FAQ: How does the balancing of the BMS or SBM work?

The balancing is done only when charging. During the charging, as soon as the voltage of any of the cells goes over the balancing voltage level, the balancing starts. The balancing is in fact working as “by-passing” (or shunting) part of the charging current.

The balancing stops as soon as the voltage level goes bellow the balancing voltage level.

If the charging current is higher than the balancing current, the voltage of the cell will keep increasing.  See also here.