Unfortunately, BMW has not explained how the i3's cell balancing works. The i3's cell balancing is reported to be the less expensive passive variety. Passive balancing generally works by discharging cells with higher voltages until the voltages of all cells have been equalized.
I have read conflicting estimates of the speed of cell balancing. One estimate claims that cell balancing is very slow to prevent the heat generated from overheating the battery management system circuit boards. Another estimate claims that so little heat would be generated that cell balancing could occur quite rapidly. So we don't know whether a significantly unbalanced battery pack could take hours or weeks to balance.
Balancing should not require active charging, so it should function while an i3 is parked and not charging. If this is true, the charging method used should not matter.
An advantage to leaving an EVSE plugged in after charging has completed would be to top up the charge if balancing dropped the charge level significantly. I would think that with a well-balanced battery pack, so little balancing would occur that no topping up would be needed.
