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.