Lithium-ion battery thermal management requirements are based on the lithium-ion battery heating mechanism, the rational design of the battery pack structure, select the appropriate thermal management methods, rational design thermal management strategies to ensure that the battery pack within the individual battery work within a reasonable temperature range At the same time try to maintain the temperature uniformity among the various batteries and battery modules in the package.
The battery thermal management system (BTMS) has a crucial influence on the dynamic performance of pure electric vehicles in various environments. Through research and analysis of lithium-ion battery heat production principle, BTMS heat transfer cooling method, and air-cooled heat dissipation and liquid-cooled heat dissipation comparison, it shows that liquid cooling effect is better than air cooling, liquid cooling will be suitable for future complex conditions Power lithium ion power battery thermal management important research direction.
As a power source for pure electric vehicles, power storage batteries are a key link to improve vehicle performance and reduce costs. Their temperature characteristics directly affect the performance, life, and durability of electric vehicles. Lithium-ion batteries are the preferred power battery for electric vehicles because of their high specific energy, long cycle life, low self-discharge rate, wide operating temperature range, and low temperature effect. Lithium-ion battery thermal management requirements are based on the lithium-ion battery heating mechanism, the rational design of the battery pack structure, select the appropriate thermal management methods, rational design thermal management strategies to ensure that the battery pack within the individual battery work within a reasonable temperature range At the same time try to maintain the temperature uniformity among the various batteries and battery modules in the package. Since the battery cells in the battery pack are connected to each other in series, the performance of any one battery will affect the overall performance of the battery pack. When the temperature difference is 5°C, 10°C, and 15°C, the state of charge of the battery pack decreases by 10%, 15%, and 20%, respectively, under the same charging conditions.
Lithium-ion battery thermal properties
A series of chemical reactions occur during charging and discharging of the battery, resulting in a thermal reaction. The main heat-generating reactions of lithium-ion power batteries include: decomposition of electrolyte, decomposition of positive electrode, reaction of negative electrode with electrolyte, reaction of negative electrode with binder, and decomposition of solid electrolyte interface film. In addition, due to the presence of internal resistance of the battery, part of the heat is generated when the current passes through. Lithium-ion batteries mainly rely on Joule heat generated by electrical resistance at low temperatures. These exothermic reactions are factors that cause battery insecurity. The thermal safety of the electrolyte also directly affects the safety performance of the battery power system of the entire lithium battery.
In the actual operating environment, the power system requires the lithium ion battery to have the characteristics of large capacity and large rate discharge, but the high temperature generated at the same time increases the operational risk. Therefore, reducing the operating temperature of the lithium-ion battery and improving the battery performance are critical.
BTMS heat transfer cooling method
According to the sources of energy provided in the BTMS, passive cooling and active cooling are used. Among them, passive cooling is used only for the cooling of the surrounding environment, and it is installed inside the system to provide a heat source at a low temperature or to provide a cold source at a high temperature. The active element includes an evaporator, a heating core, an electric heater, a fuel heater, or the like as active cooling. In accordance with the different mass transfer can be divided into air forced convection, liquid cooling, phase change material (PCM, Phase Change Material), air-conditioning refrigeration, heat pipe cooling, thermoelectric cooling and cold plate cooling. According to different discharge current rate, ambient temperature and other application requirements choose different cooling methods.
Forced air convection
As the heat transfer medium, air directly passes through the module to achieve cooling and heating. Obviously, the air natural cooling battery is ineffective. Forced air cooling is to remove the heat of the battery through the exhaust fan through the wind generated by the movement. It is necessary to increase the heat sink, the heat sink and the distance between the batteries as much as possible. The cost is low, but the battery is low. The package, installation location and heat dissipation area need to be designed. Tandem and parallel channels can be used.
The simulation results show that the battery's heat dissipation characteristics: Under natural cooling, heat radiation accounts for 43%~63% of the total heat dissipation. Enhancing heat transfer is an effective measure to reduce the maximum temperature, but the scope of expanding the heat transfer does not increase indefinitely. Temperature consistency.
The main advantages of the air-cooling method are: simple structure, relatively small weight, no leakage of possible harmful gases, effective ventilation and lower cost. The disadvantage is that the heat exchange coefficient between the wall surface and the battery is low, and the cooling and heating speed is slow.
In the series-parallel air duct, six heat-generating batteries are placed. Assuming a uniform battery density (2700 kg/m3), the heat generation rate is the same (50000 W/m3). The air flows in at a speed of 5m/s, the inlet temperature is 25°C (298K), the outlet is open freely, and the cell model uses a grid of structures, the number of which is 250,000.
The battery temperature table obtained through simulation analysis is shown in Table 1. The overall temperature difference of the series flow channel is 5.6°C, and the overall temperature difference of the parallel flow channel is 3.0°C; the heat accumulation in the middle cell of the series flow channel is more, the overall temperature is higher, and the consistency is poor; the overall temperature of the parallel flow channel is low and consistent. Better; but because the inlet air duct is horizontal at right angles in this example, the battery temperature near the inlet is higher. If the tuyere is tilted upwards at a certain angle, the cooling effect will be better. Therefore, changing the air duct design has a great impact on battery cooling.
In general conditions, the use of air cooling can meet the requirements, but under complex conditions, liquid cooling can only meet the thermal requirements of the battery. The heat exchanged between the liquid and the outside air is used to send the heat generated by the battery pack, arrange the pipeline between the modules or arrange the jacket around the module, or immerse the module in the dielectric liquid. If heat transfer tubes, jackets, etc. are used between the liquid and the module, the heat transfer medium may be water, ethylene glycol, oil or even refrigerant. If the battery module is immersed in the dielectric heat transfer liquid, insulation must be used to prevent short circuits. The rate of heat transfer between the heat transfer medium and the wall of the battery module depends primarily on the thermal conductivity, viscosity, density, and flow rate of the liquid. At the same flow rate, the heat transfer rate of air is much lower than that of direct contact fluids because of thin liquid boundary layer and high thermal conductivity.
The main advantages of the liquid cooling method include: high heat transfer coefficient between the wall surface of the battery, cooling and heating speed, and small volume. The main disadvantages are: there is the possibility of leakage; the weight is relatively large; maintenance and maintenance are complicated; water jackets, heat exchangers and other components are needed, and the structure is relatively complex.
The experimental results show that compared with the liquid cooling/heating, the air medium heat transfer effect is not very obvious, but the system is not too complicated. For a parallel hybrid vehicle, air cooling is satisfactory, whereas pure electric vehicles and tandem hybrid vehicles have better liquid cooling effects.
The battery temperature table (as shown in Table 1) was obtained through simulation analysis. In the case of different flow channel designs, the liquid cooling temperature has a good consistency. Although the overall temperature of the parallel flow channel is lower than that of the series flow channel, the temperature difference is only 0.4°C. However, considering the practical and design considerations, the structure of the series flow path is simpler and more suitable for product design.
Currently, manufacturers are reluctant to choose liquid cooling because the seal is not good enough to cause liquid leakage, so the seal design is extremely important.
Based on the finite element simulation software, this paper compares the temperature cooling effect of the battery under two different modes of air cooling and liquid cooling. The research on the above content shows that: (1) The air cooling has a greater influence on the temperature consistency of the battery under different flow paths, but the heat dissipation effect of the parallel flow path is better than that of the series flow path; (2) whether the liquid cooling is in the string, Under the flow channel, the consistency of the battery temperature is less affected, and the overall heat dissipation effect is much better than the air cooling mode. With the increase of the capacity of the battery module, the performance of the battery in harsh environments is increasingly demanding, and an efficient battery thermal management system is important. Forced air cooling can only be used under small power and good conditions because the cooling capacity is not strong; and the liquid cooling overall cooling effect is more suitable for use under large power or complex conditions. Therefore, liquid cooling is an important research direction for future battery thermal management.