Saving an over-discharged li-ion cell?

[kongfuchicken]

Hi there folks,
I've a batch of fairly recent c sized aw li-ion cells (less than a year old I believe) that were bought brand new. In the set, one pair of them was left in a light which was rarely used and thus forgotten under the bed in the guest room.
After half a year or so, I found the light while cleaning and was surprised to find that it wouldn't light up due to one of the cell being over-discharged as low as 2.5v (evidently the protection circuit's limit). The other cell was measured at a healthy 4.1v.
Is there any hope in recuperating this cell?

I've tried to charge it overnight but the latest measurement showed 2.7V which is starting to make me think it might be a lost cause.

And of course, since I'm very unfamiliar with the dangers to attempting to save dead cells as I often read about the fire risks associated with this process but seldom about the specifics, is there any dangers in charging an over-discharged cell for a long period of time using a generic li-ion charger?
Thanks for reading!

 

[VidPro]

to bad, usually if it has been at 2.4v for a while it often WOULD be recoverable at least partly.
it sounds like (not to say it is) the battery was self discharging even after it went into the protection low. A possibility for that is the cell was dead ANYWAYS and it brought itself down. If it brought itself down, then it wouldnt much matter that the protection kicked in, because it will keep on taking itself down.

the other cell (assuming it was used stored the same way) being in good shape, further indicates that the cell itself is dead.

the risk is: When it doesnt accept a charge, the power you put in, generates heat, the heat then causes thermal condition and then ignites the internals.
also it could gas out, which (depending on the cell) would expand or break the anode disconnect safety thing, once disconnected via the anode disconnect from gassing, the battery is then unconnected and completly useless.
If it gassed out (releaced), small ammounts of toxic gas will continually releace from it, this stinks enough to get a wiff of it from across the room, and would be bad to be breathing.

what you already did, a slow charge , then see if the voltage increases, if it doesnt then it wont "accept" a charge.
it IS possible to get it to squeak out of few Ma and play with it, that has been done before, but its a Total Waste Of Time. even if you recover it, its still dead, self discharges to fast, has way lowered capacity, and is dangerous to charge quickly.

check your charge parameters, make sure your charger IS slowly charging it in a SAFE location (meaning check the voltage and amperage of the charge to insure no flaws there) , if it wont take then its finished, and there isnt anything you can do.

the li-ion is Older, the Battery is showing way low compared to its teammate, the battery has been through rough times, you cant get it to work great again, and you couldnt team it up with anything either, time to move on.

 

[kongfuchicken]

So there's a chance it's partially recoverable but also a chance it was belly up from the start. Gotcha; thank you.
The cell so far hasn't heated up the least bit from charging; nonetheless I'm definitively keeping the whole setup in a safe location.

Is there some sort of indication that a cell would be refusing a charge after a long period of time and alternatively, what would be the signs of a partially successful recovery? As in, will it ever be brought back anywhere close to 4.1V?

Or do these resuscitated zombie cells pretty much carry a lower voltage for the rest of their second chance at life?

 

[VidPro]

if it takes a charge you will see it quite quickly , if it doesnt take a charge , then it will never be a good cell again.

it either does or it doesnt, when you get a recovery, its like night and day.
a recoverable cell will act like any other cell
a Sub-par cell will have self discharge and low capacity
a non-recoverable , does just what your saying, it just wont even budge, the more you put into it, you can get the voltage to rise, but then it goes right back down again.

and a sub-par cell just freaking completly croaks and is useless in a few months anyways, so after what i seen, their world is black and white

that is probably (one reason) why Silverfox says when it goes below 80% of its original capacity, its time to go, because the graph on them dying after they start declining badly, looks like a discharge graph.