4 minute read
Definition of NiMH 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).
Figure 3–6 Recovery of battery cell discharge voltage.
Under steady state conditions, the cell voltage at a known current draw is E0 - iRe, where Re is the effective internal resistance of the NiMH cell. Re is the sum of the Rh and Rd. Under transient discharge conditions, as shown in Figure 3–6, the initial voltage drops immediately to E0 - iReh and then rises exponentially, with time constant Cp ¥ Rd to a steady state voltage. This discharge condition reverses once the load being applied is removed from the battery as seen in Figure 3–6 above. Note that the slow recovery of NiMH cell voltage after removal of the load after approximately 11 minutes is attributed to the delayed resistance Rd. This behavior is identical to the effect noticed during discharge between 4 and 11 minutes.*
For most applications, unlike EV applications, the steady state voltage is adequate for describing the battery performance. This is owing to the fact that the time constant for most cells is small—typically, the time constant is less than 3% of the discharge time. Although the instantaneous resistance of the NiMH cell is comparable with NiCd cell, the delayed resistance is approximately 10% higher. For this reason, the steady-state voltage for the NiMH cell is lower than that of NiCd.
*Note: This discussion is also made in Chapter 6 to describe the battery discharge characteristics.
NiMH Battery Voltage During Discharge
The discharge voltage profile for an NiMH cell is affected by transient effects, discharge temperature, and discharge rate. Under most conditions, the voltage curve retains the flat plateau before a rapid drop off termed as the knee of discharge curve, as observed between 80% and 100% discharge. A typical discharge profile for a cell discharged at a fivehour rate (0.2C) results in the open circuit voltage drop from 1.25V to 1.2V. This discharge occurs rather rapidly. As seen by the flatness of the plateau and the symmetry of the curve, in Figure 3–7 the midpoint voltage (MPV—the voltage when 50% of the available cell capacity is depleted during discharge) provides a useful approximation to the average voltage available throughout the discharge cycle.
Figure 3–8 summarizes the operating voltages for a 90Ahr NiMH battery. As the DOD of the NiMH battery varies with the discharge rate, the amount of useful current available from the battery and thus the battery pack decreases. The discharge of the battery pack is continued till the first battery in the pack is fully discharged and reaches the cut off voltage of 8V.
Table 3–2 tabulates the operating voltage and voltage limits of the 90 Ahr NiMH battery with the operating voltage limit of 8V.
Figure 3–7 Variation of midpoint voltage (MPV) with temperature.
MPV Variation (V)
1.26 1.24 1.22 1.2 1.18 1.16 1.14 1.12 1.1 1.08
1 2 3 4 5 6 7 8
Temperature (°C)
Figure 3–8 Variation of NiMH battery voltage with respect to the depth of discharge (%DOD).
Discharged Remaining % SOC % DOD Capacity (Ahr) Capacity (Ahr) 90 10 9 81 80 20 18 72 70 30 27 63 60 40 36 54 50 50 45 45 40 60 54 36 30 70 63 27 20 80 72 18 10 90 81 9 5 95 85.5 4.5 0 100 90 0
