The biggest difference between new energy vehicles and traditional cars is that they are driven by batteries, so power batteries are the core of new energy vehicles. The power output of electric vehicles relies on batteries, and the battery management system BMS (Battery Management System) is the core. It is a system that monitors and manages the battery. It controls the battery by collecting and calculating parameters such as voltage, current, temperature, and SOC. The charge and discharge process, the management system that protects the battery and improves the comprehensive performance of the battery, is an important link between the vehicle power battery and the electric vehicle. Foreign companies that are relatively good at BMS include UMC, Continental, Delphi, AVL and FEV, etc. Now they basically follow the AUTOSAR architecture and ISO26262 functional safety requirements. The software has more functions and has higher reliability and accuracy. . Many domestic OEMs also have self-developed BMS products and applications. In the early stage, their functions and performance were far from those of first-class foreign companies. However, with the rapid development of domestic battery and BMS technology, the gap is gradually narrowing. It is hoped that in the near future Achieve success to catch up or even surpass. BMS mainly consists of three parts: hardware, underlying software and application layer software.

　　Hardware 1. Architecture The topology structure of BMS hardware is divided into two types: centralized and distributed: (1) Centralized type is to concentrate all electrical components into a large board. The sampling chip channel has the highest utilization and the sampling chip is connected to the main chip. Daisy chain communication can be used between them. The circuit design is relatively simple and the product cost is greatly reduced. However, all acquisition wire harnesses will be connected to the motherboard, which poses greater challenges to the security of the BMS. There may also be problems with the stability of daisy chain communication. question. It is more suitable for situations where the battery pack capacity is relatively small and the module and battery pack types are relatively fixed. (2) Distribution includes a main board and a slave board. One battery module may be equipped with a slave board. The disadvantage of such a design is that if the number of cells in the battery module is less than 12, it will cause a waste of sampling channels (generally there are 12 sampling chips channel), or 2-3 slave boards collect all battery modules. This structure has multiple sampling chips in one slave board. The advantages are high channel utilization, cost saving, system configuration flexibility, and adaptability to different capacities and Modules and battery packs of different specifications and types.

　　2. The design and specific selection of functional hardware should be combined with the functional requirements of the entire vehicle and battery system. Common functions mainly include collection functions (such as voltage, current, and temperature collection), charging port detection (CC and CC2) and charging wake-up ( CP and A+), relay control and status diagnosis, insulation detection, high-voltage interlocking, collision detection, CAN communication and data storage and other requirements.

　　(1) The main controller processes the information reported by the slave controller and the high-voltage controller, and at the same time judges and controls the operating status of the power battery based on the reported information, implements BMS-related control strategies, and makes corresponding fault diagnosis and processing. (2) The high-voltage controller collects and reports the total voltage and current information of the power battery in real time, realizes on-time integration through its hardware circuit, and provides accurate data for the main board to calculate the state of charge (State of Charge, SOC) and health state (State of Health, SOH). At the same time Precharge detection and insulation detection functions can be realized.

　　(3) The controller collects and reports power battery cell voltage and temperature information in real time, and feeds back the SOH and SOC of each string of cells. It also has a passive balancing function, effectively ensuring the consistency of the cells during power use.

　　(4) The sampling control wiring harness provides hardware support for various information collection of power batteries and information interaction between controllers. At the same time, a redundant insurance function is added to each voltage sampling line to effectively avoid external short circuits of the battery caused by wiring harnesses or management systems. .

　　3. Communication method There are two ways of transmitting information between the sampling chip and the main chip: CAN communication and daisy chain communication. CAN communication is the most stable, but due to the need to consider the high cost of power supply chips, isolation circuits, etc., daisy chain communication is actually It is SPI communication. The cost is very low and the stability is relatively poor. However, with the increasing pressure on cost control, many manufacturers are changing to the daisy chain method. Generally, two or more daisy chains will be used to enhance it. Communication stability.

　　4. Structure BMS hardware includes power IC, CPU, sampling IC, high drive IC, other IC components, isolation transformer, RTC, EEPROM and CAN module, etc. The CPU is the core component, and Infineon's TC series is generally used. Different models have different functions, and the configuration of the AUTOSAR architecture is also different. Sampling IC manufacturers mainly include Linear Technology, Maxim, Texas Instruments, etc., including collecting cell voltage, module temperature, and peripheral configuration balancing circuits. The underlying software is divided into many common functional modules according to the AUTOSAR architecture, reducing dependence on hardware and enabling configuration of different hardware, while the application layer software changes little. The application layer and the bottom layer need to determine the RTE interface, and consider DEM (Fault Diagnosis Event Management), DCM (Fault Diagnosis Communication Management), FIM (Function Information Management) and CAN communication reserved interfaces from the aspect of flexibility, and configure them by the application layer . The application layer software software architecture mainly includes high and low voltage management, charging management, state estimation, balance control and fault management, etc.

　　1. High and low voltage management Generally, when the power is turned on normally, the VCU will wake up the BMS through the 12V of the hard wire or CAN signal. After the BMS completes the self-test and enters standby, the VCU sends a high-voltage command, and the BMS controls the closing relay to complete the high-voltage. When powering off, VCU sends a high voltage command and then disconnects to wake up 12V. When the gun is plugged in and charging in the power-off state, it can be woken up by the CP or A+ signal.

　　2. Charging management (1) Slow charging: Slow charging uses an AC charging pile (or 220V power supply) to convert AC into DC through a car charger to charge the battery. Charging pile specifications generally include 16A, 32A and 64A, or through household power supply. to charge. The BMS can be woken up through CC or CP signals, but it should be ensured that it can sleep normally after charging. The AC charging process is relatively simple and can be developed in accordance with the detailed regulations of the national standard. (2) Fast charging: Fast charging uses DC charging piles to output DC to charge the battery, which can achieve 1C or higher rate charging. Generally, 80% of the battery can be charged in 45 minutes. It is awakened by the A+ signal of the auxiliary power supply of the charging pile. The fast charging process in the national standard is relatively complicated. There are two versions of 2011 and 2015. Moreover, charging pile manufacturers have different understandings of the technical details of the national standard process that are not clear, which also affects the vehicle charging adaptability. It is a huge challenge, so fast charging adaptability is a key indicator to measure the performance of BMS products.

　　3. Estimation function (1) SOP (StateOfPower) mainly obtains the current available charging and discharging power of the battery through temperature and SOC lookup tables. The VCU determines how to use the current vehicle based on the sent power value. It is necessary to consider both releasing battery capacity and protecting battery performance, such as limiting part of the power before reaching the cut-off voltage. Of course, this will have a certain impact on the driving experience of the entire vehicle. (2) SOH (StateOfHealth) mainly represents the current health status of the battery, which is a value between 0-100%. It is generally believed that the battery cannot be used after it is lower than 80%. It can be expressed by changes in battery capacity or internal resistance. When using capacity, the actual capacity of the current battery is estimated through the battery operating process data. The ratio to the rated capacity is SOH. Accurate SOH will improve the accuracy of other modules' estimation of battery degradation. (3) SOC (StateOfCharge) belongs to the BMS core control algorithm and represents the current remaining capacity state. It mainly uses the ampere-hour integration method and the EKF (Extended Kalman Filter) algorithm, combined with correction strategies (such as open circuit voltage correction, full correction, charging End correction, capacity correction under different temperatures and SOH, etc.). The ampere-hour integration method is relatively reliable under the condition of ensuring the accuracy of current collection, but it is not robust. Due to the error accumulation, it must be combined with a correction strategy. The EKF is more robust, but the algorithm is more complex and difficult to implement. Domestic mainstream manufacturers can generally achieve an accuracy within 6% at room temperature, but estimation at high and low temperatures and battery decay is difficult. (4) The SOE (StateOfEnergy) algorithm is currently not developed by many domestic manufacturers, or a relatively simple algorithm is used to look up the table to obtain the ratio of the remaining energy to the maximum available energy in the current state. This function is mainly used for remaining cruising range estimation.

　　4. Fault diagnosis is divided into different fault levels according to different performance conditions of the battery. Under different fault levels, BMS and VCU will take different processing measures, such as warning, power limit or direct cut off of high voltage. Faults include data acquisition and rationality faults, electrical faults (sensors and actuators), communication faults, and battery status faults.

　　5. Balance control The balance function is to eliminate the inconsistency of battery cells produced during battery use. According to the short board effect of the barrel, the cell with the worst performance reaches the cut-off condition first during charging and discharging, and the other cells There are still some capabilities that have not been released, causing battery waste. Balance includes active balance and passive balance. Active balance is the transfer of energy from more monomers to less monomers, which does not cause energy loss. However, the structure is complex, the cost is high, and the requirements for electrical components are also high. Relatively speaking, passive balance is The balancing structure is simple and the cost is much lower, but the energy will be dissipated and wasted in the form of heat. Generally, the maximum balancing current is about 100mA. Nowadays, many domestic manufacturers use passive balancing to achieve better balancing effects. Conclusion The BMS control method, as the central control idea of power batteries, directly affects the service life of power batteries and the safe operation and vehicle performance of electric vehicles. It has a significant impact on battery life and determines the future of new energy vehicles. A good battery management system will greatly promote the development of new energy vehicles.

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