BuckMkII said:
Why do cars with such seemingly similar conditions vary so much in this department?
Good question!
There can be quite a few reasons for larger amounts of cell imbalance. Here are a few:
Reasons for high cell imbalance which have nothing to do with the condition of the battery in question (these don't indicate any permanent issues):
1) The battery may not have been charged to full in a long time. I think the MY2013s still had the 80% charge setting. If that car with the red bars is set to charge only to 80%, then the cells in its battery will tend to get more out-of-balance as time goes on. Cars that charge to 100% regularly will tend keep their cells well-balanced.
2) It could be very cold, which increases the resistance of all the cells, particularly the ones on the ends of the stacks, causing rapid unbalancing during driving. (Obviously this does not apply to this situation, since it is warm and all the cars are in the same environment.)
3) The car with the more-unbalanced battery may have been driven particularly hard in its last test drive while the other two cars were driven gently during their last drives. The BMS has more difficulty keeping the cells in balance under high-power operation since that accentuates the differences between the cells.
4) I don't know if this is still true, but I have heard that there is a setting in LeafSpy that causes it to change how it interprets the shunts in the battery. Perhaps that one car needs that setting to be changed to read properly. (I doubt this is the issue, since you said the variation *is* higher with that car.)
Reasons for high cell imbalance which are due to some (permanent) condition of the battery.
5) As the battery ages, the resistance of the cells increase significantly. Some cells will have a larger increase in resistance than others as the battery ages. The result is that the BMS has more-and-more difficulty keeping the pack balanced.
6) There could be one or more cells which have lost more capacity than the rest in the stack. As a result, the BMS will have difficulty keeping the pack balanced.
7) One or more modules in the battery could have been replaced. In the case of a replaced module, there will be a pair of adjacent cells with MORE capacity than all of their neighbors. This also causes the BMS to have difficulty keeping the pack balanced.
So, how do you tell which one has the best pack? You need to get the batteries FULLY charged and balanced. The 2013s are decent at this, but it might require keeping them connected for a couple of hours after the battery gets full. Then you can to drive in similar conditions down to below *at least* very low battery warning (VLBW). That's not the first warning, but the second one. That will allow you to see if any have significantly more or less capacity than the others. It will also give the BMS a chance to "read" the battery in case it has been sitting for many months. Between your mileage numbers and the LeafSpy Ah readings, you should have a good idea what is going on.
Unfortunately, I doubt any dealership would let you do all of this! Good luck with your decision.
Finally, you should find out where these cars have spent their lives. If they all lived in Seattle, then they shouldn't be too bad, but you cannot assume they are local cars.