*3.2. Optimization of Battery Packing Design*

In addition to the number of inlets, the inlet air temperature was varied to 20, 25, and 30 ◦C to find the optimum temperature distribution inside the battery pack. Before entering the battery pack, the inlet air temperature was the conditioning of air temperature from its ambient temperature. The simulation is meant to model the battery operation in a tropical region, with no winter and summer, and where a temperature drop is unlikely to happen. Therefore, the ambient temperature does not change drastically throughout the year and can be considered a constant. We set the ambient temperature at 30 ◦C as it was the average temperature for the tropical region. Therefore, the inlet air temperature of 20 ◦C and 25 ◦C was obtained by cooling the ambient temperature. The number of inlet and inlet air temperature were fundamental factors that affected the BTMS performance, which was also simple and easy to manufacture once the design was optimized.

The simulation results of varying the inlet air temperature to each inlet configuration are presented in Figure 8 as a plot of temperature difference to the average temperature. It was fair to use average temperature as the representative parameter because one of the main objectives of the optimization was to obtain the lowest temperature difference that was still located within the optimal temperature range. The horizontal shaded area represents the optimal operating temperature, while the vertical one represents the optimal temperature difference for the lithium-ion battery, and the intersection between the two areas represents the optimal criteria for an optimized cooling strategy. From Figure 8, it can also be seen that for constant inlet air temperature, the temperature difference decreases as the number of inlets increases. The temperature difference was significantly affected by the number of inlets because higher inlet numbers had a wider inlet area that made it easier for air to reach every spot inside the battery pack.

**Figure 8.** Temperature distribution of LBM model.

Meanwhile, the average temperature was affected by the inlet air temperature more than by the inlet number. This gave the idea of adjusting the inlet number and air temperature parameters to achieve any desired performance. The best airflow configuration was the four inlets configuration with 25 ◦C air inlet temperature. This configuration gave the lowest average temperature and the temperature differences that were still in the optimum range compared to all other configurations. However, using four cooling fans meant that the configuration cost more power to operate while potentially being over capacity. It is unjustified to have a high-performance BTMS that consumes a large amount of power since the objective of BTMS is to optimize the power generation for the vehicle operation. Therefore, the power required for each cooling fan and the inlet air temperature should also be considered to obtain the most energy-efficient cooling strategy giving the best performance.

The required power was calculated as the sum of the power required for operating the cooling fans and generating the sensible heat. The sensible heat represented the power needed to obtain the inlet air temperature of 20 ◦C and 25 ◦C by cooling the air from its ambient temperature of 30 ◦C before entering the battery pack. The sensible heat and the power required were calculated from Equations (5) and (6), respectively.

$$H\_{\rm s} = \mathbb{C}\_p \times \rho \times \mathbb{V} \times dT \tag{5}$$

$$P = n \times P\_f + H\_s \tag{6}$$

where *Hs* is the sensible heat (kW), *Cp* is the specific heat of air (1.006 kJ/kg· ◦C), *ρ* is the density of air (1.202 kg/m3), ∀ is air volume flow (m3/s), *dT* is the temperature difference ( ◦C), *P* is the power required (W), *n* is the number of cooling fans, and *Pf* is the cooling fan rated power (W).

The power required and the resulting temperature of all cooling strategies are plotted in Figure 9. Each strategy is represented by a marker that indicates its required power in *x* axis, and average temperature in *y* axis. The marker shape represents the inlet number of one, two, three, and four inlets by triangle, square, diamond, and circle, respectively. The color represents the inlet air temperature of 20 ◦C, 25 ◦C, and 30 ◦C by blue, green, and red, respectively. The dashed vertical line extending from the top and bottom of the markers represent the temperature range of the battery cells resulting in each corresponding config-

uration. The green shaded area represents the area of optimal operational temperature for the lithium-ion battery.

**Figure 9.** Temperature versus required power.

The optimal cooling strategy has the lowest median temperature and minimum temperature difference that meets the optimal temperature condition while having the least power required. Therefore, the optimal cooling strategy in Figure 9 is represented by the closest marker to the objective point (0 W, 25 ◦C), whose marker and dashed line are located inside the green shaded area. The optimization utilized the Qhull algorithm to find the nearest marker to the objective point. As a result, it was found that the three inlets configuration with the inlet air temperature of 25 ◦C was the best cooling strategy in terms of performance and power consumption and was chosen as the optimized cooling strategy for air-cooled BTMS. The configuration gave a mean temperature of 33.1 ◦C, which met the optimal temperature condition for a lithium-ion battery. However, the maximum temperature difference in this configuration was 14.9 ◦C, which did not meet the requirement. In fact, the temperature differences in all simulated configurations did not meet the requirement. This is the challenge of air-cooling applications for a battery pack.

Compared to the available BTMSs, the optimized design from this study has several advantages. Firstly, the optimized cooling strategy is low cost and easy to manufacture since there are no complex parts needed and it only consists of a few parts. In addition, the optimized design can meet the optimal temperature condition for a lithium-ion battery with low power consumption compared to other cooling methods such as water cooling, heat pipe, and PCMs. Therefore, the three inlets configuration with the inlet air temperature of 25 ◦C can be proposed as a standard design for a battery pack air-cooling system. For future work, when the simplicity is not the main objective, employing variable cooling fan speed might improve the performance while increase the system complexity for the additional control system. This study only discusses the optimization from the temperature distribution perspective. Therefore, further research is required to evaluate other important aspects, such as structural strength and water protection, to complement the design optimization of the battery pack air-cooling system.
