Could it be from the damage in the car accident you got your salvaged pack from?
Definitely not a physical damage issue nor related to any kind of impact. The cells themselves really aren't affected by such things unless the pack itself, and thus the cells, were physically impacted and damaged in the accident. My testing would seem to confirm this, as well as data from non-crashed cars.
That sounds like a quality control issue in the cell manufacturing process as opposed to an inherent flaw in the general cell chemistry.
Possibly. I don't know enough about the underlying chemistry to make a real prediction on that. It's weird, because the issue seems to cause a high IR for the cell during charging only. During discharge the same cell can behave as expected. Again, I'm no expert on the chemistry itself, but my guess is a quality control problem wouldn't manifest quite like this. Could be wrong, but, don't know for sure.
That's exactly what I've seen get worse over time....
Follow-up question. I reported a problem to service where I would have observed battery percentage just disappear. 3 times I visually saw the pack at 17-19%, left the vehicle, came back 20 min later with no HVAC use in between, and the charge was at 12-15%. Could this be a possible side effect of a large uncorrectable cell imbalance?
Most likely you're seeing the range loss as a result of the pack cooling down while the car sits. This is more exaggerated at lower states of charge and is the reason for the ambiguous "There will be significantly less energy available from your battery if it gets colder" (or close to that) popup message. The "it" in that sentence refers to the pack itself, not the weather. There is a calculable amount of additional energy available to the user when the pack is warm, due to decreased IR, than when the pack cools down. The change in IR is more dramatic with temperature when the pack is at a lower SoC.
So, for example, you drive for a while, car is at 20% SoC. By then it's likely the pack has been allowed to warm to a higher working temperature, usually between 35-45C, allowing the best IR possible for discharging efficiently. Then you park the car, and it sits for a while, cools pretty quickly in Ohio winter weather, and when you get back the IR of the pack has gone up to the point where there is no way to pull the same capacity out of the pack without losing a chunk of that to IR and internal cell heating.
Hopefully that makes sense.
Is this a possible reason why they tried to put launch limits on the car, so as to slow down a scenario that could contribute to this problem?
For sure it's a contributor. They also put the launch limits in place because they seem to have found that wear and stress on the cell level fuses can be cumulative at high currents, eventually resulting in a failure of one or more fuses, and thus causing an uncorrectable pack imbalance.
