## 5 Discussion
On the contrary, the T\({}_{\rm max}\) of Design 5 (301.9 K), Design 6 (302.3 K), and Design 7 (302.38 K) in GVSG rank first, second, and third, respectively, demonstrating a significant cooling advantage of the gradient vertical spacing designs over the constant vertical spacing ones at higher flow rates. For the \(\Delta\)T, Design 7 (2.6 K) is 0.44 K lower than Design 1 (3.04 K) while Design 2 (3.44 K), Design 3 (3.33 K), Design 5 (3.34 K), and Design 6 (3.23 K) are very close to each other. As a result, Design 7 is the optimal design at 52.8 \(\times\) 10\({}^{-3}\) m\({}^{3}\)/s. As a specific reference and negative case to the GVSG, Design 4 almost came in last place in both evaluation criteria at all flow rates (except the fourth place of \(\Delta\)T at 10.56 \(\times\) 10\({}^{-3}\) m\({}^{3}\)/s). The gradient vertical spacing design with fewer cell side inlets (four-cell-side inlet/six-cell-side outlet) has been proven to be the most ineffective air-cooling BTMS design. As discussed in some previous studies, the reason could be explained by the field synergy principle (FSP); the temperature gradient field vector within the battery pack (from higher temperature side, i.e., six-cell-side, to lower temperature side, i.e., four-cell-side) is 180 degrees opposite to the air velocity field vector (from four-cell-side inlet to six-cell-side outlet), which significantly violates the essence of the FSP; reducing the intersection angle between velocity and temperature gradient field vectors could enhance the heat transfer coefficient. So, for the gradient vertical spacing design, the inlet should always be put on the side with a greater cell quantity. Generally, 2C discharging is not a small operating current for EVs. Theoretically, the average EV discharging rate should be 0.1C if the fully charged battery pack could provide a 10 h operating length. One finding in this study was that larger vertical spacing for the constant vertical spacing designs was unable to improve the cooling performances of the air-cooling BTMS. On the contrary, it would adversely increase the total volume of the battery pack. For the EV applications, it is important to find the optimal spacing value (reasonably smaller value) which could both deliver top cooling performance and save space. The gradient vertical spacing design could deliver better cooling performance when the inlet air flow rate is high. We recommend that this property is applied to the high power or performance EVs whose short-term discharging rate could reach 5C or higher. However, the compromise is that its low flow rate cooling performance is not as efficient as the constant vertical spacing design. Different spacing designs shall be applied to their favourable working scenarios according to the specific requirement and operating characteristics of the real EV applications. As mentioned in Section 2.2, a 45-cell pack is a limited yet reasonable size for a basic unit module in an EV battery pack. The size needs to be larger if the actual battery power or capacity demands of the EV are higher. Larger battery packs will also consume more computational and time resources. So, a larger air-cooling BTMS unit of around 100 cylindrical cells will be a future extension of this study. In addition, the experimental validations will also be carried out in the near future to not only prove this theoretical study, but also brighten our research in this field.