Electric vehicle battery monitoring and early warning system based on LabVIEW

As the core component of electric vehicles, how to effectively manage and utilize the energy of batteries and increase the service life of batteries has become a key issue. This paper introduces a battery information acquisition hardware platform based on STC89C52. By improving the A/D conversion, serial communication, temperature sensor control and other parts, a complete hardware data acquisition system is developed to realize the acquisition function of basic information such as single battery voltage, current, temperature, etc. LabVIEW is used to build a host computer data processing platform. The real-time monitoring and early warning system for batteries suitable for pure electric vehicles is studied, and the functions of basic battery information measurement, power estimation, fault alarm, etc. are realized, as well as reasonable and effective management and control of battery packs. The system runs stably during operation and has good reference value in practical applications.

The contradiction between energy depletion and environmental damage is becoming increasingly prominent. Based on the energy saving and environmental protection of electric vehicles, the development of electric vehicles has become an inevitable trend. The on-board battery is the core part of electric vehicles, and the key to the research of battery management system is how to effectively utilize the energy of the battery and increase the battery life. The battery management system can estimate the remaining power SOC to ensure that the SOC is within a reasonable working range, and can also make early predictions for faulty batteries to avoid reducing the service life of the entire battery group due to the failure to detect the damage of the single battery in time. Therefore, the research on battery energy management system (BMS) has received more and more attention.

Song Xuehua et al. designed a dynamic balancing battery management system (BMS) suitable for hybrid electric vehicles, optimized the SOC estimation, met the system estimation error requirement of less than 5%, and the final experimental result error was 3.3%; Wang Bo et al. designed peripheral circuits such as voltage acquisition filtering, temperature acquisition expansion, voltage balancing, SPI communication, etc. based on the LTC 6803 battery management system, minimizing the use of peripheral devices; Zhang Huahui et al. based on the DSP-based lithium-ion battery management system, realized the detection of single cell voltage, total voltage, current, and temperature, and had functions such as SOC estimation, communication, and computer monitoring.

This paper uses a battery information acquisition hardware platform based on STC89C52 to realize the collection of basic information such as single battery voltage, current, temperature, etc., builds a host computer data processing platform in LabVIEW, and studies the battery real-time monitoring and early warning system suitable for pure electric vehicles. It realizes the functions of battery basic information measurement (voltage, current, temperature), power estimation, fault alarm, etc.

1 Research Methods

The battery monitoring and early warning system must be able to monitor the battery's operating status, including the battery's voltage, current and temperature, and issue an alarm when it exceeds the operating range to maximize the battery's effectiveness.

1.1 System Solution Selection

The monitoring platform is designed based on the battery management system, including hardware and software. The hardware part is based on the STC89C52 battery information acquisition hardware platform, which collects basic information such as voltage, current, temperature, etc. of the single battery. The software part uses LabVIEW to realize functions such as basic battery information measurement, power estimation, and fault alarm.

At present, the battery monitoring and early warning systems on the market mainly include centralized, decentralized and integrated types. This study uses a lower computer with STC89C52 microcontroller as the processing chip to collect and upload the basic information of single lithium-ion batteries, and builds a monitoring and early warning platform based on LabVIEW on the upper computer to display real-time data and alarm.

1.2 Battery status monitoring

The main function of the battery monitoring and early warning system is to monitor the operating status of lithium-ion batteries in real time and feed it back to the host computer for display and processing. This study takes a single lithium iron phosphate battery as an example for operation testing.

Voltage monitoring: The normal operating voltage range of a single lithium iron phosphate battery is 2 to 3.7 V. If it is not within this range, the system will automatically alarm.

Current monitoring: The size of the discharge current is crucial to the safe operation of electric vehicles. If it exceeds the normal operating range, it may cause the battery pack to catch fire or even explode.

Temperature monitoring: The normal operating temperature of a single lithium iron phosphate battery is -20 to 50°C. If it exceeds the range, the platform will automatically indicate an alarm.

1.3 Serial port communication settings

There are usually two ways of communication between the lower computer and the PC: parallel and serial. In the design and implementation of modern single-chip microcomputer measurement and control systems, serial communication is usually used for information exchange. Among them, serial communication includes asynchronous serial communication and synchronous serial communication. In the asynchronous serial communication mode, the sending and receiving devices of the communication use their own clocks to control the sending and receiving process of data. Because this method does not require strict consistency between the clocks of the sending and receiving parties, it is relatively simple to implement and has low equipment overhead. Therefore, the communication method used in this study is asynchronous serial communication.

At present, the most commonly used serial interface standard is the asynchronous serial communication standard RS-232 developed by the American Electronics Industries Association (EIA), which can realize data communication between the microcontroller and the PC. The communication principle circuit diagram is shown in Figure 1.

 

 

Since the microcontroller uses TTL level, and RS-232 uses RS-232 level, in order to ensure communication stability, this paper uses the level conversion chip MAX232, and integrates the conversion between RS-232 level and TTL level. The circuit schematic is shown in Figure 2.

 

 

2 Hardware Implementation of Battery Monitoring and Early Warning System

2.1 Hardware structure of battery monitoring and early warning system

The hardware part uses the STC89C52 battery information acquisition hardware platform to realize the acquisition function of basic information such as single battery voltage, current, temperature, etc. The hardware structure of the voltage, current, and temperature acquisition of single lithium iron phosphate battery is shown in Figure 3.

 

 

2.2 A/D Working Principle

In the A/D conversion chip, the collected analog signal is continuous in time, while the digital signal sent to the microcontroller is discrete. Therefore, the A/D chip must sample the collected analog signal at a specified time point and convert the sampled value into a digital value and send it to the microcontroller.

A/D conversion is divided into three stages: sampling and holding, quantization and encoding. In order to improve the efficiency of data conversion, a successive comparison type A/D conversion chip is required to compare with different reference voltages several times to obtain the voltage value with the smallest difference, that is, the converted output value. The principle is shown in Figure 4.

In terms of collecting voltage and current, this study uses the ADC0804 chip, which is an integrated successive approximation A/D conversion chip.

2.3 ADC0804 voltage and current acquisition circuit

The pins and functions of ADC0804 are shown in the literature. The connection principle between the peripheral circuit of ADC0804 chip and the microcontroller in this study is shown in Figure 5.

 

(1) VIN(+) is connected to the middle sliding end of the potentiometer, and VIN(-) is grounded. When the potentiometer is adjusted, the voltage of the middle sliding end varies in the range of 0 to 5 V, and the digital output of ADC0804 varies in the range of 0x00 to 0xFF.

(2) WR and RD are connected to the P3.6 and P3.7 pins of the microcontroller respectively, and the digital output terminal is connected to the P1 port.

2.4 Temperature Acquisition Implementation Circuit

The temperature acquisition circuit uses the DS18B20 temperature sensor. The device uses a single bus protocol, occupies only one I/O port, and directly converts the external temperature into a digital signal and outputs it serially. For an introduction to DS18B20, see the literature. The measurement circuit is shown in Figure 6.

 

 

In terms of circuit connection, DS18B20 is connected to STC89C51 for communication. In practical applications, multi-point acquisition is usually used to measure the battery temperature, so it is necessary to connect the I/O ports of all sensors together. When writing the program, the internal chip serial number of the sensor is identified, multi-point expansion is performed, and a multi-channel temperature acquisition hardware system is designed.

3 LabVIEW host computer monitoring platform

3.1 Introduction to LabVIEW Virtual Instrument Software

LabVIEW is a program development environment launched by NI (National Instruments) in the United States. It pioneered the G language, a graphical programming language that uses icons instead of text lines to create applications, which is more convenient and intuitive to use. This graphical program compilation platform has multiple functions including: DLL (Dynamic Link Library), multithreading, data logging, operation control, etc. It can conduct in-depth principle analysis, detailed structural design and flexible interface implementation of program design, ensuring the efficiency and stability of the battery monitoring and early warning monitoring platform.

This study uses LabVIEW for measurement and control, and uploads the hexadecimal data of the basic information collected by the lower computer in real time to the PC through the serial port. Figure 7 shows the overall interface of the LabVIEW platform.

 

 

3.2 Analog voltage and current acquisition

FIG8 and FIG9 are real-time acquisition images of voltage and current, respectively.

 

 

Figures 10 to 12 are the program diagrams for processing voltage and current in LabVIEW. The baud rate of the serial port initialization setting is the same as that of the lower computer, which is 9 600 bit·s-1, the data bit is 8, there is no parity bit, and the stop bit is 1. This part of the program uses a cascaded sequence structure and contains 3 frames (0 to 2). The program also uses relevant knowledge of VISA serial ports, index arrays, etc.

 

 

 

 

The lower computer receives the changing analog voltage (0-5 V), and the PC receives the voltage value (hexadecimal, 1 Byte) sent by the microcontroller, converts it into decimal form, and outputs it in the form of digital and curve.

The reasonable range of voltage value setting is 2~3.7 V (normal voltage range of lithium iron phosphate battery). The red light will light up when it is lower or higher than this range. The upper limit of current value setting is 50 A (the continuous working current of lithium iron phosphate battery is about 50 A). When the current is higher than this value, the red light will light up and an alarm will sound.

3.3 Single Lithium Battery Temperature Collection

 

 

Figures 14 to 16 are the upper computer programs for temperature acquisition. The baud rate of the serial port initialization setting is the same as that of the lower computer, which is 9 600 bit·s-1, the data bit is 8, there is no parity bit, and the stop bit is 1. This part of the program has a cascading sequence structure and contains 3 frames (0 to 2). In addition, the program uses relevant knowledge such as VISA serial port and index array.

 

 

The lower computer receives the changing temperature, and the PC receives the temperature value sent by the microcontroller, converts it into decimal form, and outputs it in the form of digital and curve. The reasonable range of temperature value setting is -20～60℃. If it is lower or higher than this range, the red light will light up and an alarm will be issued.

4 Conclusion

This study is based on the battery information acquisition hardware platform of STC89C52. By improving the A/D conversion, serial communication, temperature sensor control and other parts, a complete hardware data acquisition system is developed. With the help of the multi-threading and flexible interface technology of the LabVIEW development platform, its powerful data processing capabilities and rich and practical program structure, a real-time monitoring and early warning system for batteries suitable for pure electric vehicles is studied, mainly realizing the functions of basic battery information measurement, power estimation, fault alarm, etc.