*4.1. Battery Layout Analysis*

The battery pack contains 20 type 18650 batteries. There are three possible layout configurations for a rectangular layout: 1 × 20, 2 × 10 and 4 × 5. The layouts are shown in Figure 7. To choose the best scheme, the three schemes need to be simulated and predicted.

**Figure 7.** Battery layout configuration (**a**) 1 × 20, (**b**) 2 × 10, (**c**) 4 × 5.

To fully compare the advantages and disadvantages of these three configurations and select the best battery configuration, a single variable control method was used to make a comparative analysis of the schemes. For each arrangement, there is an air inlet and an air outlet equal in size to a rectangle of 20 × 49 mm. The module is cooled with constant wind speed in both the transverse (h) and longitudinal (v) directions. The six temperature distributions of the three battery configurations are shown in Figure 8. During the 15-min discharge, the MaxT variation of the layout schemes at the end of each minute is shown in Figure 9.

**Figure 8.** Temperature distribution of the battery layout schemes.

**Figure 9.** The MaxT variation in the layout schemes over time.

In Figure 8, it can be seen that the low-temperature area appears at the entrance and the high-temperature area is distributed at the exit. This is in line with the actual situation, because the air temperature entering the inlet is room temperature (298.15 k). When the air flows through the battery surface, which has a higher temperature, it will absorb the heat of the battery leading to a gradual increase in the temperature of air from the inlet to the outlet. Additionally, the longer the air travels through the cooling system, the hotter it gets. As seen in Figure 9, the cooling e ffect of the 4 × 5 battery arrangemen<sup>t</sup> is better in both directions than the other two arrangements. Therefore, the battery layout configuration of 4 × 5 was selected as the battery layout.

### *4.2. Inlet and Outlet Position*

As mentioned above, the optimal battery layout configuration is a 4 × 5 scheme. The four sides of the battery pack can be set as an air inlet and air outlet. With each side divided into left, center and right parts, all 12 parts can be used as inlet or outlet, as shown in Figure 10.

**Figure 10.** Optional choices of inlets and outlets.

According to symmetry, the duplicate and inferior schemes were removed, leaving 18 schemes to choose from. These 18 schemes are shown in Figure 11.

**Figure 11.** Schemes for the battery pack with a single inlet and single outlet.

The MaxT change curve and TSD change values of all schemes were extracted, as shown in Figures 12 and 13. The temperature of all schemes equals room temperature (298.15k) at the initial moment. With the discharge process, the MaxT increases gradually and the rate of increase gradually slows down. It can be seen that the final MaxT of scheme 1, 2, 7, and 15 is relatively high. The final MaxT of scheme 3, 4, 12, and 13 is relatively low. In addition, the air speed at the inlet and outlet are listed in Figure 14. The speed at the inlet remained at 1 m/s while it was no more than 0.2 m/s at the outlet. The outlet air speed of scheme 6 is the lowest, while the outlet air speed is the highest in scheme 3. The difference in outlet air speed indicates the complexity of flow. The eight schemes for the fluid velocity distribution path are shown in Figures 15 and 16. It can be seen that in schemes 1, 2, 7 and 15, the inlet and outlet are too close and the pressure difference between the inlet and the outlet is the largest. After entering the inlet of the battery pack, the air fails to flow through most areas of the battery bag, resulting in most of the heat not being taken away in time. Therefore, the heat dissipation effect of the battery pack is poor. Scheme 3, 4, 12, and 13 avoid this problem. The fluid flows through almost all the cells in the battery pack.

**Figure 12.** The MaxT variation in schemes for the battery pack with a single inlet and single outlet over time.

**Figure 13.** TSD of schemes for the battery pack with a single inlet and single outlet over time.

**Figure 14.** The air speed at the inlet and outlet based on inlet and outlet position.

**Figure 15.** Velocity path line of scheme 1, 2, 7, 15.

**Figure 16.** Velocity path line of scheme 3, 4, 12, 13.

### *4.3. Number of Inlets and Outlets*

In order to obtain a better BTMS, a model with multiple outlet modes was studied. Options include one inlet and two outlets, two inlets and one outlet, two inlets and two outlets, and so on. Eighteen one inlet and one outlet options were discussed above in Section 4.2. On the basis of these one inlet and one outlet scheme, the one inlet and two outlet configurations are further discussed. Considering the symmetry, the inlet is arranged in the middle the side. The specific scheme is shown in Figure 17.

**Figure 17.** Schemes for the battery pack with a single inlet and double outlet.

According to the information shown in Figures 18 and 19, the MaxT is in scheme 23 and the minimum is in scheme 24. The largest TSD is in scheme 19 and the smallest is in scheme 24. As shown in Figure 20 by comparing scheme 19 and scheme 23, the short flow path of the fluid will remove local heat, and the temperature of the whole battery pack will be higher. The longer the fluid flow path, the more uniform the temperature distribution of the battery pack and the smaller the TSD. The MaxT and the TSD of different inlet and outlet schemes are caused by the change in the fluid flow state. The air speed at the inlet and outlet is displayed in Figure 21. The speed at the inlet was 1 m/s. There were discrepancies in the outlet speed. The zigzag change manifested by several schemes were at similar outlet speeds.

**Figure 18.** The maximal temperature of schemes for the battery pack with a single inlet and single outlet over time.

**Figure 19.** TSD of schemes for the battery pack with a single inlet and single outlet over time.

**Figure 20.** Velocity path line of scheme 19, 23, 24.

**Figure 21.** The air speed at the inlet and outlet based on the number of inlets and outlets.
