With the gradual improvement of the safety and cost performance of lithium-ion batteries and the continuous maturity of group technology, lithium-ion power batteries have gradually entered high-power applications. Especially in the field of electric vehicles, the demand for lithium-ion power batteries has grown rapidly, and has begun to gradually replace lead-acid batteries and nickel-metal hydride batteries. Lithium-ion batteries have the advantages of high voltage, low self-discharge rate, high charge-discharge efficiency, long cycle life, no memory effect, and no pollution. Float charge, and prone to overcharge and overdischarge. In addition, compared with portable equipment or fixed equipment, the working environment of power batteries for electric vehicles is more complex and harsh
1. Application characteristics of lithium-ion batteries in electric vehicles
1) Large-scale series and parallel use
In order to achieve the corresponding power and energy levels, lithium-ion batteries need to be used in a large number of series when used in electric vehicles. Taking the Austrian pure electric bus as an example, the vehicle uses lithium manganate battery as the power battery, 4 parallel 104 strings, and the battery pack capacity is 360Ah. At the same time, the larger the capacity of the battery, the higher the requirements for safety. Therefore, different from the use and management of a single battery, the battery needs to be more perfect in system management and use technology when it is applied in a group.
2) Space is limited
On the one hand, the space limitation of electric vehicles makes the placement of power batteries relatively concentrated, which brings a lot of difficulties to the installation, heat dissipation and ventilation of the power battery system. Due to the cramped space and the difficulty of heat dissipation, it is possible to make the battery run at high temperature for a long time, which will lead to faster performance degradation of the battery, and even thermal runaway, which may cause safety hazards; on the other hand, due to the small space and the large capacity and volume of the equalization equipment required for large-capacity battery packs, most electric vehicles do not have the installation conditions for online equalizers, which makes the consistency problem of the battery pack more prominent and brings more stringent tests to the use of the battery.
3) Large working current and fast change
Since electric vehicles often experience acceleration, deceleration, braking and other working conditions during operation, the power battery needs to be able to respond quickly under various working conditions to meet the power requirements of the vehicle. For example, the current of the power battery changes rapidly when the vehicle is accelerated. The conversion time of the power battery current of the Olympic bus from 0A to 300A is required to be less than 0.5s, and the maximum operating current exceeds 350A.
4) Bad working environment
Compared with other applications, the vehicle may encounter poor road conditions or bumps during operation, which puts forward higher requirements for the shock resistance and vibration resistance of the power battery. At the same time, dust, rain and line wear may cause insulation problems such as creepage, short circuit, and grounding of the battery.
The high performance and harsh working environment required for electric vehicles pose great challenges to the safety and cycle life of power batteries and the effective use of power energy. In particular, the anti-abuse ability of lithium-ion batteries is poor, and the abuse of batteries (including overcharge, overdischarge, overheating and overcurrent, etc.) may lead to serious degradation of its life, and even safety accidents such as fire or explosion. Therefore, the safety and service life of lithium-ion power batteries for vehicles (especially lithium-ion batteries used in groups) have become an urgent problem to be solved in the use and management of lithium batteries.
2. Research content and key technical points of power battery management system
The high safety, service life and utilization efficiency of lithium-ion power batteries need to be jointly solved from the perspectives of electrochemistry and electricity. On the one hand, it is necessary to improve the performance of the battery itself through research and development; on the other hand, the above indicators are also closely related to the technical level of battery use and management. Scientific and effective management technology can realize functions such as monitoring battery parameters, estimating battery status, ensuring battery safety, efficient thermal field management, and avoiding rapid cycle life decay. As one of the core components of electric vehicles, the power battery management system mainly includes a number of technologies such as the basic method of battery management, , the design of battery management system architecture, battery consistency analysis, charge/discharge control, battery state estimation, battery balance implementation method and balance strategy, vehicle CAN communication network, electrical safety design, and system testing methods. The research content and key technical points of the power battery management system mainly include the following aspects.
1) Basic theory of power battery management system
In the group application of power batteries, there are mainly the following basic problems, including battery heating model and battery box thermal field distribution, lithium-ion power battery pack model, lithium-ion power battery pack life model, battery pack consistency evaluation, tracking and optimization of power battery pack charging model parameters, charging control methods, etc. In-depth research on the above topics and analysis of a large number of experiments and actual vehicle operation data, a new balancing method aiming at battery capacity is proposed, aiming at the shortcomings of the traditional battery voltage balancing method, so as to give full play to the operating efficiency of the entire battery pack; in order to ensure the safety and intelligence of the battery pack when charging, a charging method for tracking the maximum single cell voltage of the battery pack and a charging mode in which the battery management system and the charger cooperate with each other are proposed, thereby effectively improving the safety of the battery pack charging.
2) Design of power battery management system
In order to meet the actual operation requirements of electric vehicles, the power battery management system has very important performance indicators in terms of function, reliability, stability and practicability. It mainly includes the following aspects.
①In terms of battery detection, improve the measurement accuracy of voltage, temperature and current, and complete the hardware design of battery management system based on distributed structure;
②In the software design, it is necessary to combine the basic theory and practice, and embed it into the hardware system of the battery management system to fully realize the design concept of the battery management system;
③In terms of data communication, it is equipped with a complete communication interface, which can send battery information to the vehicle controller, display interface and charger. In terms of reliability, combined with modern large-scale integrated circuit technology, the anti-interference ability of system operation is improved;
④ In terms of database management,
Since batteries and electric vehicles are both in the stage of testing and improving, most management systems are equipped with a database management system that stores and monitors battery operation and charging data, which facilitates the evaluation of battery performance and provides data support for the optimal design of vehicle batteries.
3) Consistency evaluation technology for group power batteries
On the basis of scientific experiments and engineering practice, a consistency evaluation method based on battery internal resistance and capacity in the application process of pure electric vehicle power battery pack is proposed.
This method is aimed at the traditional consistency evaluation method based on the difference of the external voltage of the battery. Due to the inability to effectively indicate the inherent differences between batteries, the problem of reducing the equalization effect is analyzed. The reasons for the inconsistency of batteries in groups are analyzed from the perspective of use, and the influence of DC internal resistance, polarization voltage, maximum usable capacity and SOC difference of each single cell on the external voltage of the battery, and find the internal cause of the difference in the external voltage of the single cell, and use the key parameters as the evaluation index to evaluate the consistency of the battery pack.
4) Estimation method of power battery SOC
Based on the analysis of the operating conditions of electric vehicles, SOC estimation methods for pure electric vehicles and hybrid electric vehicles are proposed respectively. For pure electric vehicles, optimization includes various methods such as algorithms based on open circuit voltage, ampere-hour accumulation and dynamic correction; for HEVs, the SOC estimation method based on Kalman filter is adopted for the complex working condition characteristics of HEVs, so that the SC state of the battery can be accurately estimated in real time.
5) Power battery charging/discharging safety technology
Starting from a variety of technical solutions, a reliable and stable power battery charge/discharge/electricity control method has been developed and realized, mainly including vehicle communication control technology, charge/discharge safety control technology based on the highest voltage cell and lowest voltage cell to prevent battery overcharge and overdischarge and charge/discharge control technology based on battery temperature change rate, battery system and vehicle DC high voltage bus safety technology, battery system fault diagnosis technology, battery function state estimation technology and remote monitoring technology, etc. The communication protocol of the charging process between the electric vehicle and the charger, the discharge control and intelligent use of the battery management system and the vehicle controller are established. With the help of a variety of technical means, management mechanisms such as driving process safety, charging process safety, fire safety, and electrical safety of the battery system of electric vehicles are realized.
6) Power battery management system testing technology
Based on the research on battery characteristics and vehicle operating condition data, a comprehensive test platform is established that integrates battery system testing, charging and discharging machine testing, and monitoring host computer testing. On the basis of a number of international advanced standards for automotive electronics, power batteries, electronic and electrical, structural design, etc., the type test test items, test methods and operating procedures of the power battery management system were established, and the detection method system for the large-scale application of the power battery management system was constructed.
7) Standard system
Drafted and formulated relevant national standards including electric vehicle charging standards, power battery testing standards and technical conditions for power battery management systems. The test method for the type test of the power battery management system is specified, and the design standards of the entire domestic power battery management system industry are standardized.