**1. Introduction**

In the last three decades, electric vehicles (EVs) have been developed rapidly and have brought a massive transformation in the automotive industry due to their low emission and energy-efficient advantages over internal combustion engine vehicles (ICEV) [1,2]. However, despite its rapid development, the current technology of EV still has several drawbacks compared to ICEV. One of the main disadvantages is the high battery cost, which is more than 40% of the total EV price [3]. This high cost is partly caused by the advanced technological implementation in the battery packing to assure high performance and high safety standards, such as complex cooling systems, massive structural packing protection, and advanced electrical system for energy management and control.

The batteries experience high charging and discharging rates during EV operation, increasing their surface temperature [4]. The frequency of charging and discharging also impacts battery degradation, decreasing the battery's lifetime [5]. To realize the best performance and longest lifetime, the batteries need to be operated under the optimal

**Citation:** Widyantara, R.D.; Naufal, M.A.; Sambegoro, P.L.; Nurprasetio, I.P.; Triawan, F.; Djamari, D.W.; Nandiyanto, A.B.D.; Budiman, B.A.; Aziz, M. Low-Cost Air-Cooling System Optimization on Battery Pack of Electric Vehicle. *Energies* **2021**, *14*, 7954. https://doi.org/10.3390/ en14237954

Academic Editor: Artur Bartosik

Received: 21 October 2021 Accepted: 22 November 2021 Published: 28 November 2021

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temperature condition. This optimal temperature condition for a lithium-ion battery consists of two terms: (1) the optimal operating temperature range, which states the temperature range where a battery cell gives optimal charging and discharging rate while maintaining the longest life cycle; and (2) the maximum temperature difference, which states the maximum difference in temperature between each battery cells to one another to provide relatively uniform charging and discharging rates.

The optimal temperature for lithium-ion battery cells to operate is in the range of 25 to 40 ◦C, with a maximum temperature difference among battery cells of 5 ◦C [6]. Operating outside the optimal temperature range can decrease the battery's performance significantly [7]. Moreover, safety issues like thermal runaway may arise when the battery operates in high-temperature conditions [8]. Temperature differences among battery cells may occur due to an inappropriate cooling system, especially when air cooling is applied. These differences may cause each battery cell to have different charging–discharging rates. Therefore, they are not beneficial because the battery management system (BMS) must work hard in balancing the state of charge (*SOC*) of the batteries, which can degrade its lifespan faster.

A battery thermal management system (BTMS) plays an essential role in maintaining the temperature of batteries at the optimal operating temperature [9]. An optimum BTMS can also reduce the workload of the BMS by lowering the temperature differences among battery cells. Therefore, various kinds of BTMS are applied and installed in the battery packing, such as air cooling [10], liquid cooling [11], heat pipe [12], and phase change materials (PCMs) [13]. Being a novel medium for BTMS and having high efficiency and stable performance in extreme conditions, PCMs have gained popularity in recent times. However, PCMs have the disadvantages of having low conductivity and needing to be regenerated after completely melted [14].

On the other hand, heat pipe systems do not suffer from these drawbacks, and they do not require an external power supply. They have high conductivity and efficiency in reducing battery temperature rise, but the equipment is complicated and not conducive to the practical applications of EV [15,16]. Another thermal management system, liquidcooled BTMS, is also complex with its many supporting devices, like pumps, fans, and pipes, making it costly and vulnerable to the risk of leakage that may lead to a short circuit [17].

Another cooling method in BTMS is an air-cooling system. This system has a simple configuration, low initial and maintenance cost, simple integration, and it possesses no risk of leakage, making it more favorable in the market compared to the methods mentioned above. Moreover, air-cooling systems can significantly lower battery manufacturing costs, which directly reduces the EV price. However, despite its advantages, low heat transfer coefficient, uneven temperature distribution, and low efficiency are the main drawbacks of air-cooled BTMS [18,19]. To overcome the low heat transfer coefficient of air as the cooling medium, a hybrid system of BTMS was developed either by combining air-cooling with PCMs [20] or by integrating air-cooling with mini-channel liquid cooling [21]. These studies successfully lower the battery's temperature, but on the other hand, increase the BTMS power consumption.

Adjusting the structure and flow configuration of the battery pack could also be beneficial to lower the temperature difference between battery cells and increase the efficiency of air-cooled BTMS. Several studies were conducted to achieve this result by optimizing the shape of battery pack [22,23]. Xu et al. [22] discovered that a horizontal battery pack with a double U-type duct could improve the heat dissipation performance of the air-cooling system in various conditions. Zhang et al. [23] minimized the temperature difference in battery packs for prismatic battery cells for Z-, U-, and I-types air-cooled BTMS by optimizing the widths of parallel cooling channels and divergence/convergence ducts. Other studies also have focused on improving the air-cooled BTMS by adding parts to the battery pack [24,25]. Mohammadian et al. [24] studied that thermal management of aircooling systems of high-power lithium-ion batteries could be enhanced by implementing

aluminum metal porous. Hong et al. [25] improved air-cooled BTMS performance in reducing temperature differences by adding a secondary vent. And some studies did both, optimizing the shape and adding parts to the battery pack. Jiaqiang et al. [26] improved the performance of the air-cooling strategy by locating the lateral inlet and outlet on different sides and utilizing the baffle plates. Other studies have tried to improve air-cooled BTMS performance by optimization. Liu et al. [27] performed manifold size optimization to improve J-type BTMS thermal performance under varying working conditions, resulting in the optimal configurations for each battery working condition. To achieve optimal performance for different battery working conditions, a valve control mechanism was needed to control the manifold size. Chen et al. [28] found that optimization of airflow parallel outlet position can improve the performance of J-type BTMS for prismatic battery cells; although, optimization for the airflow parallel inlet position is not as effective.

The studies mentioned above [22–28] may have successfully improved BTMS performance, but their practical implementation on EVs is still challenging, especially in reliability, energy density, and feasibility due to its complexity. Moreover, there has been no standard and design guideline regulating the battery packing until now, which leads to complex designs for battery packing. In addition, to improve the applicability, the optimization design should consider the air properties, number and position of cooling fans, spacing between cells, and other related factors [29]. These factors must be determined during the design process, together with maintaining lower manufacturing costs.

Despite the numerous studies conducted to improve the performance of air-cooling systems either by combining the air-cooling system with another type of cooling system or adjusting the structure and airflow configuration of the battery pack, not a lot have been done by optimizing the number of cooling fans and the inlet air temperature. By optimizing these parameters, this research aimed to develop a simple yet reliable air-cooling system that can simultaneously maintain high energy density by conditioning the battery temperature inside the optimal temperature condition while having low manufacturing cost.

Thermal analysis was necessary to investigate the effect of the number of cooling fans and inlet air temperature on the BTMS performance for the optimization. Some studies have performed thermal analysis for electric vehicle batteries using simulation. For example, Raharjo et al. [30] conducted a thermal analysis of modular battery by computational fluid dynamics (CFD) simulation to understand its thermal behavior, prevent overheating, and maintain battery life. Divakaran et al. [31] performed finite element simulation to analyze 18,650 lithium-ion batteries' thermal behavior under two conditions: with and without cooling systems.

To understand the cooling phenomenon, the battery packing was built in a 3D numerical model and analyzed using CFD simulation based on the lattice Boltzmann method (LBM). The effects of the number of cooling fans and inlets on air velocity and temperature distribution inside the battery pack were revealed, and the simulation results from the LBM analysis were compared. The effects of the number of cooling fans and inlets along with the inlet air temperature on the performance of BTMS were then discussed thoroughly. Furthermore, an optimized cooling strategy for air-cooled BTMS based on the consideration of temperature distribution and power consumption was developed. The results of this study are significant to develop a standardized battery packing module and enrich the literature on electric vehicle battery pack optimization.
