Abstract: The state of charge of a lithium-ion cell module including several cells is managed by observing changes in a cell's differential capacity curve exceeding or falling short of a preset limit value during a discharging or charging process of the cell. Each of the changes is a change which is specific for this particular cell and consistent with a specific state of charge of the cell, whereby the ordinal number of a differential capacity curve change is used as a basis for determining a state of charge of the cell consistent with the change or a difference in the states of charge between two or more cells. The thus determined state of charge differences between cells are then sought to be balanced by a balancing system adapted to balance a charge.