Thermal management of lithium-ion battery pack for electric vehicles

Renewable energy sources have gained importance within the scope of the impact of global warming and reducing the carbon footprint. In this context, as in all sectors, passenger vehicles with internal combustion engines that use fossil fuels are being replaced by electric vehicles. In electric vehicles, the use of which has increased in recent years, lithium-ion batteries with high energy density that can be controlled with the help of electronic equipment are preferred. In addition, lithium-ion batteries may show different reactions such as loss of performance, reduced lifespan or explosion risks if they operate at temperatures below or above ideal operating conditions. Therefore, batteries need thermal management. For this purpose, the battery must be heated at low temperatures and cooled at high temperatures. Although it is stated in the literature that the appropriate operating temperature for lithium-ion batteries should be between 15 and 35 °C, the optimum operating temperature of lithium-ion cells is approximately 23-25 °C. Another study stated that the appropriate operating temperature of batteries is between approximately 10 and 35 °C. However, most lithium-ion chemicals continue to operate down to about -20 °C and down to about 45 °C. No reduction in battery life is expected during normal operation in this temperature range. Between -20 and -40 °C, electrolytes can begin to freeze, and lower temperatures increase the impedance within the cell, resisting ion flow and reducing capacity and performance. It has been recorded in the literature that many lithium ion cell chemistries begin to change above 60 °C. The thermal management system aims to maintain a temperature difference of approximately 2–3 °C from the coldest cell to the hottest cell. However, for very large battery packs, differences of up to 6–8 °C are also acceptable. At temperatures above approximately 90 °C, the polymer-based separator may begin to melt and disintegrate, and between 90 and 130 °C the separator continues to disintegrate until a series of internal short circuits are experienced between the anode and cathode. At this point thermal runaway will begin to occur in the cell. The exact temperature at which a cell reaches its thermal runaway threshold is different for different chemistries. Some cell chemistries may experience thermal runaway temperatures exceeding 140 °C when the initial temperature is 120 °C. The development of a high-performance battery thermal management system (BMS) is very important to maintain the high efficiency and safety of the battery. In general, BTMS can be divided into three categories, including phase change materials (PCM), liquid and air, according to the physical properties of the cooling medium. Contact cooling systems have come to the fore due to the weight of FDM cooling systems and the low efficiency of air cooling systems. On the other hand, difficulties in the use of direct contact-liquid immersion systems are expressed in the literature. The thermal safety of the battery is extremely important due to the fire hazard associated with the battery system of electric vehicles in case of overheating. Therefore, issues such as the use of gas outlet valves, thermal barriers and sensors to increase the thermal safety of the battery are among the topics discussed in the literature. The thermal safety management system should include advanced monitoring and diagnostic technologies that can be used to detect abnormal temperature increases, internal pressure, voltage, and humidity. Some sensors such as fiber Bragg grating (FBG) and built-in flexible thin-film sensors are recommended. In the event of a fire occurring, the thermal safety management system must be equipped with fire suppression capabilities that can quickly suppress flames and prevent them from spreading. This may involve the use of extinguishing agents such as water or foam, or activating fire extinguishing systems built into the battery system itself. A thermal barrier must be applied to suppress heat transfer between adjacent cells during thermal runaway. While the thermal barrier can ensure excellent safety performance during abnormal operating conditions, it can also disrupt the original thermal conduction path of the battery thermal management system. In addition, passive/active valves must be used in battery packs to evacuate gases emerging from lithium ion cells in case of thermal runaway. In the conducted study, thermal management of the lithium-ion battery pack will be done with numerical methods. For this purpose, computational fluid dynamics (CFD) analysis of the cooling plates used to keep the batteries in the required temperature ranges was performed with the three-dimensional model created using the Ansys/Fluent program. CFD analysis was verified with cell temperatures taken from the battery's cooling plate performance test. The results obtained from the CFD analysis show good agreement with the test data. Compared to the reference model, with the optimum pipe placement on the cooling plate, the maximum temperature difference between the battery cells was reduced from 8 °C to 3,3 °C and a homogeneous temperature distribution was achieved. At the same time, the operating temperature of the battery pack has been reduced from 30-22 °C to 27-23,7 °C. In the CFD analysis performed with different designs obtained by changing the pipe layout on the battery pack for fixed hydraulic diameter and pipe length, it was determined that the cooling performance varied depending on the pipe layout. The pipe arrangement that gives the best cooling performance was determined and how the cooling performance was affected by the flow rate change was examined. It has been determined that the cooling performance of the cooling plate with optimum pipe arrangement increases by 16% with doubling the liquid flow rate. As a result, it has been revealed that better cooling performance can be achieved under current conditions with designs with different pipe layouts. The most important reason for this is due to the greater surface area of the cooling liquid in contact with the modules. These results show that the channel structure of the cooling plates significantly affects the heat transfer and that improvements can be made in terms of battery performance and life by optimizing the designs. In the CFD analyzes performed by shaping the cooling plates, it was observed that the cooling performance could be increased by changing the form structures, based on the test data. When the analyzes of shaped cooling plates (FSP) are compared among themselves, it is observed that FSP 4 is more efficient than other shaped structures. A comparison of cooling plates has been made, and it is seen that the cooling plate shaped in terms of cooling performance performs better in terms of efficiency compared to the others (FSP 4). Here, it was observed that FSP 4 exhibited better cooling performance than BD 4.2, which was optimized and whose flow rate was doubled. In this sense, more homogeneous and efficient cooling plates can be obtained by optimizing the shapes of the formed cooling plates. The internal resistance of lithium ion cells is directly proportional to temperature. As the temperature increases, the internal resistance of the cells also increases. In the study, the internal resistance of the cell operating at 27 °C increased by 20,6% after 8 years, while the internal resistance of the cell operating at 30 °C increased by 33%. The capacity of lithium-ion cells is inversely proportional to temperature. As the temperature increases, the capacity of the cells decreases. In the study, the cell operating at 27 °C experienced a 12,2% loss in capacity after 8 years, while the cell operating at 30 °C experienced a 15,65% loss. The optimization study is an important step in terms of its impact on energy resources and sustainability, as it reduces cell internal resistance by 12,6% and provides an improvement in capacity loss by 3,45%. This study reveals the direct effect of temperature on battery life, contributing to more efficient use of natural resources and increasing the sustainability of battery performance. The 11% efficiency increase achieved by the design and application of cooling plates makes a significant contribution to improving battery life and performance by 10 to 20%.