Electrical testing of batteries, voltammetry and surfaces analysis by X-ray diffraction (XRD) and Fourier transformation infrared (FTIR) were used to investigate the origin of the second voltage plateau. The electrical testing results show that there is a direct coupling between graphite loss and the appearance of the second voltage plateau. The graphite loss was shown to increase for float charging at elevated temperatures and high potentials. The effect of graphite particle size distribution was also investigated. Voltammetry of discharged material, Ni(OH)2, and charged material, β-NiOOH and γ-NiOOH, in carbon paste showed that the second voltage plateau did not originate from the redox properties of the material, or the presence of carbonate in the material or in the electrolyte. Therefore, the second voltage plateau is believed to appear whenever an increased internal resistance arises, may it be due to graphite loss, swelling or some other factor.