The inconsistency of lithium-ion batteries will affect the service life of the battery pack and reduce the performance of the battery pack. The inconsistency of lithium battery groups refers to the differences in parameters such as capacity, voltage, internal resistance, and self-discharge rate of single batteries, which are caused by differences in the combination structure, operating conditions, operating environment, and battery management of the battery pack.
　　The development of lithium battery technology has always focused on controlling costs, improving the energy density and power density of lithium batteries, enhancing the safety of use, prolonging service life, and improving group consistency. The improvement of these elements is still the biggest challenge facing lithium batteries. challenge. The inconsistency of single cells is an important factor affecting the performance of the battery pack, which can reduce the available capacity of the battery pack and reduce the cycle life of the battery pack.
　　After the single batteries are grouped, the cycle life will be reduced. Combining single cells with longer service life into a battery pack will increase the number of battery pack cycles. However, in order to improve the overall performance of the battery pack and obtain a longer service life, attention should also be paid to the matching consistency of the single cells to provide suitable working conditions. Conditions and proper thermal management measures, timely repair and maintenance. Based on the analysis of the causes of inconsistency of lithium-ion battery packs, improvement measures and optimization methods for battery inconsistency are proposed.
　　1. Inconsistency mechanism
　　1 Parameter difference between single batteries The
　　state difference between single batteries mainly includes initial differences of single batteries and parameter differences generated during use. There are many uncontrollable factors in the process of battery design, manufacture, storage and use, which will affect the consistency of the battery. Improving the consistency of single cells is a prerequisite for improving the performance of battery packs. The interaction of single battery parameters, the current parameter state is affected by the initial state and time accumulation.
　　Battery capacity, voltage and self-discharge rate Inconsistent battery capacity will cause inconsistent discharge depths of individual cells in the battery pack. A battery with a small capacity and poor performance will reach the full charge state ahead of time, causing a battery with a large capacity and good performance to fail to reach the full charge state. The inconsistency of the battery voltage will cause the single cells in the parallel battery pack to charge each other, and the battery with a higher voltage will charge the battery with a lower voltage, which will accelerate the attenuation of battery performance and consume the energy of the entire battery pack. A battery with a high self-discharge rate will lose a lot of capacity, and the inconsistency of the self-discharge rate of the battery will lead to differences in the state of charge and voltage of the battery, which will affect the performance of the battery pack.
　　Battery internal resistance
　　In a series system, the difference in the internal resistance of the individual batteries will lead to inconsistent charging voltages of each battery. The battery with a large internal resistance reaches the upper voltage limit ahead of time, and other batteries may not be fully charged at this time. A battery with a large internal resistance loses a lot of energy, generates high heat, and the temperature difference further increases the difference in internal resistance, leading to a vicious circle.
　　In a parallel system, the difference in internal resistance will lead to inconsistencies in the current of each battery, and the voltage of a battery with a large current changes rapidly, making the charging and discharging depth of each single battery inconsistent, making it difficult for the actual capacity of the system to reach the design value. The working current of the battery is different, and its performance will vary during use, which will eventually affect the life of the entire battery pack.
　　2 Charging and discharging conditions
　　The charging method affects the charging efficiency and charging state of the lithium battery pack. Overcharging and overdischarging will damage the battery, and the battery pack will show inconsistency after multiple charging and discharging. At present, there are several charging methods for lithium-ion batteries, but the common ones are segmental constant current charging and constant current constant voltage charging. Constant current charging is an ideal method, which can carry out safe and effective full charging; constant current and constant voltage charging effectively combines the advantages of constant current charging and constant voltage charging, and solves the problem that the general constant current charging method is difficult to charge accurately. It avoids the impact of the excessive current on the battery caused by the constant voltage charging method at the initial stage of charging, and the operation is simple and convenient.
　　3 Temperature
　　The performance of lithium batteries will be significantly attenuated under high temperature and high discharge rate. This is because when the lithium-ion battery is used under high temperature conditions and high current, it will cause the decomposition of the positive electrode active material and the electrolyte. The decomposition phenomenon is accelerated again, forming a vicious circle, and the accelerated decomposition further reduces the performance of the battery. Therefore, if the thermal management of the battery pack is improper, it will cause irreversible performance loss.
　　Differences in battery pack design and operating environment will result in inconsistent temperature environments for individual batteries. According to the Arrhenius law, the electrochemical reaction rate constant of the battery is exponentially related to the temperature, and the electrochemical characteristics of the battery are different at different temperatures. Temperature will affect the operation of battery electrochemical system, coulombic efficiency, charge and discharge capacity, output power, capacity, reliability and cycle life. At present, the main work is the quantitative research on the influence of temperature on the inconsistency of battery packs.
　　4. Battery external circuit
　　Connection mode In a large-scale energy storage system, batteries will be combined in series and parallel, so there will be many connection circuits and control components between batteries and modules. Due to the different performance and aging speed of each structural part or component, as well as the inconsistent energy consumption of each connection point, different components have different effects on the battery, resulting in inconsistent battery pack systems. Inconsistent battery decay rates in parallel circuits can accelerate system degradation.
　　The impedance of the connection piece will also affect the inconsistency of the battery pack. The resistance value of the connection piece is not the same, and the resistance value of the pole to each single battery branch is different. The battery far away from the pole has a higher resistance value due to the longer connection piece. If the current is large, the current is small, and the connecting piece will make the single battery connected to the pole reach the cut-off voltage first, resulting in a decrease in energy utilization and affecting battery performance, and the premature aging of the single battery will lead to overcharging of the battery connected to it , causing a safety hazard.
　　As the number of battery cycles increases, the ohmic internal resistance will increase, the capacity will decay, and the ratio of ohmic internal resistance to the resistance of the connecting piece will change. In order to ensure the safety of the system, the influence of the resistance value of the connecting piece must be considered.
　　BMS input circuit The battery management system (BMS) is the guarantee for the normal operation of the battery pack, but the BMS input circuit will have an adverse effect on the consistency of the battery. The battery voltage monitoring methods include precision resistor voltage division, integrated chip sampling, etc. These methods cannot avoid the external load leakage current of the sampling line due to the existence of the resistance and the circuit board path, and the voltage sampling input impedance of the battery management system will increase the battery state of charge ( SOC) inconsistency affects the performance of the battery pack.
　　5 SOC estimation error
　　The reasons for the inconsistency of SOC are the inconsistency of the initial nominal capacity of the single battery and the inconsistency of the nominal capacity decay speed of the single battery during work. For a parallel circuit, the difference in internal resistance of the single cells will cause uneven current distribution, which in turn will lead to inconsistent SOC. SOC algorithms include ampere-hour integral method, open circuit voltage method, Kalman filter method, neural network method, fuzzy logic method, discharge test method, etc.
　　The ampere-hour integration method has better accuracy when the initial state of charge SOC0 is relatively accurate, but the coulombic efficiency is greatly affected by the battery state of charge, temperature and current, and it is difficult to measure accurately, so the ampere-hour integration method is difficult. Meet the accuracy requirements of state of charge estimation. In the open circuit voltage method, after standing for a long time, the open circuit voltage of the battery has a definite functional relationship with the SOC, and the estimated value of the SOC is obtained by measuring the terminal voltage. The open circuit voltage method has the advantage of high estimation accuracy, but the disadvantage of long rest time also limits its application range.
　　2. Group inconsistency optimization method
　　1 Single cell manufacturing technology Lithium battery
　　materials The positive electrode materials of lithium ion batteries include ternary materials, lithium iron phosphate, lithium cobaltate and lithium manganate, etc., and the negative electrode materials include graphite, silicon and lithium titanate wait. The same batch of raw materials is very important to the consistency of battery performance. During the production process, parameters such as particle size distribution, specific surface area and impurity content of raw materials need to be strictly controlled to ensure batch consistency of raw materials.
　　Lithium-ion battery production process The battery production process consists of multiple processes, and each process may affect the consistency of the battery. In order for the performance of the production monomer to be consistent, each process must be reasonably designed and controlled so that it can be repeated in parallel. Design the battery production process according to the performance requirements of the battery, and analyze the influence of parameters such as raw materials, electrodes and electrolyte on the consistency of the battery, so as to reasonably control the threshold of each process parameter. The reduction of human intervention in the production line and the realization of automation can also improve the consistency of batteries.
　　2 Sorting system
　　In order to reduce the adverse impact of the initial state difference on the battery pack, it is usually necessary to screen the single batteries and combine the batteries with more consistent state parameters. The battery grouping methods mainly include single-parameter matching method, multi-parameter matching method and dynamic characteristic curve matching method. The dynamic characteristic curve matching method can reflect the battery characteristics well by comparing the difference between the charge and discharge curves of different batteries under the same rate, and the sorting effect is ideal.
　　3 battery pack external circuit
　　The connection mode of the battery pack in series and parallel mode affects the consistency of the battery. At present, there are two better connection methods: first connect two identical batteries in parallel to form a module, and then connect the modules in series (PSB); first connect two different batteries in series to form a module, and then connect the modules in parallel (SPA).
　　In order to improve the performance and service life of the battery, the battery management system needs to manage and maintain the single battery. The battery management system is an important guarantee for the normal operation of the battery system. Its main task is to ensure the performance of the battery pack, prevent battery damage, avoid safety accidents, make the battery work in a suitable area, and prolong its life. BMS consists of sensors, actuators, controllers, and signal lines. Its main functions include: data acquisition, state estimation, charge and discharge control, equalized charge, heat management, safety management, and data communication.
　　Although battery management technology has been widely used, it still needs to be improved, especially in terms of SOC estimation and data acquisition accuracy, equalization circuit, and battery fast charging. Due to the differences in the characteristics of different types of batteries, BMS applicable to all batteries is the main research direction at present.
　　Balance control In order to alleviate or even eliminate the inconsistency among the individual cells in the battery pack and improve the performance, life and safety of the battery pack, the inconsistency of the battery pack can be effectively improved through the balance circuit and balance control strategy.
　　Balanced circuit topology: The research on balanced circuit topology is mainly to design and improve the balanced circuit structure, improve the equalization efficiency and reduce the cost. According to whether the circuit consumes energy during the equalization process, the equalization circuit can be divided into energy-consuming equalization and non-energy-consuming equalization. The energy-consuming equalization circuit uses energy-consuming elements to consume the battery power with higher voltage in the battery pack, so as to achieve the consistency of the single battery. The circuit is simple, the equalization speed is fast, and the efficiency is high, but the energy utilization rate of the battery pack will not be high; Energy-consuming circuits use energy storage elements and balancing external circuits to realize energy transfer between batteries, and have high energy utilization efficiency. Non-energy-consuming equalization includes switched capacitors, converters and transformers.
　　Balance control strategy: The balance control strategy mainly determines the working mode of the balance module. At present, the working methods include maximum value equalization method, average value comparison method and fuzzy control method. The improvement of equalization ability is an important direction of battery consistency research. Equalization technology needs to be further improved, including:
　　(1) SOC is the most ideal judgment standard, and the real-time estimation accuracy needs to be further improved; (2) The topological structure of the equalization circuit should be optimized to increase the equalization speed and shorten the equalization time; (3) The equalization control strategy needs to be optimized to determine the optimal The best equalization parameters, according to the equalization circuit to find a suitable equalization path to achieve the purpose of fast equalization.
　　At present, most researches on equalization control strategies focus on the design and implementation of equalization hardware circuits. However, the parameters of the equalization circuit will affect the equalization effect. In addition, the state of charge of the battery, the equalization threshold, the charge and discharge current, the ratio of the balance current to the charge and discharge current, and the switching mode of charge and discharge conditions will also affect the equalization effect.
　　4 Charge and discharge strategy
　　Scientific and reasonable charge and discharge strategy can improve battery energy utilization efficiency. At present, the charging method with the best comprehensive performance is the coordination of the battery management system and the charger to charge in series, through the BMS to monitor the ambient temperature of the battery pack, the voltage and current of the single battery, the consistency and temperature rise, etc. Data sharing, changing the output current in real time, can prevent battery overcharging and optimize charging. This charging method is currently the mainstream, which can eliminate the problems of poor consistency, low charging efficiency and inability to fully charge lithium battery packs to a certain extent.
　　5 Battery thermal management
　　The heat production and heat dissipation of each single battery in the battery pack are unevenly distributed in space, which will cause inconsistent temperatures of the battery itself, some areas of the battery pack, and the environment in which it is located. If it is not controlled, the internal temperature of the battery pack will The temperature difference will continue to expand, thereby accelerating the degradation of battery performance. Therefore, thermal management of the battery pack is required.
　　Thermal management systems are usually required to be compact, lightweight, easy to package, reliable, low cost, and easy to maintain. Its functions are: to make the battery run in the most suitable temperature range; to reduce the temperature difference between batteries, modules and modules. There are two types of thermal management: active and passive. The heat transfer medium used in the system can be divided into three categories, namely air, liquid and phase change material.
　　At present, the research on thermal management of battery packs has limitations. For example, the thermal model of the battery is too simplified. The battery cell often adopts a zero-dimensional heat generation model. The heat generation rate of each part of the battery is the same. Performance comparison. There are few studies on the low-temperature characteristics of lithium-ion batteries and low-temperature thermal management technology.