# Adaptive Thermal Control of Cell Groups to Extend Cycle Life of Lithium-Ion Battery Packs
## 2 Experimental
Three thermometers were used to monitor the temperatures of the three cooling plates by attaching one thermometer to the flat side of each plate. The three remaining thermometers were used to monitor the ambient  
Figure 2: Close-up image of the cells (21 mm diameter \(\times\) 70 mm height) and assembly including spot welds and reinforced steel epoxy. temperature, the chiller water reservoir temperature, and the resistor bank reservoir water temperature (used for cooling the high C-rate discharge resistors). Thermal Control--A 30 W submersible pump (Growneer, WP550-3, Amazon, Bellevue, WA, USA) was used to pump water through the cooling plates at approximately 8 L/min. The submersible pump was placed in the cooling reservoir of the chiller (Nestlab RTE 740, Thermo Fisher Scientific, Waltham, MA, USA) which was set to 15 \({}^{\circ}\)C. Clear polyethylene tubing (3/8-inch ID) was used to supply chilled water from the pump to the cooling plates. In addition, the resistor load bank was submerged in a chilling reservoir, which maintained a constant temperature thereby preventing a change in resistance after the current began flowing during discharge. Without this precautionary step, the rapid rise in resistor temperature would have resulted in a resistance change which would have impacted the voltage measurement, and thus the accuracy of the capacity measurement. Cycling Parameters--Cells within the 12 V pack were charged using a 12 V, 50 A switching power supply (s-XHD12V, Mieshile, Amazon, Bellevue, WA, USA) which resulted in an approximate 3C average charge rate per cell (see Molicel data sheet [19] for charge curves). The voltage was fixed at 12 V while the current varied according to the state-of-charge (current starts high at the beginning of charge and then tapers off as state-of-charge increases). This fixed 12 V charging supply resulted in each cell group being charged to approximately 4 V (or \(\sim\)90% SOC). This was performed deliberately to allow a safety factor to prevent any group from exceeding 4.20 V, given the potential for slight imbalances between cell groups. If the power supply was instead set to 12.6 V, then any voltage imbalance would cause some cell groups to exceed 4.20 V, a potentially dangerous situation. The battery pack was discharged using a bank of 16 1-ohm resistors, with an 8 parallel, 2 series configuration. This created an equivalent resistance of 0.25 ohm, which resulted in an average 5C discharge rate per cell. This setup, consisting of a fixed voltage power supply along with a fixed load resistance, although simple, was adequately designed and equipped to conduct the proposed experiments. These high rates of charge and discharge were chosen to deliberately stress the cells and also to complete the experiment consisting of 100 cycles within a reasonable timeframe. During discharge, the stopping criterion for the end of the cycle was determined to be 2.5 V per cell group under load, which resulted in a steady-state voltage of approximately 2.9 V per cell group once the load was removed and 10 min had passed. The discharge stoppage point was automated by commanding the Arduino microcontroller to open the main discharge relay once the voltage of any group of cells reached 2.5 V.  
For capacity measurements, each cell group was discharged over a 1-ohm resistor for a 1C discharge rate capacity test. Capacity measurements were first performed at the outset before starting fast cycling, and then again after 25, 50, 75, and 100 cycles. Two additional capacity tests were conducted after the 10 th and 85 th cycles to assess cell group capacities prior to completing 25 full cycles.