Reversed polarity 18650 cell.

I've picked up some discarded laptop battery packs from my local campus.
One battery was 3S2P 18650 cells, two of the pairs are OK ( they had ~4.0v and I get from them approx 2050 mah per cell (tested with Imax B8),
But the last pair had reversed -0.27v !!!
First question is: how could this happen in the first place? There's big PCB in the laptop battery to prevent this kind of situation
I've tried to charge these - 0.27v cells, but my charger won't accept them (it says reverse polarity).
Maybe I just charge them manually, with 4.2v and some resistor in series to limit the current?
Or connect to healthy full battery in parallel, through a 10ohm resistor ?
Is it safe to use them?

Hello Junglemike,

If you have a fire proof area that is well ventilated, you may be able to get away with playing with those cells that have reversed polarity, but they are not safe to use them.

This happens because protection circuits are not perfect. As cells age their properties change. The protection circuits are designed for the properties of new cells. During discharge reverse polarity occurs when a cell that is completely discharged is left in the circuit. The cells that have some capacity left reverse charge the empty cell. When the chemistry within the cell gets all screwed up you end up with a cell with reverse polarity.

Reverse polarity in a Li-Ion cell indicates that damage has been done to the cell. Using a Li-Ion cell that has been damaged is asking for additional problems.

With that said, I had some Li-Ion cells that I had discharged, then clamped a piece of wire between the + and - terminals. The idea was to discharge the cell completely and let it sit for a week or two to check for the effects of over discharge. I aborted the attempt to charge them because of overheating in the cells.

Tom

Wait, so does this only happen when cells are used in multiples?

I wouldn't try to charge them, they will be in very bad condition. I accidentally reverse charged one AW IMR RCR123 cell to -0.1V (left it in well regulated 2 cell light for too long). And I wasn't able to revive it at all. I've even tried to charge it from power supply set to various voltages up to 4.2V and it didn't get any current.

LEDAdd1ct said:

Wait, so does this only happen when cells are used in multiples?

Click to expand...

Yes, it can happen to all kinds of cell when used in series. You can also damage NiMH cells this way.

Okay, thanks. I knew it could happen in series with other cells, but I had not yet heard of it happening to one lonesome cell.

Poor little guy. It's like SilverFox's signature says:

"Most batteries don't die - they are tortured to near death, then murdered..."

LEDAdd1ct said:

Okay, thanks. I knew it could happen in series with other cells, but I had not yet heard of it happening to one lonesome cell.

Click to expand...

In this case there were three (pairs of) cells in series. The potential for problems gets worse the more cells in series there are.

I just received a battery pack for a laptop off Ebay, and the device had no charge coming off it from the start. Cracking it open and found CGR18650D MH12210 2250mAh batteries in S3P2 configuration, and none had a charge beyond a stirring in my multimeter.

I thought about the protection circuit and being CGR batteries it would have kicked in and prevented further loss and so I tried charging but the charger would detect them.

So I was stumped until I remembered that cells of different charge used in series reverse charge the lower voltage cells and so I chucked one of the bad cells (foolishly in the correct polarity) in my 3x CREE CM-L T6 torch with two other good cells. After clicking it on and off about 10 seconds total I extracted the battery and viola! A 2.7v charge!

In the wrong direction -_-. I felt trolled by the battery cause it had the polarity printed on either end .

I figured I could undo this reversal by chucking it the reverse polarity back into the device and after the same process it came back and was rechargeable!

I did this for all 6 cells and they are all back up to capacity.

I am waiting on my IMax B6 LiPo charger to give capacity tests but as a general rule for my current chArger it charges Ultrafire so-labelled "3200mAh" batteries in 2-4 hours and Samsung 2000mAh batteries in over 8 hours, and these batteries charge for about the same time.

Quite upset with the low capacity ultrafires and so my advice is avoid those brands altogether and recycle multicell laptop batteries cause you can usually get a better deal with more capacity from a half dead pack than a 8 pack of lightweight Ultrafire 18650s.