Analysis of key technologies for new energy vehicle battery management

　　1. Analysis on the Necessity of New Energy Vehicle Battery Management

　　To break through the above bottlenecks, electric vehicle battery systems face "four high" requirements: high specific energy, high safety, long life, and high state accuracy. How to improve the "four high" indicators based on existing batteries? This is what battery management does.

　　The main functions of the battery management system are divided into four parts: the first is collection, the main function includes the collection of voltage, temperature, current and other information; the second is output, that is, whether the remaining mileage can be accurately calculated; the third is balancing, that is, a large number of batteries are strung together How to make the performance work better; the fourth is thermal management, that is, ensuring that the battery works at the appropriate temperature and has better performance.

　　2. Power battery status estimation and fault diagnosis analysis

　　Power battery status description indicators include SOC estimation, SOH estimation, SOP estimation, SOE estimation, etc. Xu Jun pointed out that battery status cannot be directly measured by sensors, and the battery system has strong nonlinearity and time variability. At the same time, complex and changeable usage environments and usage conditions increase the difficulty of status estimation.

　　Common SOC estimation methods include ampere-hour integration method, data-driven method, model-based method, etc. According to Xu Jun, the main problem with the ampere-hour integration method is that the initial SOC is difficult to measure. The current solution is the ampere-hour integration method plus correction, which is more commonly used. There are many data-driven methods, such as neural network models, etc. This method requires a large amount of experimental data to train the model and high-performance computing, and is not universal, so it is rarely used in practice. The main problem with the model-based method is that as the battery decays, the model changes at any time, resulting in inaccurate estimation. This method has received a lot of research, and some of it has been put into practical use.

　　Common SOH estimation methods include: direct measurement method, online estimation, indirect method, etc. The direct measurement method refers to directly measuring the characteristic parameters of the battery to evaluate the battery SOH. It mainly includes capacity/energy measurement and impedance measurement methods, which are usually performed under laboratory conditions. The key issue in online estimation is the accuracy of SOC. The indirect method is obtained by using the relationship between other quantities and the actual capacity.

　　3. Power battery balancing structure and strategy analysis

　　Balancing is mainly to solve the problem of battery inconsistency, which is caused by many reasons, including inconsistencies caused by the manufacturing process and inconsistencies caused by the use process. Inconsistent batteries can easily lead to overcharging or over-discharging, which may lead to thermal runaway or even explosion.

　　Xu Jun said equalization and reconstruction are effective methods to solve battery inconsistencies. The balancing topology is the hardware foundation for battery balancing. The design of the topology is the initial step in the design of the battery balancing system, providing a design basis for the subsequent formulation of balancing control strategies and the construction of experimental platforms.

　　There are two ways of balancing: passive balancing and active balancing. Passive equalization is to dissipate excess energy in the battery in the form of heat energy through resistors until all battery states reach the same state. Its advantages are simple structure and low cost, but its disadvantages are low equilibrium efficiency and serious energy consumption.

　　Consistency control strategies include voltage-based, SOC-based, capacity-based and other methods. Among them, the advantage of the voltage-based method is that it is convenient, intuitive, simple, and widely used. The disadvantage is that the battery terminal voltage gap is small and the equalization effect is poor; the SOC-based method can effectively avoid excessive equalization, but it requires more controller design. High, it is difficult to use; the capacity-based method can obtain the maximum usage capacity, but the calculation is complex and it is difficult to use.

　　4. Power battery structural design and thermal management analysis

　　Xu Jun pointed out that batteries need to work within a very suitable temperature range. If the temperature is too high or too low, it will affect the performance characteristics of the battery. Working in a high-temperature environment will cause the battery temperature to be too high, leading to thermal runaway. In severe cases, the battery may even explode. When the temperature is too low, the battery can release and charge very little power. When used at low temperatures, the battery will have an internal short circuit, and the internal short circuit may cause thermal runaway.

　　The purpose of thermal management is to ensure battery safety and enable the battery to perform better. The main functions of thermal management are: (1) Effective heat exchange when the battery temperature is too high to prevent thermal runaway accidents; (2) Temperature preheating when the battery temperature is low to ensure charge and discharge performance; (3) Reduce The temperature difference within the battery pack inhibits the formation of local hot zones. Therefore, battery thermal management is of great significance to improving vehicle performance.

　　Battery thermal management methods include air cooling, liquid cooling, phase change materials, heat pipes, etc. Xu Jun admitted that with large subsidies for new energy vehicles, many power batteries currently on the market do not have thermal management. He believes that after subsidies are withdrawn, everyone will speak for themselves with their products. Whoever has better technology will get a larger market, and the higher the recognition of the product by actual users.

　　Air cooling means air cooling, and the cooling medium is air. Xu Jun said that the Prius air-cooling system is currently more famous in this regard. Some domestic manufacturers call air cooling just by adding a few fans, and the effect is not particularly good.

　　Liquid cooling is gradually recognized in China, and more and more manufacturers are launching products using liquid cooling.

　　Phase change material cooling is to immerse the battery pack directly in phase change material (PCM). It can also adopt a jacket structure and put a layer of ring-shaped PCM on the outside of the single battery to form a slightly larger single battery, and then form a larger single battery. Battery. When the battery is discharging, the system stores heat in the PCM in the form of phase change latent heat, thereby absorbing the heat released by the battery and rapidly reducing the battery temperature.

　　Heat pipe cooling uses a hollow tube with a sealed structure and uses evaporative phase change to transfer heat. The main advantage of heat pipe cooling is that it can conduct heat from one side to the other instantly. According to Xu Jun, heat pipes have been used in consumer electronics products, but are relatively rarely used in electric vehicle battery management systems.

　　In addition to cooling, battery thermal management also includes low-temperature heating. Low-temperature heating methods include external heating and internal heating. External heating methods include air/liquid heating methods, film heating methods, other heating methods, etc., and internal heating methods include AC heating methods, internal self-heating methods, etc.

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