The electrical power is determined by the electrode material and the electrode surface area in the cells. The assembly of several cells results in the energy converter unit, the so-called stack. At the electrodes, the electric potential and voltage are generated through interaction of the two electrolytes. In order to avoid mixing of the electrolytes, the half-cells are separated by a membrane. The membrane is only permeable to certain ions and ensures the charge balance in the electrolytes. Under electrical load the battery can supply electricity. To charge the battery, a power source is connected and the reaction in the half cells proceeds in the opposite direction.