As the core of new energy electric vehicles, on-board batteries are directly related to the vehicle's lifespan, mileage, economy, and safety, all of which depend on the performance of the battery management system. The accuracy and reliability of battery management system monitoring rely on various sensors, so it is particularly necessary to research and analyze sensor technology.
1、 New Energy Electric Vehicle Battery Management System
The Battery Management System (BMS) is a system that monitors the voltage, current, load, temperature, and other states of vehicle batteries, and provides safety, communication, battery cell balancing, and management control. It also provides communication interfaces with application devices. BMS has the ability to monitor the total voltage and current data of the battery system, obtain the voltage of individual batteries, battery cell groups, and battery modules, and master the temperature and morphology of the battery pack. It mainly consists of three parts, including hardware architecture, underlying software, and application software.
1.1 Hardware Architecture
BMS hardware includes CPU, power supply and sampling IC, isolation transformer, CAN module, EEPROM and RCT, etc. Its core is CPU. The hardware structure of BMS , where centralized and distributed are the topological structures of BMS hardware. The centralized approach categorizes electronic components within the board, and the sampling chip communicates with the main chip through a daisy link. The link is simple, the cost is low, and the disadvantage is insufficient stability. Distributed consists of motherboard and slave board, with flexible system configuration and high channel utilization rate, suitable for various battery packs. The disadvantage is that channel waste occurs when the number of battery modules is insufficient.
The main controller of BMS has the functions of processing reported information, comprehensively judging battery operation, implementing control strategies, and processing fault information. The high-voltage controller has the ability to collect and report the total voltage and current, and provide the required data for the load condition (SOC) and health condition (SOH) of the motherboard, achieving two detection functions: pre charging and insulation. The controller has the function of collecting and reporting individual battery information, and has dynamic balancing function, which can maintain the consistency of power output of the battery cells. The sampling control harness has the function of adding redundant insurance on each voltage sampling line simultaneously, which can avoid external short circuit faults of the battery .
1.2 Underlying Software
According to the Automotive Open System Ar chip architecture (AUTOSAR), in order to reduce dependence on hardware devices, the BMS is divided into many common functional blocks. Capable of configuring different hardware with minimal impact on application layer software. It needs to be connected to the application layer software through the RET interface, and should be configured from the application layer due to flexibility requirements such as fault diagnosis event management (DEM), fault diagnosis communication management (DCM), functional information management (FEM), and CAN communication reserved interface.
1.3 Application layer software
The application layer covers high and low voltage management, charging management, state estimation, balance control, and fault management.
1) High and low voltage management mainly involves when power is needed, the VCU excites the BMS through a hard wire (CAN signal) of 12V, and after the latter completes self inspection, closes the relay to apply high voltage; When it is necessary to power down, the VCU issues a command to disconnect the 12V signal, or it is triggered by the CP (A+) signal during charging.
2) The slow charging process in charging management is relatively simple, while fast charging requires 80% charging capacity to be completed within 45 minutes, which needs to be triggered by the charging auxiliary power supply A+signal. Currently, the national standard for fast charging has not yet been unified, that is, there are two fast charging versions in 2011 and 2015.
3) SOC is the core control algorithm of the state estimation function, representing the remaining capacity of the battery, calculated through a specific ampere hour integration method; SOH is used to determine the lifespan and capacity of a battery in its fully charged state. Generally, batteries below 80% should not be used anymore; SOP needs to be calculated based on temperature and SOC, and can timely send signals to limit some functions of the power system before the battery is critical; The SOE algorithm is used to estimate the remaining range, which is currently relatively simple to develop. Therefore, the range of new energy electric vehicles is often inaccurate, commonly known as the "air electric" phenomenon.
4) The function of balance control is to balance the inconsistent discharge of individual batteries, as the circuit will inevitably be cut off due to the cut-off of the worst performing individual battery, resulting in waste of storage capacity of other fully performing batteries. Balanced control is divided into active and passive control, where active control transfers energy between individual units, which has a complex structure and high cost. Passive control, in addition to wasting some energy, has more obvious advantages and is currently favored by manufacturers.
5) Fault diagnosis mainly divides different fault levels based on data collection, general faults, electrical equipment faults, communication faults, and battery faults, and takes corresponding measures.
2、 Application of Sensors in Battery Management Systems
The main sensors used in BMS include current sensors, temperature and humidity sensors, voltage sensors, position sensors, and gas sensors.
2.1 Current sensors
2.1.1 Hall current sensor
The Hall Effect sensor converts the changing magnetic field into the changing voltage, which is an indirect measurement. It can be divided into two types: open loop and closed loop, with the latter having higher accuracy. The Hall current sensor simplifies the circuit by only connecting the positive and negative poles of the DC power supply. By passing the measured current bus through the sensor, the isolation detection of the main circuit and control circuit is completed. The output signal of the sensor is the secondary side current, which is proportional to the primary side current (input signal) and has a small value, requiring A/D conversion. Hall current sensors combine the advantages of transformers and diverters, and have a simpler structure. However, they are susceptible to interference and are no longer suitable for increasingly complex new energy electric vehicle power supply environments.
2.1.2 Fluxgate current sensor
The principle of flux gate is that under the influence of excitation current, the excitation current of a easily saturated magnetic core changes the inductance strength, thereby changing the magnetic flux. The magnetic flux then opens or closes like a gate.
The accuracy of ordinary Hall current sensors is between 0.5% and 2%, while the flux gate current sensor is made using the principle of flux gate, and the accuracy can reach 0.1% or even higher. Therefore, it is also known as a high-precision current sensor. Structurally, there are two types: open loop and closed loop. This article focuses on introducing the closed-loop fluxgate current sensor, which amplifies the second harmonic signal of the fluxgate excitation current, drives the compensation coil, cancels out the magnetic flux of the focusing magnetic core and the magnetic flux of the primary current, and maintains a "zero magnetic flux" state; For the HPIT series, the magnetic flux is not zero and is a symmetric shape without second harmonic.
Fluxgate current sensors are structurally divided into four categories: single magnetic ring, double magnetic ring, double magnetic ring (shielding), and multiple magnetic rings (nested).
Due to its high sensitivity, strict correspondence between closed-loop magnetic balance and turn ratio output, overall magnetic core sealing, and probe compensation to eliminate the impact of oscillation and harmonic on output cleanliness, closed-loop fluxgate current sensors are widely used in various types of new energy electric vehicle products, such as Tesla Model 3, BYD Han, Ideal ONE, Xiaopeng P7, and other best-selling models.
2.1.3 Tunneling magnetoresistive effect current sensor
Tunneling magnetoresistance effect (TMR) current sensor is a new generation of magnetic sensor. Compared with Hall device, anisotropic magnetoresistance (AMR) and Giant Magneto Resistance (GMR), it has the advantages of low energy consumption, low temperature drift and high sensitivity, and can significantly improve the sensitivity and temperature characteristics of current detection.
2.2 Temperature and humidity sensors
2.2. Temperature sensor
Temperature is of great significance for the performance of BMS. In order to further improve battery utilization, prevent excessive discharge (charging) of the battery, control battery operating conditions, and increase battery life, an NTC temperature sensor is installed to monitor temperature.
The NTC temperature sensor is mainly made of oxidized compounds of high-purity metal elements such as Mn through a combination of ceramic technology and semiconductor technology. The working principle is that these materials have fewer charge carriers and higher resistance. As the temperature increases, the number of charge carriers correspondingly increases and the resistance correspondingly decreases. It has the advantages of high resistivity, small heat capacity, fast response, excellent linear relationship between resistance and temperature, flexibility, low price, and long service life. There are three commonly used types: ground ring shell NTC temperature sensors, commonly known as "ground ring type"; Epoxy resin encapsulated NTC temperature sensor, commonly known as "water droplet head" or "small black head"; Thin film NTC temperature sensor.
2.2.2 Humidity sensor
A humidity sensor is a device or device that converts environmental humidity into an electrical signal that can be labeled. A common humidity sensor measures relative humidity. The commonly used humidity sensors for new energy electric vehicle BMS now include resistive humidity sensors and capacitive humidity sensors. The principle is to apply a layer of moisture sensitive material film on the substrate. When water vapor is adsorbed on the film in the environment, the component resistivity and resistance value will change, and humidity can be measured.
The humidity factor is particularly difficult to capture in the battery management system of new energy electric vehicles, but it has a significant impact on the performance and lifespan of the battery. Temperature compensation is applied to the humidity output of the sensor to obtain a linear voltage, which is input into the BMS of a new energy electric vehicle with ADC.
2.3 Voltage Sensors
The battery pack of the electric vehicle power supply system is connected by hundreds of series cells, so there is a large demand for measuring voltage channels. The series battery pack is a cumulative voltage, but the electromotive force of a single battery is not the same, and the error cannot be simply eliminated using one-way compensation method. The collection of battery voltage requires high accuracy, reaching 1mV, while the current collection accuracy is only 5mV.
A voltage sensor can convert the voltage of the tested battery into an output signal. The electroluminescent effect voltage sensor used in new energy electric vehicles measures the luminous intensity of the luminescent material at the measured voltage to obtain the effective value of the measured voltage. Compared with the traditional optical voltage sensor, the voltage sensor based on the electroluminescent effect will no longer use the carrier light source. On the one hand, it will eliminate the instability of the carrier light source measurement, on the other hand, it will also simplify the sensor structure and reduce the production cost.
2.4 Position Sensors
The position sensor in BMS is mentioned in a utility model patent titled 'Battery Temperature Control Management System and Electric Vehicle', and has not yet been widely used in new energy electric vehicles.
The position sensor is mainly used to detect the position of the cooling liquid level in the water cooling device in the BMS system. The position sensor is installed on the cooling water float to detect the position of the coolant relative to the liquid level of the expansion kettle, and to obtain the contact between the outlet of the expansion kettle and the liquid. Usually, at least three floats are required, and position sensors are installed on each float to facilitate the BMS to adjust and control the main and auxiliary water pumps in a timely manner when the vehicle passes through steep slopes or when there are a large number of bubbles in the cooling system.
2.5 Gas Sensors
The thermal runaway of the power battery of new energy vehicles usually generates a large amount of abnormal gases (carbon monoxide/hydrogen/hydrogen fluoride/TVOC) before the battery catches fire. After diagnosing the fault through CO sensors and hydrogen sensors, a warning is issued and the vehicle controller is required to effectively handle it. The Battery Management System (BMS) comprehensively monitors the health status of batteries. Different sensors have their own advantages and disadvantages, and they usually detect thermal runaway of power batteries through multiple different sensors.
2.5.1 Carbon monoxide sensor
In order to minimize casualties and losses, timely detection of fires and early warning are crucial. The thermal runaway of the power battery usually generates a large amount of CO before the battery catches fire, so monitoring the concentration of CO is undoubtedly an effective solution. Once the alarm threshold is exceeded, the alarm will be activated to evacuate personnel and initiate fire extinguishing, in order to gain more valuable time.
2.5.2 Hydrogen sensor
For new energy vehicles, hydrogen sensors can not only be used to monitor the leakage of hydrogen in hydrogen storage bottles and fuel cell systems, but also to detect the concentration of hydrogen in exhaust emissions. New energy vehicles can also analyze the performance and responsiveness of the stack in real-time based on these monitored information, thereby adjusting relevant input indicators or data configurations in a timely manner to achieve safe and efficient operation of the vehicle.
3. Development Trends of Battery Management System Sensor Technology
3.1 Trend of Functional Integration
New energy electric vehicles have been developing in the direction of lightweight, while the integration requirements for components are becoming more stringent. BMS is a complex and functionally integrated management system, with a small volume. Therefore, sensors are required to have multifunctional integration, allowing for comprehensive monitoring of the battery system with a minimum number of sensors. When an abnormality occurs, it can also quickly and accurately locate the fault point.
3.2 Trends in Monitoring Precision
In the future, the monitoring data accuracy of sensor technology in products will become increasingly precise, requiring more accurate data collection for current, voltage, temperature and humidity, in order to improve users' accurate understanding of battery system conditions. The next step is to start from both theoretical simulation and experimental research, and explore a new generation of monitoring efficient and high-precision BMS sensors.
3.3 Product Safety Trends
Functional safety is a fundamental requirement for new energy electric vehicles and an inevitable trend in the development of sensor technology. On the one hand, it is necessary to ensure the safety of the sensor product itself, and on the other hand, the safety of the entire BMS supported by the sensor will directly or indirectly affect driving safety, affecting the user's driving experience and personal safety.
