New record! -2.69V on a cell group!

[Syonyk]

Every now and then when I tear down a battery pack, I find a very dead pack that has clearly been drained out the terminals, and isn't the result of normal self discharge. Usually a bad BMS seems involved.

I was playing with one of these recently, and one parallel group of 3 18650s was -2.69V across the bank.

Yes, I checked polarity. Repeatedly.

I even pulled the pack apart to verify that there wasn't some funky routing going on.

Nope. It was just over -2.69V initially, dropping slightly before I got this photo taken.

What's the chemistry of a lithium cell going negative? And how far negative will they actually go in a worst case condition? I have no desire to go messing around with these cells (I expect they're rather damaged internally), so... has anyone else done the experiments?

 

[nutnspecial]

I am imagining a new major power mode/source . . . negative voltage- as close to dark energy as we have got

Seriously though, those cells are pretty effed

 

[redilast]

Interesting. I wonder how the cells got reversed charged? I've seen this from Nickel-based cells Ni-Mh and Ni-Cd's but don't recall seeing negative, or this far negative before from Li-ions. I wonder how much energy they are storing if you were to reverse the leads and put the cell(s) on a battery analyzer to try to pull some power from them?

 

[heathyoung]

Interesting - usually reverse-charging results in a shorted cell.

I don't think they would have any real capacity, probably acting as capacitors more than anything else.

 

[liveforphysics]

I've seen -9vdc

 

[dogman dan]

Ok being the dummy I am,,, what exactly is this? is it now charged to 2.69v but the poles are now reversed?

This is why they call it cell reversal? Even after all the years, huge gaps in my knowledge.

 

[nutnspecial]

I did a tiny bit of reading b4 posting, so let's see if it's close (for layman).

The positive has become shorted to ground. It is 'more' grounded than the original negative, thus a negative voltage. The cell is basically destroyed at this point and although it can hold and show voltage, it will not store much energy again.

How it happens I'm not exactly sure.

___________
On that subject, get this. . . . it's spooky.

Do you guys ride @ night much? I have, enough to notice more times than can be normal, a few particular street lights will turn off as I approach. This happened many times, and although I didn't wait to see how long they stayed off, they were always on the next day I rode by (and often turned off or blinked again).

My garage door opener. . . the switch is just two bare ended wires that I press together. Being copper they retain shape and a gap of about 1/8". MANY times when I open it, as I reach up to grasp the wires to stop progress, it stops before I touch them! Sometimes right before, sometimes a foot away.

I think these two phenomina are related, and also of course to the subject (spooky emoji)

 

[Syonyk]

Seriously though, those cells are pretty effed

Yup. They go straight in my "Oh hell no" box of batteries for recycling. I'm not going to mess with those. I'm curious, but I don't have a highly fireproof location to play with them, and I rather like my office not on fire.

Interesting. I wonder how the cells got reversed charged?

Somewhat different capacities on cells in a string, followed by draining the entire pack down to 0V (or, 2V, close enough). This bank hits 0V first, the rest of the cells keep pushing current through, and magic happens.

I wonder how much energy they are storing if you were to reverse the leads and put the cell(s) on a battery analyzer to try to pull some power from them?

Given that I've seen the voltage tending towards 0V with a high impedence voltmeter attached, I suspect very, very little.

I've seen -9vdc

*eep*

That's when you calmly put the battery in a concrete bunker and go find a stiff drink, right?

Ok being the dummy I am,,, what exactly is this? is it now charged to 2.69v but the poles are now reversed?

Pretty much. The cells have a higher potential on the negative end than the positive end. This shouldn't happen, but, obviously, does. It probably does some amazing damage to the insides of the cell in the process.

This is why they call it cell reversal? Even after all the years, huge gaps in my knowledge.

Yes - the cell is "reversed," voltage-wise. It's now a negative voltage source if you look at it normally.

 

[999zip999]

My friend had a Volkswagen we looked at it at auto shop and the battery was hooked up backwards it was working so my shop teacher said just leave it alone. The teacher also own an electrical shop. Don't know how long it was hooked up backwards. But it was working.? ??

 

[Punx0r]

IIRC Lead-acid uses the same plates for positive and negative so it's possible to drain a battery to zero and then reverse charge it. Apparently it was done periodically on submarine batteries to extend their working lives.

 

[Syonyk]

IIRC Lead-acid uses the same plates for positive and negative so it's possible to drain a battery to zero and then reverse charge it. Apparently it was done periodically on submarine batteries to extend their working lives.

I believe they start out the same way, though they end up somewhat different over time. It wouldn't surprise me if the Navy did something like reverse the battery polarity on occasion - anything beats having to pull the cells out of a sub.

 

[999zip999]

The batteries are hard to service on the submarine but in Seal Beach they change the nukes out of the submarines all the time. Just south of Long Beach.