Figure 7: Equivalent circuit model representing lead-acid battery. 20
The impedance value, x-intercept from the Nyquist plot (Figure 6 a-d), is observed to increases gradually with cycling for all the temperatures (Figure 8a). However, the increase is higher at low temperatures (-10 & 0 °C) than at higher temperatures (25 & 40 °C) even though the total active material availability and utilization is significantly reduced at the lower temperatures. The lower real impedance at higher temperatures (25 and 40 °C) essentially means that battery activity is improved despite higher degradation.
The ohmic and charge transfer resistance were calculated by fitting the EIS data to equivalent circuit model using ZSimpWin software. The ohmic resistance and charge transfer resistances exhibited similar behavior as impedance values (Figure 8b and 8c). However, it is reported that charge transfer resistance is critical in representing the degradation mechanism as it reflects the reaction mechanism at electrodes.10,20,22 The voltage at 50% discharge cycle (Vdis,50%) was analyzed to further evaluate the capacity degradation with cycling (Figure 8d). These values increase with cycles at all temperatures, however, more rapidly at 25 and 40 °C than at 0 and -10 °C. Higher battery degradation correlates to higher slope of the Vdis,50% vs cycle profiles, which essentially shows that the cell discharge capacity and SOH are decreasing with cycling.