# Adaptive Thermal Control of Cell Groups to Extend Cycle Life of Lithium-Ion Battery Packs
## 3 Results  
Five calibration cycles were performed at the beginning of the experiment to determine the temperature rise of the cells, cooling plates, and water reservoirs during charge and discharge. A high level of temperature uniformity was expected across all three cell groups (denoted as CG1, CG2, and CG3) due to the high flow rate of water through the system. Figure 3 confirms that all three cell groups stayed within about 1 \({}^{\circ}\)C of each other for the duration of this 5C-rate discharge. It should be noted that although the cooling water was pumped through the system during this calibration cycle, the chiller was not turned on as with subsequent test cycles. All three cell groups started at a room temperature of 20 \({}^{\circ}\)C and rose to approximately 43 \({}^{\circ}\)C during the 400 s discharge. As the chiller reservoir was reasonably large, its temperature only rose from about 16 \({}^{\circ}\)C to 18 \({}^{\circ}\)C during this period. The resistor water reservoir temperature rose from about 14 \({}^{\circ}\)C to 18 \({}^{\circ}\)C. The resistor reservoir did not require active cooling during the initial calibration cycles since its maximum temperature was verifiedto not exceed 35 \({}^{\circ}\)C. Another calibration test was performed to ensure voltage uniformity of the three groups throughout discharge. It was confirmed that the voltage difference between cell groups did not exceed 0.1 V during discharge. After performing the calibration cycles, the chiller was turned on and the system was set up to run autonomously for 100 charge/discharge cycles. The average maximum temperature of actively cooled cells during charge was 27 \({}^{\circ}\)C, compared with un-cooled cells which reached a maximum of 34 \({}^{\circ}\)C during charge. During discharge, the peak average temperature for cooled cells reached 40 \({}^{\circ}\)C, compared to un-cooled cells which reached 45 \({}^{\circ}\)C.  
Figure 4 shows the capacity degradation of each cell group throughout the experiment. Since each cell group had two cells in parallel and the voltage ranged from 4.0 V down to 2.9 V under load, the starting capacity before the first fast cycle test (denoted as the 0 th cycle) was just under 7 Ah for each cell group (the manufacturer's specification sheet stated 4.2 Ah per cell for a voltage range of 4.2 V to 2.5 V [19]). This voltage range was chosen to deliberately not span the full voltage range to ensure a safety margin in case the cells became unbalanced. This was especially important to restrict the cells below 4.2 V to prevent a thermal runaway. The accuracy of the capacity measurement was estimated to be approximately 0.03 Ah after repeated testing on the same cell group. For the first 50 cycles, all three cell groups were actively cooled with the chiller reservoir set to 15 \({}^{\circ}\)C. The capacity of all cell groups declined steadily at about 3 mAh/cycle but remained within \(\pm\)0.03 Ah of the mid-range value (CG1) which is within the capacity measurement accuracy. Hence, it can be inferred that there is no significant difference between cell group capacities after the first 50 cycles. In order to deliberately induce a capacity imbalance, from the 50 th to 75 th cycle, only CG1 was cooled while CG2 and CG3 were not cooled. This resulted in a faster degradation rate of about 8 mAh/cycle for the two latter groups, as shown in Figure 4 while CG1 continued to degrade at the same slower rate of 3 mAh/cycle. The higher degradation rate  
Figure 3: Fifth calibration cycle temperatures during discharge (5C-rate). The chiller was not turned on during the calibration experiment, however, water was flowing through the system with an uncooled reservoir. is due to a significantly higher operating temperature of the cell which causes the thermal breakdown of various components of the cell (for example: anode, cathode, electrolyte, and SEI layer breakdown).