Readers may be interested in a recent study which provides an interesting glimpse into what occurs when Li-ion cells are severely overdischarged (e.g. as may occur when a cell in series is reverse charged). The study linked below examines what occurs when cells are discharged to –100% SOC.
One interesting result is that overdischarges < 12% SOC reliably cause internal short-circuits caused by copper foil dissolution and deposition, as illustrated in the diagram below.
Below is the associated voltage profile.
Notice how the voltage rebounds in Stage II. This occurs because the anode has reached so high potential (3.5-3.6V) that it enables dissolution of the copper current collector. Then copper ions dissolve into the electrolyte, pass through the separator, and deposit onto the cathode, which increases potential as the copper ions are reduced. This process continues in Stage III. and the internal short(s) become more severe, while the voltage asymptotically approaches zero.
One implication of this is that the cell voltage alone cannot be used as a reliable indicator of how overdischarged the cell was since, e.g. above –0.5V could be either –9% or –100% SOC. All the more reason to be very careful with cells that have been severely overdischarged.
All the more reason to be very careful with lithium power packs. I’ve come across clones where the balance circuits were entiry missing from the pack protection pcb, a recipe for disaster only prevented by a failed solder tab resulting in refusal of the charger to accept the battery. 18, 24, and 36V packs can only multiply the power of possible disasters. Power walls represent both a powerful resource and a powerful danger. You just know some wannabe Darwin Award winner will prove once again that nothing is truly idiot proof.
It is, of course, interesting that scientists have found out what happens when a LiIon cell is reverse charged, but most people here know already that it is a very bad idea.
When dealing with the uninitiated, it is sometimes helpful to be able to explain the process behind the problem. Knowing the science can lend you a bit of authority when someone asks why it’s so important while you’re trying to impress the safety rules on them.
Hey still a bit dizzy
So shoot it out here to be safe
Hd two Pana GAs drained by a anoying parasitic non stop flashlight
They were at 0.6V
Charged OK, did not took longer then before, did not get warm, IR had jumped from 15 to 49
used and did another charge, again good no heat of slow, they keep charge.
if the info of the OP has penetrated to my now thicker skull, still dangerous? toss or keep?
Not sure i get the ‘reverse charging’ thing… Did those cells simply get completely discharged or where they charged the wrong way? How can that happen? Battery packs…? Sorry for the noob question. I guess i have to read more!
The chart shows down to –2V for a battery.
This can happen to batteries in series without individual protection if one cell has less capacity than the other cells.
@gauss163: Thanks for the details regarding the experiment.
Still, this seems an unlikely scenario…? One cell at 0% and four others at 100% in a 5S setup. How could this happen in real life?
I understand that when the cells have different capacities, at the lowest, one will be completely discharged while some others will still have something to give out. But the difference should not be as large as a 100% charge state. Hence the reverse charging episode should not be as long and dramatic as in this experiment. What would be interesting to know is how large can be the capacity difference before it becomes a problem.
