# Adaptive Thermal Control of Cell Groups to Extend Cycle Life of Lithium-Ion Battery Packs
## 2 Experimental  
Figure 1 shows the experimental setup, consisting of six cells with three cooling plates. Using groups of two parallel cells allowed for small initial differences in capacity to be minimized. Cell Selection and Cooling Plates--Molicel INR-21700-P42A [19] cylindrical lithium-ion cells were used for the experiment due to their high discharge rating (45 A continuous). Cooling plates were obtained by disassembling a Smart Fortwo electric car battery pack and repurposing them for this experiment. Nylon spacers were placed between the cooling plates to electrically insulate them from each other and eliminate the risk of a short circuit in the pack. An image of the experimental setup with epoxy-bonded cells is shown in Figure 2.  
Series/Parallel Battery Pack Assembly--The experimental battery pack comprised six 21700-type cells (21 mm diameter, 70 mm height). The nominal capacity and voltage of each cell was 4.2 Ah and 3.6 V, respectively [8]. The pack was assembled into a 3 series, 2 parallel (3S2P) configuration. Three groups of cells in series were chosen to enable electrical measurements of a pack with cell groups located at the negative and positive terminal ends, as well as a middle-sandwiched cell group, in addition to their thermal aspects. Having two cells in parallel within each group reduced the capacity variability across groups due to manufacturing discrepancies in individual cells. The cells were thermally bonded to  
Figure 1: Test system layout: Pack of six cells (three pairs in series) with a thermal management system consisting of three cooling plates. the cooling plates by spot welding nickel tape (40 mm long \(\times\) 10 mm wide \(\times\) 0.2 mm thick) first to the negative side of the cylindrical shell casing (overlapping the diameter by a large margin), and then directly spot welding the overlapping nickel tape to the cooling plates. This method ensured excellent thermal contact between the cells and base plates, as nickel has a thermal conductivity of about 97 W/mK. In addition to spot welding, steel-reinforced epoxy was used to strengthen the physical bond between the base of the cells and the cooling plates. The thermal conductivity of epoxy by itself is quite low (\(<\)1 W/mK), which is why the nickel spot welding technique was employed. One difference between this experimental setup and a typical commercial battery pack is that we could incorporate a metal-to-metal connection between the cell casing and cooling plates as the cooling plates are electrically insulated from each other. In commercially produced packs, an electrically insulating layer must be inserted between the cell casing and cooling plate because many groups of cells are mounted in series onto the same cooling plate. In our case, we employed multiple cooling plates since our goal was to achieve the best possible thermal contact between the cell and the plate. _Electronics_--An Arduino Uno (Arduino AG, Scarmagno, Italy) was used as a simple microcontroller to control the relays and log data to a serial monitor via USB. Small relays (10 A) were used for capacity measurement while large relays (100 A) were used for high-current cycling. Voltage sensors (3-01-1125, HiLetgo, Shenzhen, China) were used to monitor voltage to within 4.89 mV resolution. [PERSON]'s law was used to calculate the current from the known resistance and voltage values. A 5 s time step was used for data acquisition. Nine digital thermometers (DS18B20, Gikfun, China) were used to monitor temperatures with \(\pm\)0.5 \({}^{\circ}\)C accuracy. In the DS18B20 specification manual [20], a typical error curve is shown in Figure 1, detailing the tendency for below-actual temperature readings at low temperatures and above-actual temperature readings at high temperatures, but all fall well within the 0.5 \({}^{\circ}\)C specification. Three digital thermometers were used to monitor the temperatures of the three cell groups by placing one thermometer at the top-center of each pair of cells and securing it with electrical tape.