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Cell triggering high voltage disconnect

lettuceman

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Apr 26, 2021
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I haven't used my battery pack in a year, and when I started charging it again 2 of my cells immediately jumped to 3.7v and triggered the bms overvoltage disconnect. After it disconnects the voltage of the cell drops back to 3.1v, at which point the pack starts charging again, which immediately makes the cell spike up to 3.7v, triggering the high voltage disconnect again. Besides those two cells, the other ones also jump up in voltage, but not nearly as significant. Still, some of them go from 3.1 to 3.4v when charging. Any ideas on why this happened? I already top balanced them when I received the cells new, but I'll do it again just to be sure. I really hope it's not a faulty cell, but it seems like that's what's happening. Attached is a photo of the cell voltages when charging.
 

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I haven't used my battery pack in a year, and when I started charging it again 2 of my cells immediately jumped to 3.7v and triggered the bms overvoltage disconnect. After it disconnects the voltage of the cell drops back to 3.1v, at which point the pack starts charging again, which immediately makes the cell spike up to 3.7v, triggering the high voltage disconnect again. Besides those two cells, the other ones also jump up in voltage, but not nearly as significant. Still, some of them go from 3.1 to 3.4v when charging. Any ideas on why this happened?

Yes. You let a DIY battery sit for a year. You likely purchased from a questionable source.

I already top balanced them when I received the cells new, but I'll do it again just to be sure. I really hope it's not a faulty cell, but it seems like that's what's happening. Attached is a photo of the cell voltages when charging.

You definitely need to re-balance. I've seen cells go imbalanced enough to have a runner when only 0.1% out of balance after sitting for just a few weeks.

Your cells fail to meet self-discharge specs, which is typically 1% per month. IMHO, you have some bad cells. That doesn't mean they won't work if used regularly, but they are suspect.
 
Cell voltages are all over the place, they need to be top balanced before any conclusions about cell health can be determined.
Yup, that's what I figured, top balancing is going to take awhile because all of the cells are at low soc though, and I don't think I can precharge them because the bms will trigger the high voltage disconnect again.
 
Yup, that's what I figured, top balancing is going to take awhile because all of the cells are at low soc though, and I don't think I can precharge them because the bms will trigger the high voltage disconnect again.

@BentleyJ is being optimistic. The only explanation for the huge cell disparity is that most don't meet the self-discharge spec. The two "runners" are probably the only cells that come close to meeting spec.
 
Yes. You let a DIY battery sit for a year. You likely purchased from a questionable source.



You definitely need to re-balance. I've seen cells go imbalanced enough to have a runner when only 0.1% out of balance after sitting for just a few weeks.

Your cells fail to meet self-discharge specs, which is typically 1% per month. IMHO, you have some bad cells. That doesn't mean they won't work if used regularly, but they are suspect.
They were lishens that I used a couple times before I had to go overseas for a year, told my friends to help me cycle it a few times, but evidently it wasn't enough :ROFLMAO: . Thanks for the help, you're probably right but fingers crossed, I'll report back once they're done top balancing.
 
They were lishens that I used a couple times before I had to go overseas for a year, told my friends to help me cycle it a few times, but evidently it wasn't enough :ROFLMAO: . Thanks for the help, you're probably right but fingers crossed, I'll report back once they're done top balancing.

A Human BMS is barely better than none at all. :)
 
Question to you experts from an interested onlooker.
Would an active balancer in circuit such as the Heltec, either have prevented, or helped fix this problem?
 
If a cell sits at very low SoC for months it can grow lithium metal dendrite leakage paths and corrode copper foil interface to graphite electrode. These defects are permanent and non-recoverable.

Possible more benign cause is a corroded busbar- cell terminal connection. Check them first.

Measure voltage directly on cell terminal under load or charge current. Don't probe on top of bus bars as you may have connection resistance.

Should not use bare copper bus bars or bare copper terminal lugs to aluminum cell terminals. The inter-metallic interaction will cause pitting into the cell aluminum terminals over time. Should always use nickel or nickel-tin plated bus bars and terminal lugs. Nickel provides an inter-metallic barrer layer.
 
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If a cell sits at very low SoC for months it can grow lithium metal dendrite leakage paths and corrode copper foil interface to graphite electrode. These defects are permanent and non-recoverable.

Possible more benign cause is a corroded busbar- cell terminal connection. Check them first.

Measure voltage directly on cell terminal under load or charge current. Don't probe on top of bus bars as you may have connection resistance.

Should not use bare copper bus bars or bare copper terminal lugs to aluminum cell terminals. The inter-metallic interaction will cause pitting into the cell aluminum terminals over time. Should always use nickel or nickel-tin plated bus bars and terminal lugs. Nickel provides an inter-metallic barrer layer.
would oxyguard or something of that nature help in this?
 
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