1 introduction

Lithium ion batteries used in electric vehicles have a large capacity, a large number of serial and parallel sections, a complex system, and high performance requirements for safety, durability, power and other properties, which are difficult to implement. Therefore, they have become the bottleneck affecting the popularization of electric vehicles. The safe working area of lithium-ion batteries is limited by the window of temperature and voltage. If the range of the window is exceeded, the battery performance will be accelerated to decay and even safety problems will occur. At present, most of the vehicle lithium ion battery, the reliable operating temperature is required, discharge -20~55C, charging 0~45C (for graphite negative), and the lowest temperature of negative LTO charging is -30C; The working voltage is generally about 1.5~4.2V (about 2.5~4.2V for LiCoO2/C, LiNi0.8Co0.15Al0.05O2/C, LiOxNiYMnZo2 /C and LiMn2O4/C material systems). About 1.5~2.7V for LiMn2O4/Li4Ti5O12 material system and about 2.0~3.7V for LiFePO4/C material system).

Temperature has a decisive influence on the performance of lithium-ion batteries, especially the safety. According to the different electrode material types, lithium-ion batteries (C/LiMn2O4, C/ LMo, C/ LiOxNiYMnZO2, C/NCM, C/LiFePO4, C/LiNi0.8Co0.15Al0.05O2, C/NCA) typical operating temperatures are as follows: discharge at -20-55℃, charging at 0-45℃; When the cathode material is Li4Ti5O12 or LTO, the minimum charging temperature can often reach -30℃.

When the temperature is too high, the battery life can be adversely affected. When the temperature is high enough, it may cause safety problems. As shown in Fig. 1, when the temperature is 90~120℃, the SEI film will begin exothermic decomposition [1~3], while some electrolyte systems will decompose at a lower temperature of about 69℃[4]. When the temperature exceeds 120℃, the SEI film can not protect the negative carbon electrode after decomposition, so that the negative electrode reacts directly with the organic electrolyte and combustible gas will appear [3]. When the temperature is 130 ° C, the membrane will begin to melt and close the ion channel, making the positive and negative electrodes of the battery temporarily without current flow [5,6]. When the temperature rises, the cathode material begins to decompose (LiCoO2 begins to decompose at about 150℃[7], LiNi0.8Co0.15Al0.05O2 at about 160℃[8,9], LiNiCoYMnZo2 at about 210℃[8], LiMn2O4 at about 265℃[1], LiFePO4 presents oxygen at about 310℃[7]). When the temperature is higher than 200℃, the electrolyte will decompose and produce flammable gas [3], which will react violently with the oxygen generated by the decomposition of the positive electrode [9], thus leading to thermal runaway. Charging at below 0℃ will cause lithium metal to form an electric coating on the surface of the negative electrode, which will reduce the cycle life of the battery. [10]

Too low a voltage or too high a discharge can cause the electrolyte to decompose and produce flammable gas, which can lead to a potential safety risk. Excessive voltage or overcharging may cause the cathode material to be inactivated and produce a large amount of heat; Common electrolytes decompose at voltages higher than 4.5V [12]

In order to solve these problems, people trying to develop ability to work in a very bad situation for the new battery system, on the other hand, the current commercial lithium ion battery must connect management system, make the lithium ion battery can get effective control and management of every single battery under the condition of appropriate work, fully guarantee the security and durability of the battery and power performance.

2 battery management system meaning

The important task of battery management system is to ensure the design performance of battery system, which can be divided into the following three aspects:

1) Safety, protect the battery monomer or battery pack from damage, to prevent safety accidents;

2) Durability, make the battery work in a reliable safety area, prolong the service life of the battery;

3) Power performance, to keep the battery working in a state that meets the requirements of the vehicle. The safe working area of the lithium-ion battery is shown in Figure 1.

Figure 1 shows the safe operation window of the lithium-ion battery

BMS consists of various kinds of sensors, actuators, controllers and signal lines, etc. In order to meet the relevant standards or specifications, BMS should have the following functions.

1) Battery parameter detection. Including total voltage, total current, single battery voltage detection (to prevent the occurrence of overcharge, overdischarge or even reverse phenomenon), temperature detection (it is best to have a temperature sensor for each battery, key cable joints), smoke detection (electrolyte leakage monitoring, etc.), insulation detection (leakage monitoring), collision detection, etc.

2) Battery state estimation. Including state of charge (SOC) or discharge depth (DOD), health state (SOH), functional state (SOF), energy state (SOE), failure and safety state (SOS), etc.

3) On-line fault diagnosis. Including fault detection, fault type judgment, fault location, fault information output and so on. Fault detection refers to the acquisition of sensor signals, the use of diagnostic algorithm to diagnose the type of fault, and early warning. Battery fault refers to sensor fault of battery pack, high voltage circuit, thermal management and other subsystems, actuator fault (such as contactor, fan, pump, heater, etc.), as well as network fault, hardware and software fault of various controllers, etc. The battery pack fault itself refers to overvoltage (overcharge), undervoltage (overdischarge), overcurrent, ultra-high temperature, internal short circuit fault, joint loosening, electrolyte leakage, insulation reduction, etc.

4) Battery safety control and alarm. Including thermal system control, high voltage safety control. After the fault is diagnosed, the BMS notifies the vehicle controller through the network and requires the vehicle controller to deal with the fault effectively (when the fault exceeds a certain threshold, the BMS can also cut off the power supply of the main circuit) to prevent damage to the battery and human body caused by high temperature, low temperature, overcharge, overdischarge, overcurrent and leakage.

5) Charging control. BMS has a charging management module, which can control the charger to charge the battery safely according to the characteristics of the battery, the temperature and the power level of the charger.

6) Battery balance. The existence of inconsistencies makes the capacity of the battery pack smaller than that of the smallest cell in the pack. Battery equalization is to make the battery pack capacity as close as possible to the capacity of the minimum cell by using the balancing methods, such as active or passive, dissipative or non-dissipative, according to the information of the single cell.

7) Thermal management. According to the temperature distribution information in the battery pack and the charge and discharge requirements, determine the intensity of active heating/cooling, so that the battery can work at the most suitable temperature as far as possible, and give full play to the performance of the battery.

8) Network communication. BMS should communicate with the vehicle controller and other network nodes; At the same time, it is not convenient to disassemble the BMS on the vehicle, so online calibration, monitoring, automatic code generation and online program download (program update without disassembling the product) should be carried out without disassembling the shell. Generally, the vehicle network adopts CAN bus technology.

9) Information storage. Used for storing key data, such as SOC, SOH, SOF, SOE, cumulative charge/discharge Ah number, fault code and consistency, etc. A real BMS in a vehicle may only have some of the hardware and software mentioned above. Each battery unit shall have at least one battery voltage sensor and one temperature sensor. For battery systems with dozens of batteries, there may be only one BMS controller, or even integration of BMS functionality into the vehicle's main controller. For battery systems with hundreds of cell units, there may be a master controller and multiple slave controllers that manage only one cell module. For each battery module with dozens of cell units, there may be a number of module circuit contactors and balance modules, and the same management of the battery module from the controller like measuring voltage and current, control the contactor, balance the battery unit and communicate with the main controller. Based on the reported data, the master controller will perform battery state estimation, fault diagnosis, thermal management, etc.

10) EMC. Due to the harsh operating environment of electric vehicles, BMS is required to have good anti-EMI capability and low external radiation. The basic framework of electric vehicle BMS software and hardware is shown in Figure 2.

Figure 2 Basic framework of hardware and software of vehicle-mounted BMS

Key issues with 3BMS

Although a BMS has many functional modules, this article only analyzes and summarizes its key issues. Currently, key issues relate to battery voltage measurement, data sampling frequency synchronization, battery state estimation, battery uniformity and equalization, and accurate measurement of battery fault diagnosis.

3.1 Battery voltage measurement (CVM)

The difficulties of battery voltage measurement lie in the following aspects:

(1) The battery pack of an electric car has hundreds of cells connected in series, requiring many channels to measure voltage. Because the measured cell voltage has a cumulative potential, which varies from cell to cell, it is impossible to eliminate the error with a one-way compensation approach.

Figure 3OCV curve and change in SOC per millivolt (measured at 25 ° C, 3 hours rest time)

(2) Voltage measurement should be high precision (especially for C/LiFePO4 batteries). SOC estimation requires high accuracy of battery voltage. Here we take C/LFP and LTO/NCM batteries as examples. Figure 3 shows the open circuit voltage (OCV) of battery C/LiFePO4 and LTO/NCM and the corresponding SOC changes per mV. As can be seen from the figure, the slope of the LTO/NCM OCV curve is relatively steep, and the maximum SOC rate range corresponding to the voltage change per mV is less than 0.4% in most SOC ranges (except SOC60-70%). Therefore, if the measurement accuracy of the battery voltage is 10mV, the SOC error obtained by the OCV estimation method is less than 4%. Therefore, for LTO/NCM batteries, the measurement accuracy of battery voltage should be less than 10mV. However, the slope of the C/LiFePO4OCV curve is relatively flat, and in most ranges (except SOC< 40% and 65~ 80%), and the maximum corresponding SOC change rate per millivolt reaches 4%. Therefore, the collection accuracy of battery voltage is very high, up to about 1mV. Currently, most battery voltages are measured with an accuracy of only 5mV. In literature [47] and [48], the voltage measurement methods of lithium-ion battery and fuel-powered battery are summarized respectively. These methods include the resistive voltage divider method, the optically coupled isolation amplifier method, the discrete transistor method [49], the distributed measurement method [50], the optically coupled relay method [51], and so on. At present, the battery voltage and temperature sampling has formed the industrialization chip, and the performance of the chip used in most BMS is compared in Table 1.

Table 1 statistics battery management and equalization chips

3.2 Synchronization of data sampling frequency

The sampling frequency and synchronization of signal have influence on real-time data analysis and processing. When designing BMS, the sampling frequency and synchronization precision of the signal should be required. However, in the current BMS design process, there is no clear requirement for signal sampling frequency and synchronization. There are a variety of battery system signals, and the battery management system is generally distributed, if the current sampling and single-chip voltage sampling are on different circuit boards; In the process of signal acquisition, signals of different control subboards will have synchronization problems, which will affect the real-time monitoring algorithm of internal resistance. The same single chip voltage acquisition sub-board, generally adopts the patrol inspection method, the single voltage also exists synchronization problem, which affects the inconsistency analysis. System data of different signal sampling frequency and synchronization of different requirements, the low inertia big parameter requirements, such as the rise of temperature of the pure electric vehicle battery normal discharge order of magnitude of 1 ℃ / 10 min, considering the safety monitoring of temperature, at the same time considering the BMS temperature (about 1 ℃), the precision of the temperature of the sampling interval can be classified as 30 s (for hybrid lithium-ion batteries, The temperature sampling rate is higher).

The voltage and current signal change rapidly, so the sampling frequency and synchronization are very high. According to the AC impedance analysis, the internal ohmic resistance response of power lithium battery is in the MS level, the ion transport resistance voltage response of SEI membrane is in the 10ms level, the charge transfer (double capacitance effect) response is in the 1-10s level, and the diffusion process response is in the min level. At present, when the electric vehicle accelerates, the response time of the driving motor current from the minimum change to the maximum change is about 0.5s, and the current accuracy is required to be about 1%. Considering the variable load condition comprehensively, the current sampling frequency should be 10~200Hz. The number of voltage channels on the monolithic information acquisition subboard is usually a multiple of 6, and the current maximum is 24. General pure electric passenger car battery is composed of about 100 batteries in series, single battery signal acquisition to multiple acquisition subboards. In order to ensure voltage synchronization, the voltage sampling time difference between the cells in each acquisition sub-board should be as small as possible, and one inspection cycle should be within 25ms. Time synchronization between child boards CAN be achieved by sending a CAN reference frame. Data update frequency should be above 10Hz.

Battery state estimation, including an overview of SOC estimation methods, an overview of SOH estimation methods, an overview of SOF estimation methods, an overview of battery consistency and equalization methods, and an overview of fault diagnosis, will also be covered in the next two days.