Analysis of thermal management technology of electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah system

1. Background of thermal management of electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah system 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 dependence on oil and exhaust pollution in the automobile industry. In order to promote the new energy automobile industry, the Chinese government has issued a series of development plans, financial subsidies and tax incentives to promote the development of the new energy automobile industry.

The 18650 rechargeable battery lithium 3.7v 3500mah pack is the main energy storage component of electric vehicles, composed of lithium batteries, which directly affects the performance of electric vehicles. Due to the limited space for loading batteries on the vehicle and the large number of batteries required for normal operation, the 18650 rechargeable battery lithium 3.7v 3500mah will discharge at different rates and generate a lot of heat at different heat generation rates. In addition, the accumulation of time and space will accumulate a lot of heat, resulting in complex and changeable temperature conditions in the operating environment of the 18650 rechargeable battery lithium 3.7v 3500mah pack. The temperature rise in the 18650 rechargeable battery lithium 3.7v 3500mah pack seriously affects the operation of the electrochemical system of the 18650 rechargeable battery lithium 3.7v 3500mah pack, cycle life, charging acceptability, 18650 rechargeable battery lithium 3.7v 3500mah pack power and energy, safety and reliability, etc. If the electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah pack cannot dissipate heat in time, the temperature of the 18650 rechargeable battery lithium 3.7v 3500mah pack system will be too high or unevenly distributed, which will reduce the efficiency of the 18650 rechargeable battery lithium 3.7v 3500mah charge and discharge cycle, affect the power and energy of the 18650 rechargeable battery lithium 3.7v 3500mah, and in severe cases, lead to thermal runaway, affecting the safety and reliability of the system; in addition, due to the dense placement of the heat-generating 18650 rechargeable battery lithium 3.7v 3500mah body, the middle area will inevitably accumulate more heat, and the edge area will have less heat, which will increase the temperature imbalance between the units in the 18650 rechargeable battery lithium 3.7v 3500mah pack, which will cause the performance of each 18650 rechargeable battery lithium 3.7v 3500mah module and monomer to be unbalanced, and ultimately affect the consistency of 18650 rechargeable battery lithium 3.7v 3500mah performance and the accuracy of 18650 rechargeable battery lithium 3.7v 3500mah state of charge (SOC) estimation, affecting the system control of electric vehicles.

Lithium 18650 rechargeable battery lithium 3.7v 3500mah generates heat

The internal reaction process of lithium 18650 rechargeable battery lithium 3.7v 3500mah The working principle of lithium-ion 18650 rechargeable battery lithium 3.7v 3500mah is essentially the redox reaction between the internal positive and negative electrodes and the electrolyte. At low temperatures, the rate of lithium insertion reaction of active substances on the electrode surface slows down and the concentration of lithium ions inside the active substances decreases, which will cause the 18650 rechargeable battery lithium 3.7v 3500mah equilibrium potential to decrease, the internal resistance to increase, and the discharge capacity to decrease. In extreme low temperature conditions, the electrolyte may even freeze and the 18650 rechargeable battery lithium 3.7v 3500mah cannot be discharged, which greatly affects the low temperature performance of the 18650 rechargeable battery lithium 3.7v 3500mah system, causing the power output performance of electric vehicles to decay and the driving range to decrease. In addition, when charging in a low-temperature environment, lithium deposition is easily formed on the surface of the negative electrode. The accumulation of metallic lithium on the surface of the negative electrode will pierce the 18650 rechargeable battery lithium 3.7v 3500mah separator and cause a short circuit between the positive and negative electrodes of the 18650 rechargeable battery lithium 3.7v 3500mah, threatening the safety of 18650 rechargeable battery lithium 3.7v 3500mah use. The safety problem of low-temperature charging of electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah systems has greatly restricted the promotion of electric vehicles in cold areas. Therefore, in order to improve the performance of the whole vehicle and enable the 18650 rechargeable battery lithium 3.7v 3500mah pack to achieve the best performance and life, it is necessary to optimize the structure of the 18650 rechargeable battery lithium 3.7v 3500mah pack and design an electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah pack thermal management system BTMS that can adapt to high and low temperatures. 2. Current status of thermal management technology of electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah systems Power 18650 rechargeable battery lithium 3.7v 3500mah heat dissipation research can be divided into air heat dissipation, liquid cooling, solid phase change material heat dissipation and heat pipe heat dissipation. The main existing heat dissipation technologies are mainly the first three. 1. Air-cooled heat dissipation system Air-cooled heat dissipation system is also called air-cooled heat dissipation system. The air-cooled heat dissipation method is the simplest. It only needs to let the air flow through the 18650 rechargeable battery lithium 3.7v 3500mah surface to take away the heat generated by the power 18650 rechargeable battery lithium 3.7v 3500mah to achieve the purpose of heat dissipation of the power 18650 rechargeable battery lithium 3.7v 3500mah pack. According to different ventilation measures, air-cooled heat dissipation has two methods: natural convection heat dissipation and forced ventilation heat dissipation. Natural convection heat dissipation does not rely on external forced ventilation measures (such as adding fans, etc.), but only uses the airflow generated by the temperature change of the internal fluid of the 18650 rechargeable battery lithium 3.7v 3500mah pack to cool and dissipate the heat. The forced convection cooling system is a cooling system that adds corresponding forced ventilation technology to the natural convection cooling system. At present, there are two main systems for air cooling of power batteries: series and parallel. However, this method has poor effect and it is difficult to achieve high 18650 rechargeable battery lithium 3.7v 3500mah temperature uniformity.

Series air cooling/parallel air cooling 2. Liquid cooling system The liquid cooling system of power batteries refers to a cooling system in which the refrigerant directly or indirectly contacts the power 18650 rechargeable battery lithium 3.7v 3500mah, and then takes away the heat generated in the 18650 rechargeable battery lithium 3.7v 3500mah pack through the circulation of liquid fluid to achieve a heat dissipation effect. The refrigerant can be water, a mixture of water and ethylene glycol, mineral oil and R134a, etc. These refrigerants have high thermal conductivity and can achieve better heat dissipation effect. The current liquid cooling technology of power batteries also has a fairly mature technology, and it has also been relatively widely used in the cooling system of electric vehicles. For example, Tesla 18650 rechargeable battery lithium 3.7v 3500mah packs use liquid cooling of a mixture of water and ethylene glycol to dissipate heat, and BMW i3 uses R134a for heat dissipation. Liquid cooling systems often require more complex and more stringent structural designs to prevent leakage of liquid refrigerant and ensure uniformity between 18650 rechargeable battery lithium 3.7v 3500mah cells in the 18650 rechargeable battery lithium 3.7v 3500mah pack. The complex structure of the liquid cooling system also makes the entire cooling system very bulky, which not only increases the weight of the vehicle and greatly increases the burden of the 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 cost also increases accordingly.

Liquid cooling system diagram

Power 18650 rechargeable battery lithium 3.7v 3500mah pack liquid cooling structure cooling method

Tesla 18650 rechargeable battery lithium 3.7v 3500mah pack liquid cooling cooling diagram 3. Phase change material cooling system The phase change material cooling system uses phase change material as a heat transfer medium, and uses the characteristics of phase change material that can store and release energy when phase change occurs to achieve the effect of low-temperature heating and high-temperature cooling of power batteries. However, the thermal conductivity of phase change materials is relatively low. In order to change the inherent defects of the material, people fill some metal materials into the phase change material. For example, in some studies, very thin aluminum plates are filled into the phase change material to achieve the purpose of improving thermal conductivity. In order to improve the thermal conductivity of phase change materials, some people have proposed filling carbon fiber, carbon nanotubes, etc. into the phase change material.

Phase change material wrapped 18650 rechargeable battery lithium 3.7v 3500mah structure 4. Heat pipe cooling system As an efficient heat conducting element, heat pipe can quickly and efficiently transfer heat energy from one place to another, that is, it can quickly and effectively transfer heat between two objects. In the thermal management system of electric vehicles, many scholars at home and abroad have also applied heat pipes, a heat conducting element, to the heat dissipation of power batteries. Compared with the traditional forced convection cooling system, in the heat dissipation system introduced with heat pipes, the power 18650 rechargeable battery lithium 3.7v 3500mah can not only maintain the normal working temperature range, but also maintain the temperature uniformity between each 18650 rechargeable battery lithium 3.7v 3500mah cell, which is an effect that the forced cooling and 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 18650 rechargeable battery lithium 3.7v 3500mah heating system The above introduces four methods of cooling the 18650 rechargeable battery lithium 3.7v 3500mah. The following introduces the heating method to adapt the 18650 rechargeable battery lithium 3.7v 3500mah to the low temperature environment. The heating system is mainly composed of heating elements and circuits, among 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 18650 rechargeable battery lithium 3.7v 3500mah silicone heating film PTC is widely used due to its safety, high heat conversion efficiency, rapid heating, no open flame, and automatic constant temperature. Its low cost is a favorable factor for the current high-priced power batteries. However, the PTC heating element is large in size and will occupy a large space inside the 18650 rechargeable battery lithium 3.7v 3500mah system. Insulated flexible electric heating film is another heater that can be bent according to the arbitrary shape of the workpiece to ensure close contact with the workpiece and ensure maximum heat transfer. Silicone heating film is a thin surface heating element with flexibility, but it needs to be in complete close contact with the heated object, and its safety is worse than PTC. The research team led by Hu Xuegong, a researcher at the Institute of Engineering Thermophysics, Chinese Academy of Sciences, successfully developed a prototype of an electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah pack thermal management system (BTMS) with a high energy density of more than 120Wh/kg using micro-groove group composite phase change technology. The micro-groove group composite phase change technology uses the micro-scale groove group structure composite phase change to enhance the heat transfer mechanism 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 of high energy density 18650 rechargeable battery lithium 3.7v 3500mah group monomers in the electric vehicle industry that it is difficult to maintain uniform temperature. Its technical indicators are better than Tesla (temperature difference between 18650 rechargeable battery lithium 3.7v 3500mah monomers ≤±2℃), and the cost advantage is huge, which is at the leading level in the electric vehicle industry.

Micro-slot group thermal management system for electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah packs III. Development direction of thermal management technology for electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah systems From the perspective of the country's support for electric vehicles, the thermal management system of electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah packs will inevitably develop in the direction of lightweight, high specific energy and high uniform temperature. The Ministry of Science and Technology's "13th Five-Year Plan" also proposed to carry out mechanical-electrical-thermal design of 18650 rechargeable battery lithium 3.7v 3500mah systems based on vehicle integration, develop advanced and reliable 18650 rechargeable battery lithium 3.7v 3500mah management systems and compact and efficient thermal management systems. By 2020, the maximum temperature difference between monomer batteries should be ≤2℃, and the specific energy of the 18650 rechargeable battery lithium 3.7v 3500mah system should be ≥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, and a large number of waste power batteries will be generated, which will bring a lot of work for the disassembly and recycling of power batteries. Therefore, when designing the thermal management system of electric vehicle 18650 rechargeable battery lithium 3.7v 3500mah packs, it should be considered that the 18650 rechargeable battery lithium 3.7v 3500mah pack is easy to disassemble and has no additional pollution, so as to achieve the green design of the 18650 rechargeable battery lithium 3.7v 3500mah pack thermal management system.

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