Pack imbalance is probably correlated better with the condition of the battery cells. If the internal resistance of a cell is higher than the others, more of the charging/discharging current passing through the cell would be converted to heat rather than to energy. The result would be that this cell would have a lower charge level when the pack is fully charged.
Cell capacities are never perfectly matched, so those cells with lower capacities won't have the same charge levels as those with higher capacities.
The temperatures of the cells will not always be the same, maybe due to their positions in the pack and how well cooling effects each cell. This temperature difference would affect the cell performance which would gradually result in charge level differences.
All battery packs with cells connected in series need to have the cells' voltages equalized, usually at a high charge level ("top balancing"). Our 2000 Honda Insight's BMS did not include cell charge level balancing, so the charge levels of its 120 battery cells gradually diverged over time. As a result, the usable capacity gradually decreased as the BMS prevented the cells at the highest charge levels from overcharging even though cells at lower charge levels could have accepted more energy, and when a cell's voltage decreased to the lower limit allowed by the BMS, other cells at higher charge levels were unable to discharge farther. When the usable capacity decreased to a certain level, a diagnostic trouble code (DTC) was stored and a battery pack trouble light illuminated. Honda's only fix was to replace the battery pack even though the battery cells weren't damaged but only needed their charge level's equalized. Insight owners believed that the battery packs that Honda was installing under warranty were packs rebuilt with used cells that had all been fully charged and thus were in balance. The battery pack in our Insight was replaced twice under the 8-year battery pack warranty.
A couple of enterprising Insight owners designed and build battery pack balancers that took advantage of the fact that NiMH battery cells aren't damaged significantly when occasionally overcharged. They merely convert the energy to heat, so the battery pack cooling fan was activated to prevent overheating during balancing. So charging continued until all cells were fully charged and thus their charge levels were balanced. I bought one of these cell balancers and began using it shortly after Honda installed my 3rd battery pack just after my battery pack warranty expired. This battery pack was still performing well 9 years later when we sold our Insight along with the cell balancer.
Fortunately, the i3 implements a much more sophisticated passive cell balancing protocol that seems to work very well.
I wish that BMW had provided some guidance in this respect. I think the good news is that cell balance seems to be maintained well regardless of how the battery pack is treated. Gonville's experience with a badly unbalanced battery pack seems to be unusual, or maybe he's one of the few i3 owners who would have realized that cell imbalance was reducing the usable battery pack capacity.
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