Figure 4: Charge curves at C/10 rate for (a ) -10 °C, (b) 0 °C, (c) 25 °C and (d) 40 °C
Discharge capacity of the cells calculated from the discharge profiles is illustrated in Figure 5a. It was observed that discharge capacity is close to the theoretical capacity (5Ah) for 25 °C and 40 °C, while the initial discharge capacity at -10 °C is higher than the result at 0°C, the discharge capacity drop is much significant in -10 °C because lower temperature increases the internal resistance and reduces the capacity.19 Steep slope confirms that degradation is higher at 40 °C due to enhanced ageing of the electrodes from faster reaction kinetics at elevated temperature. The capacity degradation after 130 cycles are 92, 74, 69 and 17% for 40, -10, 25 and 0 °C, respectively. Moreover, the cell degradation is insignificant at 0 °C even though the total discharge capacity is low. The state of health (SOH) is a measurement that reflects the general condition and ability to deliver the specified performance compared to that of a fresh battery. SOH takes into consideration factors such as charge acceptance, internal resistance, voltage, and self-discharge.4,20,21 It is a measure of the battery’s long-term capability and available lifetime energy throughput. Health of the battery tends to deteriorate gradually due to irreversible physical and chemical changes, hence SOH is estimated by multiple methods including impedance, conductance and discharge capacity (current/actual).4,20 SOH was calculated using instant discharge capacity and initial capacity (after 1stcycle) as illustrated in Figure 5b. SOH was observed to be highest for 0 °C ranging from 95% (1st cycle) to 89% (130th cycle). However, the SOH decreased sharply for cells discharged at -10 °C (78% to 46%), 25 °C (67 to 28%), and 40 °C (73 to 10%). Significant SOH loss was observed for 40 °C confirming major degradation.