An arrangement for charging batteries, the arrangement comprising: cell groups that include one or more cells, a battery pack that comprises two or more cell groups connected in series, monitoring units for the cell groups, wherein each monitoring unit is arranged to manage the voltage ( U B) of a cell group by controlling a part, i.e. a bypass current ( I S) of a charging current to pass through a bypass circuit of the monitoring unit bypassing the cell group when the voltage ( U B) of the cell group reaches a reference voltage, and wherein each monitoring unit comprises means for determining control data ( ctrl) to be transmitted to a charger, and the charger arranged to charge the battery pack and comprising means for charging the battery pack in response to control data ( ctrl) determined by any monitoring unit, wherein each monitoring unit comprises an amplifier (4.1), an inverting input of each such amplifier (4.1) is coupled to receive, via a first summing resistor (4.4) and a second summing resistor (4.5), a sum of an actual cell voltage value ( U B,0) of the respective cell group and a voltage ( U BIS,0) proportional to the voltage ( U B) of the respective cell group and the respective bypass current ( I S), a non-inverting input of each such amplifier is coupled to receive the reference voltage ( U REF), each monitoring unit is arranged to allow the voltage ( U B) of the cell group to increase in response to an increase in the bypass current ( I S) of the bypass circuit of the monitoring unit, the means for determining control data are arranged to determine the control data ( ctrl) in response to the voltage ( U B) of the cell group and the charger comprises means for changing a charging voltage in response to the control data (ctrl) determined by any monitoring unit. cell groups that include one or more cells, a battery pack that comprises two or more cell groups connected in series, monitoring units for the cell groups, wherein each monitoring unit is arranged to manage the voltage ( U B) of a cell group by controlling a part, i.e. a bypass current ( I S) of a charging current to pass through a bypass circuit of the monitoring unit bypassing the cell group when the voltage ( U B) of the cell group reaches a reference voltage, and wherein each monitoring unit comprises means for determining control data ( ctrl) to be transmitted to a charger, and the charger arranged to charge the battery pack and comprising means for charging the battery pack in response to control data ( ctrl) determined by any monitoring unit, wherein each monitoring unit comprises an amplifier (4.1), an inverting input of each such amplifier (4.1) is coupled to receive, via a first summing resistor (4.4) and a second summing resistor (4.5), a sum of an actual cell voltage value ( U B,0) of the respective cell group and a voltage ( U BIS,0) proportional to the voltage ( U B) of the respective cell group and the respective bypass current ( I S), a non-inverting input of each such amplifier is coupled to receive the reference voltage ( U REF), each monitoring unit is arranged to allow the voltage ( U B) of the cell group to increase in response to an increase in the bypass current ( I S) of the bypass circuit of the monitoring unit, the means for determining control data are arranged to determine the control data ( ctrl) in response to the voltage ( U B) of the cell group and the charger comprises means for changing a charging voltage in response to the control data (ctrl) determined by any monitoring unit.