<p>The system (50) has a measuring unit (52) determining voltages lying at individual capacitor cells (32-1 to 32-n) by voltage measurements. An evaluation unit (100) determining a reference voltage for the capacitor cells. Discharging branches (80-1 to 80-n) are connected to capacitor cells and controllable by a discharging unit (102). The discharging unit discharges the respective capacitor cells according to the fixed reference voltage and the lying voltage in the capacitor cells. The evaluation unit determines deviations of a middle voltage from the determined voltage at the capacitor cells.</p>