## 1 Introduction  
Lithium-ion batteries (LIB) have been widely adopted as the onboard power battery cell type for commercial electric vehicles (EV) [1, 2]. To provide an ideal operating environment, the battery thermal management system (BTMS) is essential to maintain the battery cell temperatures within a certain range. Air cooling and liquid cooling are two dominant technical routes for commercial EV BTMS. Presumably, the two methods have pros and cons in different aspects. Liquid-cooling BTMS has higher cooling efficiency and capacity at the cost of its high expense and complex structure. Air-cooling BTMS is well-known for its simple structure, robust performance, and safe character (no leakage risk) but delivers a much less efficient cooling performance than its liquid-cooling counterpart [3]. With the fierce competition of commercial EV manufacturing around the world, the overall cost has been more and more underscored by many original equipment manufacturers (OEMs), not only for survival but also for more profits. The strong urge for cost reduction has aroused interest in the air-cooling BTMS. To cope with this economical and technical impulse, this study aims to improve the heat transfer efficiency and cooling performance of a basic air-cooling BTMS design. As can be seen in the relevant review articles, there are many approaches to improve the cooling efficiency of air-cooling BTMSs [4]. Changing the cooling channels, modifying the inlet and outlet, adopting more thermally conductive materials, adding fins and vortex generators, and cell-cooling methods are all effective to improve overall cooling performances. For the cooling channel improvement, one novel approach is to adjust the LIB cell spacings to form variable cooling channels, obtaining different cooling performances. Many types of research are about optimizing the battery pack cell spacings for either prismatic or cylindrical cells. For prismatic cell battery packs, [PERSON] et al. [5] found smaller gap spacing between cells could lower the temperature rise. Moderate gap spacing was beneficial to improve the temperature uniformity. Uneven gap spacing could improve the temperature distribution but hardly reduce the maximum temperature rise. [PERSON] et al. [6] proposed a wedge-shaped layout air-cooling BTMS in a row of 12 parallel prismatic cells. The cell clearance was found to affect the flow rate remarkably and cause a higher level of noise. The simulation results also show that smaller clearance (3 mm clearance design is better than both 5 mm and 7 mm clearance designs) was better for heat dissipation at the cost of greater energy consumption. [PERSON] et al. [7] optimized the battery cell spacings of a parallel rectangular battery pack of 12 \(\times\) 2 prismatic cells. They found the spacing around the hottest cell should be increased while the spacing around the coolest cell should be reduced to obtain a lower maximum temperature and smaller temperature differences. The results show that the adjusted cell spacings remarkably reduced the maximum temperature difference by 42%. [PERSON] et al. [8] investigated the influences of cell number and spacing distribution on the cooling performance of a typical parallel air-cooling BTMS with a rectangular battery pack of N \(\times\) M prismatic cells. They adjusted the cell spacings to acquire more homogeneous air flow rates within the cooling channels by using the optimization strategy of reducing the gap spacing of the high air flow rate cooling channel and increasing the gap spacing of the low air flow rate cooling channel. The air flow rates within two channels adjacent to the inlet and outlet are suggested to be 70% of the other channels. The maximum temperature and temperature difference were reduced by 4.0 K and 69% by using the cell spacing optimization approach. They also investigated the cooling performance of a symmetrical air-cooling BTMS with an uneven cell-spacing structure [9].