A brief introduction to electric vehicle button cell battery cr1620 system thermal management technology

　　1. Background of Thermal Management of Electric Vehicle button cell battery cr1620 Systems

　　With the rapid development of manufacturing industry, China's automobile industry is facing the challenges of industrial transformation, emission reduction, energy crisis and low-carbon development. The development of new energy vehicles has become the only way to reduce the automobile industry's oil dependence and exhaust pollution. The Chinese government has To promote the new energy automobile industry, a series of development plans, financial subsidies and tax incentive plans have been released to promote the development of the new energy automobile industry.

　　The button cell battery cr1620 pack is the main energy storage component of electric vehicles. It is composed of lithium batteries and directly affects the performance of electric vehicles. Since the space for loading batteries on the vehicle is limited, the number of batteries required for normal operation is also large. The batteries will discharge at different rates and generate a large amount of heat at different heat generation rates. In addition, time accumulation and space effects will accumulate a large amount of heat. , resulting in complex and changeable button cell battery cr1620 pack operating environment temperature conditions. The temperature rise in the button cell battery cr1620 pack seriously affects the operation, cycle life, charge acceptability, button cell battery cr1620 pack power and energy, safety and reliability of the button cell battery cr1620 pack's electrochemical system. If the electric vehicle button cell battery cr1620 pack cannot dissipate heat in time, it will cause the temperature of the button cell battery cr1620 pack system to be too high or unevenly distributed, which will reduce the button cell battery cr1620 charge and discharge cycle efficiency, affect the button cell battery cr1620's power and energy performance, and in severe cases will lead to thermal runaway, affecting System safety and reliability; in addition, due to the dense placement of heating button cell battery cr1620 bodies, more heat will inevitably accumulate in the middle area, and the smaller edge areas will increase the temperature imbalance between the units in the button cell battery cr1620 pack, which will cause each button cell battery cr1620 to The imbalance of module and cell performance ultimately affects the consistency of button cell battery cr1620 performance and the accuracy of button cell battery cr1620 state-of-charge (SOC) estimation, affecting the system control of electric vehicles.

　　Lithium batteries generate heat

　　Internal reaction process of lithium button cell battery cr1620

　　The working principle of lithium-ion batteries is essentially a redox reaction between the internal positive and negative electrodes and the electrolyte. At low temperatures, the lithium insertion reaction rate of the active material on the electrode surface slows down and the lithium ion concentration inside the active material decreases, which will cause the button cell battery cr1620 balance potential The internal resistance increases, the discharge capacity decreases, and extreme low temperatures may even cause the electrolyte to freeze and the button cell battery cr1620 to fail to discharge, which greatly affects the low-temperature performance of the button cell battery cr1620 system, resulting in the attenuation of the power output performance of electric vehicles and the reduction of driving range. In addition, charging in low-temperature environments can easily form lithium deposits on the surface of the negative electrode. The accumulation of metallic lithium on the surface of the negative electrode will pierce the button cell battery cr1620 separator and cause a short circuit between the positive and negative electrodes of the button cell battery cr1620, threatening the safety of the button cell battery cr1620. The low-temperature charging safety of electric vehicle button cell battery cr1620 systems is greatly restricted. promote the promotion of electric vehicles in cold regions.

　　Therefore, in order to improve the performance of the vehicle and maximize the performance and life of the button cell battery cr1620 pack, it is necessary to optimize the structure of the button cell battery cr1620 pack and design an electric vehicle button cell battery cr1620 pack thermal management system BTMS that can adapt to high and low temperatures.

　　2. Current status of thermal management technology for electric vehicle button cell battery cr1620 systems

　　Power button cell battery cr1620 heat dissipation research can be divided into air heat dissipation, liquid cooling heat dissipation, solid phase change material heat dissipation and heat pipe heat dissipation. The existing main heat dissipation technologies are mainly the first three.

　　1. Air-cooling cooling system

　　Air-cooled cooling system is also called air-cooled cooling system. The air-cooling heat dissipation method is the simplest. It only needs to let the air flow through the button cell battery cr1620 surface to take away the heat generated by the power button cell battery cr1620 to achieve the purpose of cooling the power button cell battery cr1620 pack. Depending on the ventilation measures, air cooling has two methods: natural convection heat dissipation and forced ventilation heat dissipation. Natural convection heat dissipation does not rely on additional external forced ventilation measures (such as air fans, etc.), but is a cooling and heat dissipation system that uses the air flow generated by the temperature change of the fluid inside the button cell battery cr1620 pack itself. The forced convection cooling system is a cooling system that adds corresponding forced ventilation technology on the basis of the natural convection cooling system. Currently, there are two main types of air-cooled heat dissipation systems for power batteries: series and parallel. However, this method is less effective and difficult to achieve high button cell battery cr1620 temperature uniformity.

　　Series air cooling/parallel air cooling

　　2. Liquid cooling system

　　The liquid-cooled heat dissipation system of the power button cell battery cr1620 refers to a heat dissipation system in which the refrigerant contacts the power button cell battery cr1620 directly or indirectly, and then takes away the heat generated in the button cell battery cr1620 pack through the circulation of liquid fluid to achieve the heat dissipation effect. The refrigerant can be water, a mixture of water and ethylene glycol, mineral oil, R134a, etc. These refrigerants have high thermal conductivity and can achieve better heat dissipation effects. The current liquid cooling technology of power batteries is also quite mature and has been widely used in the heat dissipation system of electric vehicles. For example, Tesla button cell battery cr1620 packs use liquid cooling with a mixture of water and ethylene glycol to dissipate heat. BMW i3 uses R134a for heat dissipation. Liquid-cooling systems often require more complex and stringent structural designs to prevent leakage of liquid refrigerant and ensure uniformity between button cell battery cr1620 cells in the button cell battery cr1620 pack. The complex structure of the liquid-cooling system also makes the entire heat dissipation system complex. It is very bulky, which not only increases the weight of the entire vehicle and greatly increases the burden on the entire vehicle, but also makes the maintenance and upkeep of the liquid cooling system relatively difficult due to its structural complexity and high sealing, and the maintenance costs also increase accordingly.

　　Liquid cooling system diagram

　　Power button cell battery cr1620 pack liquid cooling structure heat dissipation method

　　Tesla button cell battery cr1620 pack liquid cooling heat dissipation diagram

　　3. Phase change material cooling system

　　The phase change material heat dissipation system uses phase change materials as the heat transfer medium. It uses the characteristics of the phase change material to store and release energy when phase change occurs to achieve the effects of low-temperature heating and high-temperature heat dissipation of the power button cell battery cr1620. However, the thermal conductivity of phase change materials is relatively low. In order to change the inherent defects of the materials, people fill the phase change materials with some metal materials. For example, in some studies, very thin aluminum plates are filled into the phase change materials to improve the thermal conductivity. the goal of. In order to improve the thermal conductivity of phase change materials, some people have proposed filling carbon fibers, carbon nanotubes, etc. into phase change materials.

　　Phase change material wrapped button cell battery cr1620-like structure

　　4. Heat pipe cooling system

　　As an efficient thermal conductive element, heat pipes can quickly and efficiently transport heat energy from one place to another, that is, they can quickly and effectively transfer heat between two objects. In the thermal management system of electric vehicles, many scholars at home and abroad also apply heat pipes, a thermal conductive element, to the heat dissipation of power batteries. Compared with the traditional forced convection heat dissipation system, in the heat dissipation system that introduces heat pipes, the power button cell battery cr1620 can not only be maintained within the normal operating temperature range, but also the temperature uniformity between each button cell battery cr1620 cell can be maintained. This is forced cooling. The effect that the cooling system cannot achieve. However, its mass and volume are too large and there is a heat exchange limit.

　　heat pipe cooling

　　5. Electric vehicle button cell battery cr1620 heating system

　　The above introduces four methods for dissipating heat to the button cell battery cr1620. Next, we will introduce the heating method to adapt the button cell battery cr1620 to the low-temperature environment. The heating system mainly consists of heating elements and circuits, of which the heating element is the most important part. Common heating elements include variable resistance heating elements and constant resistance heating elements. The former is usually called PTC (positive temperature coefficient), and the latter is a heating film usually composed of metal heating wires, such as silicone heating film, flexible electric heating film, etc. .

　　PTC for electric vehicles

　　Power button cell battery cr1620 silicone heating film

　　PTC is widely used due to its safe use, high heat conversion efficiency, rapid temperature rise, no open flame, and automatic constant temperature. Its low cost is a favorable factor for the current higher-priced power batteries. However, the heating element of PTC is large and will occupy a large space inside the button cell battery cr1620 system. Insulating flexible electric heating film is another type of heater that can be bent according to any shape of the workpiece, ensuring close contact with the workpiece and ensuring maximum heat energy transfer. Silicone heating film is a flexible thin surface heating element, but it needs to be in complete close contact with the object to be heated, and its safety is worse than PTC.

　　A scientific research team led by researcher Hu Xuegong from the Institute of Engineering Thermophysics, Chinese Academy of Sciences, successfully developed a prototype of an electric vehicle button cell battery cr1620 pack thermal management system (BTMS) with a high energy density of over 120Wh/kg using micro-groove group composite phase change technology. The technology utilizes the composite phase change enhanced heat transfer mechanism of the micro-scale groove group structure to achieve high-intensity heat transfer. It is currently an advanced passive micro-scale phase change enhanced heat transfer technology in the world. This achievement solves the technical problem in the electric vehicle industry that it is difficult to maintain uniform temperature between high-energy-density button cell battery cr1620 cells. Its technical indicators are better than Tesla (the temperature difference between button cell battery cr1620 cells is ≤±2℃), and The cost advantage is huge and it is at the leading level in the electric vehicle industry.

　　Electric vehicle button cell battery cr1620 pack micro-slot group thermal management system

　　3. Development direction of thermal management technology for electric vehicle button cell battery cr1620 systems

　　Judging from the direction of national support for electric vehicles, the thermal management system of electric vehicle button cell battery cr1620 packs will inevitably develop towards lightweight, high specific energy and high temperature uniformity. The Ministry of Science and Technology’s “Thirteenth Five-Year Plan” also proposes to carry out mechanical-electrical-thermal design based on vehicle-integrated button cell battery cr1620 systems and develop advanced and reliable button cell battery cr1620 management systems and compact and efficient thermal management systems. By 2020, The maximum temperature difference between single cells is ≤2°C, and the specific energy of the button cell battery cr1620 system is ≥210Wh/kg.

　　On the other hand, by the end of the 13th Five-Year Plan, the number of electric vehicles in my country will reach 5 million, resulting in a large number of waste power batteries, which will bring a lot of work to the dismantling and recycling of power batteries. Therefore, when designing the button cell battery cr1620 pack thermal management system for electric vehicles, it should be considered that the button cell battery cr1620 pack is easy to disassemble and has no additional pollution, so as to achieve a green design of the button cell battery cr1620 pack thermal management system.

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