Temperature is an important factor affecting the aging of lithium-ion batteries. In fact, many researchers have pointed out that for most lithium-ion batteries, the appropriate and safe operating temperature window is between 15 ° C and 35 ° C. 25 ° C is the optimal temperature to reduce the aging mechanism caused by high/low temperature. In addition, due to the limited temperature tolerance of relatively unstable chemicals in automotive battery components, not only the lifespan, but also the performance of lithium-ion batteries is significantly affected by temperature. For example, experimental results show that the charging performance of 18650 nickel cobalt aluminum batteries is independent of temperature between 20 ° C and 40 ° C, while the charging and discharging performance of automotive batteries significantly decreases when the temperature is below 20 ° C; The experimental results show that as the temperature increases, the discharge capacity of nickel manganese cobalt batteries will increase to a certain extent, while the internal resistance will decrease to a certain extent; A study was conducted on the influence of temperature on the capacity, internal resistance, and open circuit voltage (OCV) of automotive batteries at temperatures ranging from -30 ° C to 50 ° C. The results showed that the performance of automotive batteries was optimal at moderate temperatures, while at extreme temperatures, The performance of automotive batteries has not been optimized.
In a cell pack composed of multiple automotive batteries, if the temperature distribution is uneven for a long time, it will cause uneven performance of each automotive battery module and individual, ultimately affecting the consistency of automotive energy battery performance. Poor consistency will lead to different thermal effects between automotive batteries, further expanding the temperature distribution differences of each automotive battery, Forming a vicious cycle that ultimately has a serious impact on automotive batteries, many researchers believe that the temperature difference between battery packs should not exceed 5 ° C.
Therefore, in order to extend the lifespan of automotive batteries and ensure their performance, it is crucial to develop battery thermal management (BTM) technology for automotive batteries. In fact, as early as 2004, Cosley et al. summarized thermal management systems for lead-acid batteries and predicted that with the development of electric vehicles, thermal management technology for automotive batteries will become a focus of future research. In recent years, thermal management technology for automotive batteries has mostly developed based on the demand for electric vehicle (including hybrid vehicles) power vehicle batteries. Due to the small installation space, high distribution density, and poor heat dissipation performance of on-board power batteries, most research is dedicated to improving the heat dissipation performance of battery packs. In addition, due to the poor performance of lithium-ion batteries at low temperatures, many researchers are dedicated to studying the heating/pre heating technology of automotive battery packs or the design of integrated heating and cooling thermal management systems. Starting from the needs of this research, this section only conducts research on the current research status of automotive battery cooling systems at home and abroad. According to the different working media of thermal management systems, automotive battery cooling systems can be divided into three categories: air cooling systems, liquid cooling systems, and phase change cooling systems.
(1) Air cooling system
Air cooling, as the most traditional cooling method, has been studied for many years and has been widely applied in reality. According to different operating modes, air cooling can be divided into two categories: passive air cooling (using natural convection) and active air cooling (using forced convection). In order to improve the performance of the air cooling system, researchers have optimized two types of air cooling from three aspects: automotive battery layout, airflow channel layout, and cooling fan layout/operation. For example, in terms of automotive battery layout, Wang et al. used computational fluid dynamics (CFD) methods to explore the thermal performance of automotive battery modules under different automotive battery arrangement structures, including rectangular, hexagonal, and circular arrangements, The proposal of a hexagonal structure is the best choice when considering both module space utilization and cooling effectiveness, while Chen et al. optimized the thermal management effect of automotive battery packs by adjusting the spacing between automotive batteries.
In terms of airflow channel layout, Pesaran et al. simulated and studied series ventilation cooling and parallel ventilation cooling systems using finite element method, and found that the performance of parallel ventilation cooling systems was better than that of series ventilation cooling systems. Chen et al. further optimized the flow channel structure of parallel ventilation cooling systems. In addition, Wang et al. also studied the effect of fan position on the temperature distribution of a 5×5 automotive battery pack, while Mahamud et al.’s study showed that reciprocating airflow has a better cooling effect on automotive battery packs than single airflow.
Overall, for power batteries with high energy density, due to the much lower convective heat transfer coefficient of natural convection compared to forced convection, the cooling effect of forced convection is better than that of natural convection. The advantage of an air cooling system is its simple structure, resulting in a light weight, low cost, and maintenance costs. However, due to the low specific heat capacity of the air and limited space in the car, the cooling performance is not good whether it is natural convection or forced convection.
(2) Liquid cooling system
Liquid cooling uses liquids with a much higher specific heat capacity than air, so its cooling performance is better than air cooling. It has also been applied to the heat dissipation of electric vehicle battery packs. According to the direct contact between the surface of the automotive battery and the heat transfer fluid, liquid cooling can be divided into direct contact and indirect contact.
The direct contact liquid cooling system uses insulating liquid as the cooling working fluid, which is more compact in structure and has higher cooling efficiency than air cooling systems. The performance optimization method of direct contact liquid cooling system is similar to that of air cooling system, with most of it being structural optimization. For example, Karimi and Dehghan studied the effects of coolant type and flow channel structure on the temperature of automotive battery packs when using air, silicone oil, and water as working fluids for direct cooling. They improved the cooling effect of the system by optimizing the flow channel structure to increase the heat transfer area. In addition, some researchers have increased the thermal conductivity of the working fluid by improving the coolant, and have achieved better cooling effects. Compared with direct contact liquid cooling, indirect contact liquid cooling has slightly lower performance. However, considering the weight/volume, control, and cost of the system, Indirect contact liquid cooling is more common in practical applications. Indirect contact liquid cooling systems mainly have three channel forms: cold plate, discrete pipe, and jacket. In addition to improving coolant performance, the cooling effect is mainly improved through pipeline layout and channel structure optimization. The channels of the cold plate can be roughly divided into two forms: parallel design and serpentine design. For example, Rao et al. designed a parallel designed microchannel cold plate, pointing out that the heat dissipation needs of automotive battery packs can be met by increasing mass flow rate, and the energy consumption of the system can be reduced by increasing channel width; Jairett et al. optimized the channel shape of the serpentine channel cold plate through parametric modeling and applied it to the thermal management of electric vehicle batteries, achieving good cooling effects; Chen et al. comprehensively compared the cooling performance of another type of serpentine channel cold plate with air cooling, direct liquid cooling, and fin cooling methods, and pointed out that the maximum temperature rise of the system is the lowest when using serpentine channel cold plate cooling. In terms of research on discrete tubes, Xu et al. studied the cooling performance of aluminum mini channel tubes at the level of square battery cells and modules, and found that the arrangement of discrete tubes can prevent the diffusion of temperature rise between automotive batteries; Another application of discrete tubes is to combine with thermal conductive components, first transferring heat to the thermal conductive components and then to the liquid cooling channel. This method can avoid damage to the automotive battery caused by liquid leakage. Jacketed liquid cooling is often combined with phase change materials. Huang Juhua et al. have demonstrated that their proposed phase change materials and water jacket liquid cooling structure can ensure the reasonable and normal operation of automotive battery packs, while also improving the safety and durability of automotive battery packs.
Overall, the cooling effect of liquid cooling systems is good, but the complexity of their structure, cost, and potential leakage problems hinder the promotion and application of technology.
(3) Phase change cooling system
In theory, phase change cooling has the potential for better cooling performance than air cooling and liquid cooling, and can also reduce the complexity and weight of structures by eliminating liquid paths and additional heat exchangers, making it a recent research hotspot. However, phase change cooling directly in contact with automotive batteries poses a risk of leakage due to volume changes and non-uniformity during the phase change process, and the properties of phase change materials themselves are crucial, so practical applications are not easy to achieve; The phase change cooling system through heat pipes often needs to optimize the structural arrangement of the heat pipes or add additional cooling plates due to insufficient contact area, and the research and optimization of the structural arrangement of the heat pipes and cooling plates are very similar to indirect liquid cooling.