Electrical testing of negative limited mono-pocket plate cells, cyclic voltammetry and surface analysis by X-ray diffraction were used to investigate the origin of the second discharge plateau at the cadmium pocket plate electrode. The electrical testing results show that there is a direct coupling between the appearance of a second discharge plateau and a high carbonate concentration in combination with high discharge rates (1 C 5A). An exchange to a carbonate free electrolyte results in recovering of the cell. Voltammetric measurements show that the reduction of CdCO 3 takes place in the same potential region as the reduction of Cd(OH) 2. Thus, the presence of CdCO 3 in the discharged negative electrode cannot give rise to a second voltage plateau or capacity loss. However, a decrease in the hydroxide concentration results in a positive shift of the oxidation potential. These results are interpreted in the context of the porous structure of the pocket plate cadmium electrode. For a high carbonate concentration and large discharge rates, the hydroxide concentration within the pores are depleted leading to a positive potential shift and the appearance of a second discharge plateau.