Battery pack/module replacement (out of main)

[mongo]

Fun discussion.

In principal, you are not dealing "with the same level of imbalance". The batteries were designed and manufactured at the same time, with the same materials and operated with the same currents in the same environment so they should, for the most part, degrade the same way. Meaning the capacity imbalances are small and the BMS re-balancing never allows them to have very different SOC.

In the case where one module is markedly different from another, it is a different story. There is no reason to expect that the self-discharge, impedance (thus local temperatures), and other properties would be similar between modules with different materials, different histories and specifically different capacity.

What I meant was that a x Ah capacity difference acts the same whether it is due to a new module being swapped in or an original module losing a cell or two (in the same group).

I think that you would want to pre-condition the modules to a common SOC (thus different voltage due to different capacity) upon installation, and the BMS would need to realize one group was markedly stronger than the others and NOT discharge the weaker groups to match the lower voltage of the strong module.

Agreed, that was what I meant by the BMS targeting being balanced at 100% SOC (versus always keeping the voltages the same)

Keeping the SOC at the low end is easy because it can be charged with whatever the other cells are taking because it's in no danger of reaching its upper limit voltage; if for some reason the strong module goes into a lower SOC than the weak modules (unlikely because of it's lower voltage should reduce the self-discharge), the balancing circuit can discharge the weak modules much more easily because they are low capacity and you can do balancing discharge whether or not the car is being driven (in case it takes a long time). That's not true if you are trying to force it to the high end.

If the high capacity is anywhere in the middle it is easy. If it is too low, then you hit the under voltage limit, if too high, then the over voltage. The amount of balancing needed goes back to the difference in leakage for the two module types. Keeping the higher capacity module at a higher SOC should increase it's leakage which may assist balancing.