*2.1. Air Cooling—Prismatic Cells*

One of the easiest ways to control the battery pack temperature is by utilizing aircooling systems. These can be realized with natural ventilation or with forced ventilation. Several simulations and experimental tests are available in the literature, which evidence the effect of different airflow duct modes [22] or important construction details such as the air-inlet angle, the air-outlet angle, and the width of the airflow channel between battery cells [16]. Many studies deal with the temperature distribution and streamlines obtained in different arrangements of the air-flow for battery pack made with densely packed prismatic cells.

Interesting results can be extrapolated from the research of Xu et al. [22]. In a parallel flow arrangement, the air flows parallel to the battery cells and is expelled by the fans from the air-outlets. The highest temperature areas are inside the battery near the air-outlet. With this configuration, the heat dissipation performance requirements are not satisfied with any of the environmental temperatures considered (*Tenv*,1 = 20 ◦C, *Tenv*,2 = 27 ◦C, *Tenv*,3 = 40 ◦C).

In a cross-flow arrangement, the air flows perpendicularly to the battery cells and the shorter air-flow paths improve the heat dissipation. The temperature peaks are lower, showing that this configuration is better than a parallel-flow one in terms of heat dissipation. Mixed approaches, such as adding double-passage or U-passage channels at the bottom of the battery pack, can increase the heat transfer performances. Using the double-passage channel, the temperature values are lower with respect to the previous tests but not enough to satisfy the heat dissipation performance requirements for all the environmental temperatures considered (only for 27 ◦C and 40 ◦C). In the last case of a bottom U-passage, it clearly appears that the temperature field distribution is more uniform with respect to the double passage. Using the U-passage, the temperatures are lower with respect to all the previous cases, satisfying the heat dissipation performance requirements for all the environmental temperatures considered (the value of *Tris*,*max* = 10.10 ◦C for *Tenv* = 20 ◦C is sufficiently close to the specification). In Table 1, the values of *Tris*,*max* (A) and *Tdi f* ,*max* (B) in the parallel flow test are compared with those recorded during the other tests in terms of temperature decreasing.

With the U-passage, the heat dissipation requirements are satisfied for SOC = 70% and SOC = 100%, but the results are better for the lowest SOC percentage: this means that in case of insufficient heat dissipation condition, it could be helpful to work with lower SOC values [22]. The heat dissipation requirements are satisfied for the charge and discharge rates of 0.6C, 0.8C and 1C with the best results for the lowest of these values: this means that, in the case of insufficient heat dissipation condition, it could be helpful to work with lesser charge and discharge rates [22]. The angle between air-inlet and air-outlet channels and the battery cells, as well as the air-flow channel width, are crucial for different aspects. Xie et al. [24] focus their attention on these aspects of prismatic lithium-ion cells arrangements. The environmental temperature for the experimental tests is set at 25 ◦C. During the tests, the air-inlet angle and the air-outlet angle are changed on the basis of the size of the inlet and outlet channels, respectively, while the layout of the air-flow channels is

changed by the gap in the battery pack. Another important factor considered is the distance between the cells [24]. By taking into account all these parameters, the lowest values of *Tris*,*max* and *Tdi f* ,*max* are obtained for the cases of the air-inlet angle of 2.5◦ and air-outlet angle of 2.5◦, with evenly-spaced air flow channel between battery cells. The values of *Tris*,*max* and *Tdi f* ,*max* are dropped by 12.82% and 29.72%, respectively, by the optimization approach [24].

**Table 1.** Temperature decreasing obtained moving from a parallel flow cooling method to a U-passage technology at different environmental temperatures [22].


Air Cooling—Cylindrical Cells

In this section, some air-cooling techniques tested on cylindrical-cell bricks are described. Zhou et al. [16] take into account 18,650 Li-ion batteries, monitoring the temperature of every single cell with k-type thermocouples in three different points: the top, the middle, and the bottom. The air distribution pipes have orifices arranged all along with the axial direction of the pipes themselves. Three kinds of pipes are tested: the first one presents three orifices with 1 mm diameter, the second one has four orifices with 15 mm diameter, and the third pipe has five orifices with 2 mm diameter. The air enters into the pipes from their inlets and flows out from the orifices. Considering a constant discharge rate process, it is shown that the temperature increases from the negative pole to the positive one, and this is explainable with a high battery cap internal resistance (high heating due to the Joule's effect). After this observation, four main factors are considered to optimize the air distribution pipe system: the number and the diameter of the orifices, the air inlet pressure, and the discharge rate. For all the diameters of the orifices, the temperature obtained in the middle of the cells is lower than that on the top and the bottom, and, in particular, it decreases as the orifice diameter increases. This is the effect of an enhanced heat transfer area between the battery surface and the airflow. Choosing pipes with bigger orifices leads to an increase in the forced cooling air inlet area. At the same time, the power consumption increases but it is not a significant effect. The effect of inlet pressure is considered. Higher inlet pressure values lead to higher inlet airflow rates and, consequentially, by increasing the inlet pressure, *Tdi f* ,*max* decreases, while the power consumption increases [16]. As shown by the experiments of Xu et al. [22] on prismatic cells, the discharge rate affects the temperature. This is because as the heat dissipation rate remains the same, the heat generation rate changes. There is a degree of discharge (DOD) value for each discharge rate from which the temperature starts to drop, but, at the same time, the higher the discharge rate is, the higher the maximum temperature of the battery: the cooling rate declines with the increase of the discharge rate. The maximum temperature difference also increases with the discharge rate. The effect of baffle installation is important. Jiaqiang et al. [26] study the influence of the air-flow inlet and outlet positions and the benefits of a baffle in terms of heat dissipation. The tested module is made of 18,650 Li-ion batteries. The battery cells are 2 mm spaced from the others, 5 mm from the case bottom, and 20 mm from the case top. The inlet airflow speed is 2 m/s. Figure 1 shows the temperature distribution

obtained within the brick under different forced ventilation conditions: lateral inlet and outlet, same side, lateral inlet and outlet, opposite side, and a baffle installation.

**Figure 1.** Temperature distribution obtained under different forced-convection conditions obtained in [26]. From left to right: lateral inlet and outlet, same side, lateral inlet and outlet, opposite side, and a baffle installation.

In the first condition, the cells near the air inlet have the lower temperatures, but the highest peaks are obtained on the right, where the inlet air hardly flows through the cells. *Tris*,*max* and *Tdi f* ,*max* are still unacceptable. In the second condition, with the inlet and outlet air intakes in these positions, the temperature distribution remains the same, and the higher temperatures obtained before decrease but not enough to consider the values of *Tris*,*max* and *Tdi f* ,*max* acceptable. In the third condition, it is possible to narrow and concentrate the airflow with the installation of a simple and cheaper baffle: the result is an improved airflow cooling ability. The temperature distribution is the same as the previous cases without the baffle with optimal working temperature range with *Tris*,*max* = 38.9 ◦C. Problems of temperature uniformity still have to be fixed to let the battery work in the optimum temperature range and, at the same time, to satisfy the requirement of *Tdi f* ,*max* < <sup>5</sup> ◦C.
