery catches on fire.

Answer 2

Your cells are very likely irrecoverable, but there is a method that MAY recover them.

Using one cell connect a charger or DC voltage source of up to about 5V via a say 1,000 ohm resistor. This should provide a maximum current of under 5 mA which MAY trickle the battery back into a recoverable state. If this does not work after a day or so then IF you can locate the cell in a location where "vent with flames" / fire / modest explosion is acceptable you could use a 100 ohm resistor - upping max current to under 50 mA.
Neither of these methods is liable to cause "vent with flames" BUT it is possible.

If the cell voltage does rise to 2.5V or more, allow it to rise to say 3V and then attempt charging in the normal manner. If the voltage does rise above 2.5V then DO NOT leave connected indefinitely to a voltage source above 4V.