The battery pack is the most important component of an electric vehicle, providing energy for its operation, and its performance directly affects the working performance of the electric vehicle. Having reliable safety performance is a basic requirement for power batteries used in electric vehicles. During the charging and discharging process of batteries, various stresses are generated due to electrochemical reactions accompanied by a large amount of heat. Especially when the charging and discharging current is too large, it is more likely to produce polarization effects, causing the battery voltage to be artificially high, and at the same time, the heat generation will increase significantly, and the thermal effect will be enhanced. The rapid increase in internal heat of the battery will cause irreversible damage to the active substance ions inside the battery, seriously affecting the performance of the battery and greatly reducing its service life. Therefore, analyzing the thermal characteristics of battery packs is helpful for the safe use of electric vehicles.
1 Simulation of battery pack
The internal resistance of a single lithium iron phosphate battery pack for electric vehicles is 7 mΩ, the capacity of the battery is 10 A • h, the charging cut-off voltage is 4.23 V, and the length, width, and height of the single battery with a mass of 0.27 kg are 65 mm, 22 mm, and 104 mm, respectively; The radius of the pole is 2.25 mm and the height is 6.00 mm. The battery pack is in four parallel and twelve series. The total discharge current at a 1 C rate is 40 A, and the actual current through a single battery is 10 A. Establishing a thermal model of battery pack by ANSYS.
1.1 Boundary Conditions of Heat Flow Field in Battery Chamber
The thermal flow field inside the battery chamber is solved using a fluid thermal coupling field. Convective heat transfer conditions can be directly used in the flow field, without the need to set boundary conditions for the thermal field. The import and export boundary conditions include temperature, air velocity, and air pressure. Set the inlet temperature to ambient temperature of 20 ℃ and the outlet pressure to 0. Air flow rate is one of the main factors affecting the heat dissipation of battery chamber. This article will study the effects of natural ventilation and forced ventilation on the thermal characteristics of batteries. Due to the small temperature difference between the chamber and the outside, and the small heat exchange with the outside, the wall boundary condition is set as a constant temperature adiabatic boundary.
1.2 Thermal characteristics analysis of lithium iron phosphate battery pack
1) Under the condition of constant current discharge at 1 C rate at an ambient temperature of 20 ℃, the maximum temperature of the battery pack is 32 ℃, the minimum temperature is 26 ℃, and the temperature difference is 6 ℃. The lowest temperature occurs at the edge of the battery pack. These locations are generally well ventilated, allowing for more sufficient convective heat transfer; The highest temperature occurs at the core position inside the individual battery. Due to the lack of direct connection with the outside world, thermal conduction can only rely on the polymer and electrolyte inside the battery, which can easily cause temperature accumulation. Therefore, the temperature at the core position is slightly higher than that at the edge position
2) When discharging at a constant current rate of 2 C under an ambient temperature of 20 ℃, the maximum temperature of the battery pack is 43 ℃, the minimum temperature is 31 ℃, and the temperature difference is 12 ℃. The lowest temperature occurs at the edge of the battery pack, while the highest temperature occurs at the internal core of the individual battery. Because the overall temperature difference inside the battery pack exceeds the designed standard temperature difference (10 ℃). If the 2 C rate discharge is used for a long time for starting, climbing, accelerating, etc., it will affect the service life of the battery.
2 Thermal Test
The temperature field experiment of lithium iron phosphate batteries for electric vehicles can measure the temperature difference at typical points of the battery pack. Not only can we understand the actual temperature distribution and temperature rise of lithium iron phosphate batteries. At the same time, the accuracy of simulation calculation results can also be verified based on experimental data. Provide reliable basis for the thermal management design of battery packs.
In order to accurately measure the actual surface temperature of the battery pack in the battery chamber at different discharge rates. The head of the temperature sensor needs to be tightly adhered to the side surface of the battery pack. To minimize temperature reading errors as much as possible without affecting the air flow inside the battery chamber.
Place a temperature sensor at center point A on the side of the battery pack. Place a temperature sensor at edge B on the side of the battery pack. Conduct discharge tests at 1 C and 2 C discharge rates respectively. To ensure the accuracy of the experiment, 5 tests were conducted at different discharge rates, and the average highest temperature at the same time was taken for the 5 tests.
By comparing and analyzing the experimental and simulation results, it can be seen that the error between the simulation results and the experimental results is within 8%, indicating that the lithium battery simulation model established in this paper is accurate and practical, and can be used to improve the thermal management of battery packs
3 Improvement of thermal management
Electric vehicles require high current discharge of the battery during acceleration, hill climbing, and other working conditions. At this time, the current inside the battery pack increases exponentially, and the temperature field inside the battery pack will also significantly change. Therefore, it is necessary to improve the heat dissipation of the existing battery pack.
The thermal management system using air as the medium has a simple structure, low cost, and simpler manufacturing process compared to the thermal management system using liquid as the medium. Moreover, harmful gases generated due to various reasons can be discharged in a timely manner, resulting in better safety. Therefore, this article adopts forced ventilation to dissipate heat in the system. To ensure good heat dissipation, the speed at the air inlet of the cooling system is set to 30 m/s. Through ANSYS simulation calculation, the temperature field distribution of the improved battery pack is obtained.
After improving the thermal management of the battery pack, the temperature difference and maximum temperature during discharge at different discharge rates have been reduced. And all are within the permitted scope. When the battery pack is discharged at a rate of 1 C, the maximum temperature of the battery decreases by 1 ℃ without any change in temperature difference;
When the battery pack is discharged at 2 C rate, the maximum temperature of the battery pack decreases by 8 ℃, and the temperature difference drops to 8.5 ℃;
When the battery pack is discharged at a 3 C rate, the maximum temperature of the battery pack drops to the allowable range, and the temperature difference also drops to the allowable range.
By comparison, it can be seen that the maximum temperature and temperature difference of the improved battery pack are within the allowable range. The improvement plan is feasible and effective.
4 Conclusion
This article takes a certain type of electric vehicle lithium iron phosphate battery pack as the research object, establishes its simulation model using ANSYS software, and conducts simulation analysis. The results showed that the highest discharge temperature of the battery pack at a 2C rate was 43 ℃, with a temperature difference of 12 ℃, exceeding the allowable temperature difference of 10 ℃.
Long term use can affect the lifespan of the battery. The thermal management method of the battery pack has been improved, and the maximum temperature and temperature difference of the battery pack during discharge at 2 C and 3 C magnification have been reduced, meeting the heat dissipation requirements of the battery pack during electric vehicle starting, climbing, and acceleration.