54
ELECTRIC VEHICLE BATTERY CAPACITY the PbSO4 crystals. In addition, larger current densities result in smaller PbO2 particles. At higher current levels, the formation of a more porous surface layer on the positive grid. During the process of anodic polarization of the metal electrode, an insoluble anodic layer is formed at the surface of the electrode. This layer may be polycrystalline or a homogeneous nonporous film. Even at high charge current levels, the passivation layer builds up to the point where discharge capacity can be severely limited. The formation of the crystalline layer is determined by the changes in potential and resistance. When the entire electrode surface is covered by PbSO4 crystals, the potential of the electrode increases rapidly and the resistance remains constant. The electrode is passivated with an increase in the battery potential. This increase in the battery potential does not affect the capacitance and resistance values. The PbSO4 layer tends to undergo a conversion to PbO2. Under open circuit conditions, the battery potential takes values lying between the equilibrium potentials of the PbSO4 and the PbO2/PbSO4 electrodes. Thus the VRLA battery undergoes PbSO4 passivation in two ways: by anodic polarization of the electrode and by self-passivation under open circuit conditions. In order to achieve the maximum cycle life from the VRLA batteries, it is both required that the DOD be kept at low as possible and that the charge current limit is as high as possible. This ensures that the passivation of the battery electrodes is at a minimum.
DEFINITION OF NIMH BATTERY CAPACITY NiMH batteries are rated with an abbreviation C, the capacity in Ahr. The C rating for the NiMH battery is obtained by thorough conditioning of the individual NiMH cells. This can be established by subjecting the cell to a constant-current discharge under room temperature. Since the cell capacity varies inversely with the discharge rate, capacity ratings depend on the discharge rate used during the discharge process. For NiMH batteries, the rated capacity is normally determined at a discharge rate that fully depletes the cell voltage in five hours. For the purpose of electrical analysis of the battery cell, the Thevenin equivalent circuit is used. This circuit models the circuit as a series combination of the voltage source (E0), a series resistance (Rh = the effective instantaneous resistance), and the parallel combination of a capacitor (Cp = the effective parallel capacitance) and the resistor (Rd = the effective delayed resistance).
