What functions does a typical 3.7v 18650 lithium battery management system need to pay attention to?

Battery management system, BMS (Battery Management System), is an important component of the 3.7v 18650 lithium battery system of electric vehicles. On the one hand, it detects, collects and preliminarily calculates the real-time status parameters of the battery, and controls the on-off of the power supply circuit according to the comparison relationship between the detection value and the allowable value; on the other hand, it reports the collected key data to the vehicle controller, receives the instructions of the controller, and coordinates with other systems on the vehicle. Battery management system, different types of battery cells often have different requirements for management systems. So, what functions does a typical 3.7v 18650 lithium battery management system need to pay attention to? Today, I translated and sorted out an article. Let's take a look at the key technologies of BMS. The overall content is divided into three parts: upper, middle and lower.

What functions does a typical 3.7v 18650 lithium battery management system need to achieve (Part 1)

1 Introduction

The lithium-ion battery used in electric vehicles has a large capacity, a large number of series and parallel sections, and a complex system. In addition, the performance requirements such as safety, durability, and power are high and difficult to implement, so it has become a bottleneck affecting the promotion and popularization of electric vehicles. The safe working area of lithium-ion batteries is limited by temperature and voltage windows. If the range of this window is exceeded, the battery performance will accelerate attenuation, and even safety problems will occur. At present, most automotive lithium-ion batteries require a reliable operating temperature of -20~55°C during discharge and 0~45°C during charging (for graphite negative electrode), while the minimum temperature for negative electrode LTO charging is -30°C; the operating voltage is generally around 1.5~4.2V (about 2.5~4.2V for LiCoO2/C, LiNi0.8Co0.15Al0.05O2/C, LiCoxNiyMnzO2/C and LiMn2O4/C material systems, about 1.5~2.7V for LiMn2O4/Li4Ti5O12 material systems, and about 2.0~3.7V for LiFePO4/C material systems).

Temperature has a decisive influence on the performance of lithium batteries, especially safety. Depending on the type of electrode material, the typical operating temperature of lithium batteries (C/LiMn2O4, C/LMO, C/LiCoxNiyMnzO2, C/NCM, C/LiFePO4, C/LiNi0.8Co0.15Al0.05O2, C/NCA) is as follows: discharge at -20-55°C, charge at 0-45°C; when the negative electrode material is Li4Ti5O12 or LTO, the lowest charging temperature can often reach -30°C.

When the temperature is too high, it will have an adverse effect on the life of the battery. When the temperature reaches a certain level, it may cause safety problems. As shown in Figure 1, when the temperature is 90~120°C, the SEI film will begin to decompose exothermically [1~3], while some electrolyte systems will decompose at a lower temperature of about 69°C [4]. When the temperature exceeds 120°C, the SEI film decomposes and cannot protect the negative carbon electrode, causing the negative electrode to react directly with the organic electrolyte to produce flammable gas [3]. When the temperature is 130°C, the separator will begin to melt and close the ion channel, so that the positive and negative electrodes of the battery temporarily have no current flow [5,6]. When the temperature rises, the positive electrode material begins to decompose (LiCoO2 begins to decompose at about 150°C [7], LiNi0.8Co0.15Al0.05O2 at about 160°C [8,9], LiNixCoyMnzO2 at about 210°C [8], LiMn2O4 at about 265°C [1], LiFePO4 at about 310°C [7]) and produces oxygen. When the temperature is higher than 200°C, the electrolyte will decompose and produce flammable gas [3], and react violently with the oxygen produced by the decomposition of the positive electrode [9], leading to thermal runaway. Charging below 0°C will cause lithium metal to form an electroplating layer on the negative electrode surface, which will reduce the cycle life of the battery. [10]

Too low voltage or over-discharge will cause the electrolyte to decompose and produce flammable gases, leading to potential safety risks. Too high voltage or overcharging may cause the positive electrode material to lose its activity and generate a lot of heat; ordinary electrolytes will decompose when the voltage is higher than 4.5V [12]

In order to solve these problems, people are trying to develop new battery systems that can work under very harsh conditions. On the other hand, current commercial lithium-ion batteries must be connected to a management system so that lithium-ion batteries can be effectively controlled and managed, and each single cell works under appropriate conditions to fully ensure the safety, durability and power of the battery.

2 Definition of battery management system

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

1) Safety, protect the battery cell or battery pack from damage and prevent safety accidents;

2) Durability, make the battery work in a reliable safety area and extend the battery life;

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

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

1) Battery parameter detection. Including total voltage, total current, single cell voltage detection (to prevent overcharging, overdischarging, or even reverse polarity), temperature detection (it is best to have a temperature sensor for each battery string, key cable connector, etc.), smoke detection (monitoring electrolyte leakage, etc.), insulation detection (monitoring leakage), collision detection, etc.

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

3) Online fault diagnosis. Including fault detection, fault type judgment, fault location, fault information output, etc. Fault detection refers to the use of diagnostic algorithms to diagnose fault types and provide early warnings through the collected sensor signals. Battery failure refers to sensor failures, actuator failures (such as contactors, fans, pumps, heaters, etc.) of various subsystems such as battery packs, high-voltage circuits, and thermal management, as well as network failures, various controller software and hardware failures, etc. Battery pack failures refer to overvoltage (overcharging), undervoltage (overdischarging), overcurrent, ultra-high temperature, internal short circuit failures, loose joints, electrolyte leakage, insulation reduction, etc.

4) Battery safety control and alarm. Including thermal system control and high-voltage electrical safety control. After the BMS diagnoses a fault, it notifies the vehicle controller through the network and requires the vehicle controller to take effective measures (the BMS can also cut off the main circuit power supply when it exceeds a certain threshold) to prevent high temperature, low temperature, overcharging, overdischarging, overcurrent, leakage, etc. from damaging the battery and human body.

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

6) Battery balancing. The existence of inconsistency makes the capacity of the battery pack smaller than the capacity of the smallest cell in the group. Battery balancing is to make the battery pack capacity as close to the capacity of the smallest single cell as possible by using active or passive, dissipative or non-dissipative balancing methods based on the information of the single cell.

7) Thermal management. According to the temperature distribution information in the battery pack and the charging and discharging requirements, the intensity of active heating/heat dissipation is determined so that the battery can work at the most suitable temperature as much as possible and give full play to the performance of the battery.

8) Network communication. BMS needs to communicate with network nodes such as the vehicle controller; at the same time, BMS is not convenient to disassemble on the vehicle, and it is necessary to perform online calibration, monitoring, automatic code generation and online program download (program update without disassembling the product) without removing the shell. Generally, the vehicle network uses CAN bus technology.

9) Information storage. It is used to store key data such as SOC, SOH, SOF, SOE, cumulative charge and discharge Ah number, fault code and consistency. The real BMS in the vehicle may only have some of the hardware and software mentioned above. Each battery cell should have at least one battery voltage sensor and one temperature sensor. For a battery system with dozens of batteries, there may be only one BMS controller, or even the BMS function is integrated into the main controller of the vehicle. For a battery system with hundreds of battery cells, there may be a master controller and multiple slave controllers that manage only one battery module. For each battery module with dozens of battery cells, there may be some module circuit contactors and balancing modules, and the slave controller manages the battery module like measuring voltage and current, controlling the contactors, balancing the battery cells and communicating with the master controller. Based on the reported data, the master controller will perform battery state estimation, fault diagnosis, thermal management, etc.

10) Electromagnetic compatibility. Due to the harsh use environment of electric vehicles, BMS is required to have good anti-electromagnetic interference ability, and at the same time, BMS is required to have low external radiation. The basic framework of electric vehicle BMS hardware and software is shown in Figure 2.

3 Key issues of BMS

Although BMS has many functional modules, this paper only analyzes and summarizes its key issues. At present, the key issues involve battery voltage measurement, data sampling frequency synchronization, battery state estimation, battery uniformity and balancing, and accurate measurement of battery fault diagnosis.

3.1 Cell Voltage Measurement (CVM)

The difficulties of cell voltage measurement are as follows:

(1) The battery pack of electric vehicles has hundreds of cells connected in series, requiring many channels to measure the voltage. Since the measured cell voltage has a cumulative potential, and the accumulated potential of each cell is different, it is impossible to eliminate the error using a unidirectional compensation method.

(2) Voltage measurement requires high accuracy (especially for C/LiFePO4 batteries). SOC estimation places high demands on battery voltage accuracy. Here we take C/LFP and LTO/NCM batteries as examples. Figure 3 shows the open circuit voltage (OCV) of batteries C/LiFePO4 and LTO/NCM and the SOC change per mV voltage. From the figure we can see that the slope of the OCV curve of LTO/NCM is relatively steep, and in most SOC ranges, the maximum SOC rate range corresponding to the voltage change per millivolt is less than 0.4% (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 the battery voltage needs to be less than 10mV. However, the slope of the C/LiFePO4 OCV curve is relatively gentle, and in most ranges (except SOC < 40% and 65~80%), the maximum corresponding SOC change rate per millivolt voltage reaches 4%. Therefore, the acquisition accuracy of the battery voltage is required to be very high, reaching about 1mV. At present, most of the acquisition accuracy of battery voltage is only 5mV. In references [47] and [48], the voltage measurement methods of lithium battery packs and fuel cell packs are summarized respectively. These methods include the resistor divider method, the optical coupling isolation amplifier method, the discrete transistor method [49], the distributed measurement method [50], the optical coupling relay method [51], etc. At present, the voltage and temperature sampling of the battery has formed chip industrialization. Table 1 compares the performance of the chips used in most BMS.

3.2 Data sampling frequency synchronization

The sampling frequency and synchronization of the signal have an impact on the real-time analysis and processing of the data. When designing a BMS, it is necessary to put forward requirements for the sampling frequency and synchronization accuracy of the signal. However, in the design process of some BMS, there are no clear requirements for signal sampling frequency and synchronization. There are many types of battery system signals, and the battery management system is generally distributed. If the current sampling and the single-chip voltage sampling are on different circuit boards; during the signal acquisition process, there will be synchronization problems in the signals of different control sub-boards, which will affect the real-time monitoring algorithm of internal resistance. The same single-chip voltage acquisition sub-board generally adopts the inspection method, and there will also be synchronization problems between the single-cell voltages, which will affect the inconsistency analysis. The system has different data sampling frequency and synchronization requirements for different signals, and has lower requirements for parameters with large inertia. For example, the temperature rise of a normal discharge of a pure electric vehicle battery is 1°C/10min. Considering the safe monitoring of temperature and the accuracy of the BMS temperature (about 1°C), the temperature sampling interval can be set to 30s (for hybrid batteries, the temperature sampling rate needs to be higher).

The voltage and current signals change rapidly, and the sampling frequency and synchronization requirements are very high. From the AC impedance analysis, it can be seen that the ohmic internal resistance response of the 3.7v 18650 lithium battery is at the ms level, the SEI membrane ion transfer resistance voltage response is at the 10ms level, the charge transfer (double capacitance effect) response is at the 1~10s level, and the diffusion process response is at the min level. At present, when the electric vehicle accelerates, the response time of the drive motor current from the minimum to the maximum is about 0.5s, and the current accuracy requirement is about 1%. Considering the variable load condition, the current sampling frequency should be 10~200Hz. The number of voltage channels of a single-chip information acquisition sub-board is generally a multiple of 6, and currently the maximum is 24. Generally, the battery of a pure electric passenger car is composed of about 100 batteries in series, and multiple acquisition sub-boards are required for single-cell battery signal acquisition. In order to ensure voltage synchronization, the voltage sampling time difference between the monomers in each acquisition sub-board should be as small as possible, and a patrol cycle should preferably be within 25ms. Time synchronization between sub-boards can be achieved by sending a CAN reference frame. The data update frequency should be above 10Hz.

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