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2 pack faling behind with charging and get unbalanced (update 9/07/2020)
#11
Please post a photo of the positive terminal of the cell with the ruptured sidewall. Are its vents clogged by debris?  Did any debris  appear to erupt from the vents? Did you mess with the CID on any cell, e.g. attempt to reset it?
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#12
@not2bme

Thanks for your explanation and yes i caught it in time pfff... i do not know how it happend, the cell was not dented or damaged in some way what i could see when i installed it.
the strange thing is the voltage was 4.15 volt but its slowly goes down in voltage now
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#13
(08-10-2020, 03:31 PM)not2bme Wrote: [...] Deviation between the packs may change during the day, and that's OK. You will never create an exactly balanced pack. So at certain battery state or SoC during the day, especially when reaching full charge or low charge, one battery may reach that goal faster so if you look at the charge curve the voltage will shoot up creating this deviation from the rest of the pack [...]

The hourly variations of 0.01V or so shown in your graph likely do not correspond to SOC differences. Rather, they are more likely due to thermal drift in the voltage meters, and possibly also variations in IR, which will serve to perturb the readings when measured under (dis)charge.
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#14
(08-10-2020, 03:48 PM)gauss163 Wrote: Please post a photo of the positive terminal of the cell with the ruptured sidewall. Are its vents clogged by debris?  Did any debris  appear to erupt from the vents?  Did you mess with the CID on any cell, e.g. attempt to resent it?

CID and fuse are intact Huh i will take a photo.
there were no debris from the hole in the sidewall..

AND NO I DID NOT RESET THE CID THAT IS THE ******** YOU COULD DO ,i am not that stupid Big Grin
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#15
(08-10-2020, 03:59 PM)barry Wrote: the strange thing is the voltage was 4.15 volt but its slowly goes down in voltage now

Yes, it will continue to self discharge due to (micro) internal shorts. Since it is still at high voltage it still has the capability to (violently) vent with flame, so treat it with caution. Neighboring cells may have been highly heated so may also have suffered internal damage.

It will be interesting to see a photo of the vent. Check to see if there is any debris under the vent that clogged it. Don't disturb the cell in any way. Try to preserve it in the name of science in the hope that someone will offer to do a careful analysis.

I've seen reports of such sidewall ruptures when internal shorts occur near the can, where gases can get trapped and be unable to reach the vent, or when the vents are blocked or damaged. I may have time to dig up some links later.
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#16
i have taken some photo's it look all good.


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#17
(08-10-2020, 03:59 PM)barry Wrote: @not2bme

Thanks for your explanation and yes i caught it in time pfff... i do not know how it happend, the cell was not dented or damaged in some way what i could see when i installed it.
the strange thing is the voltage was 4.15 volt  but its slowly goes down in voltage now

Yes it's dropping and self discharging because of a short and giving out heat around that area. Energy can't be lost and needs to be transferred. So in this case it's transferring the loss through heat. That's the one thing that boggled my mind when someone made me realize that computers don't use any energy, and all of that is turned into heat. Yes it computed something for you, but it's a by-product and it's actually generating heat.


(08-10-2020, 04:04 PM)gauss163 Wrote:
(08-10-2020, 03:31 PM)not2bme Wrote: [...] Deviation between the packs may change during the day, and that's OK. You will never create an exactly balanced pack. So at certain battery state or SoC during the day, especially when reaching full charge or low charge, one battery may reach that goal faster so if you look at the charge curve the voltage will shoot up creating this deviation from the rest of the pack [...]

The hourly variations of 0.01V or so shown in your graph likely do not correspond to SOC differences. Rather, they are more likely due to thermal drift in the voltage meters, and possibly also variations in IR, which will serve to perturb the readings when measured under (dis)charge.

The chart is daily so each peak is when the packs are heading towards full charge during the day. I'm sure there's some variations in IR that would affect the voltage, but it's still due to the SoC.
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#18
(08-10-2020, 05:46 PM)not2bme Wrote: The chart is daily so each peak is when the packs are heading towards full charge during the day. I'm sure there's some variations in IR that would affect the voltage, but it's still due to the SoC.

SOC imbalances generally don't evolve under short (hour/day) timescales (unless you have a high-rate self-discharging cell, or huge temp imbalances).  Li-ion cells have nearly 100% coulombic efficiency, i.e. charge_in = charge_out. So if they start out well-balanced they will remain that way over short periods since there are no charge losses.

But over much longer periods they can become (significantly) imbalanced due to differing self-discharge rates, and also due to various degradation processes.

The minor hourly variations you are seeing are almost surely due to the reasons I mentioned in my prior post (a poor balancing circuit may also play a role).
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#19
(08-10-2020, 06:04 PM)gauss163 Wrote: SOC imbalances generally don't evolve under short (hour/day) timescales (unless you have a high-rate self-discharging cell, or huge temp imbalances).  Li-ion cells have nearly 100% coulombic efficiency, i.e. charge_in = charge_out. So if they start out well-balanced they will remain that way over short periods since there are no charge losses.

But over much longer periods they can become (significantly) imbalanced due to differing self-discharge rates, and also due to various degradation processes.

The minor hourly variations you are seeing are almost surely due to the reasons I mentioned in my prior post (a poor balancing circuit may also play a role).

Of course it does. If everything was equal then yes they would stay the same. But if one of my packs are not properly balanced at the exact same level then one pack will reach 100% faster than the other. Like I said before, if you have two packs in series that is 100Ah and one pack that is 200Ah. If at 50% SoC on both of them it reads at exactly 3.8565554V on both packs (i.e balanced at 50% SoC). When the 100Ah pack reaches 100% SoC at 4.20000002V, what would the 200Ah pack read? 4.2000002V as well? No. It will read somewhere in between. There's no logic that states that the SoC for all my packs will remain equal in all ranges.
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#20
^^^ So the differences are due to different assumptions. As I wrote: assuming they "start out well-balanced".... Are the packs in your graph very far off in capacity, etc?

Btw, one other point worth stressing. Chemistry also plays a role in balancing. If you create a pack using cells of different chemistries then you end of with a bit of a Frankenstein voltage vs SOC profile, no two of which will likely be the same. So even if their SOCs remain balanced, their voltages may not due to the differing voltage profiles. In that case, a balancer can do more harm than good in some cases - by trying to correct a SOC imbalance that doesn't actually exist.
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