Battery pack state monitoring system based on wireless communication

Taking lithium ion battery as the DC motor power supply system as the research object, using the ARMv7 series microprocessor STM32F103 VET6 as the main control chip, combining the DC current transmitter SIN-DZI-20A and the DC voltage transmitter SIN-DZU-30V, it is proposed A set of embedded battery pack condition monitoring system design scheme. The monitoring system can realize real-time measurement of battery pack voltage, current, power and other status parameters, battery pack safety management, data and data waveform display, battery pack charging and discharging status control, and wireless WiFi data communication. Through the joint debugging of the hardware and software systems, the experimental results show that the battery pack state measurement system runs stably, the state measurement accuracy reaches 0.5 level, and it has a certain degree of anti-electromagnetic interference capability.


1 Introduction 

With the maturity of commercial battery technology, secondary batteries such as lithium-ion batteries have been widely used in electric vehicles, mobile phones, notebook computers, industrial mobile robots, wind farm energy storage systems, grid frequency modulation, distributed power sources, and microgrids.[1] . Compared with other secondary batteries, lithium-ion batteries have the advantages of high energy density, small size, light weight, no memory effect, less self-discharge, long cycle life and environmental friendliness [2]. These characteristics determine that it has great development prospects in terms of storing electric energy.

The working status of the battery pack includes physical parameters such as voltage, current, power, and temperature [3]. Whether it is a traditional lead-acid battery or a lithium battery with better performance, when the heat dissipation rate is less than the heat accumulation rate, it will inevitably cause the internal temperature to rise and generate a lot of heat energy. In order to ensure the safe operation of the battery pack and prolong its service life, it is necessary to monitor the battery pack's physical parameters such as voltage, current, power, and temperature in real time, warning of danger and automatic fault removal.

In 1991, the United States Advanced Battery Consortium (USABC, United States Advanced Battery Consortium) established a laboratory specializing in the development and research of battery management systems (BMS, Battery Management System) [4]. The basic functions of the developed BMS include: limiting the overcharge and undercharge of the battery; ensuring the balance between the batteries in the battery pack; and maintaining the safe operation of the battery pack [5]. With the development of industrial technology, in order to meet the needs of monitoring battery packs in complex situations, research on battery SoC (State of Charge) prediction, battery safety management, energy balance of battery packs, and battery thermal management have been introduced. [6]. In 2008, Tesla developed a battery management system while improving the cell structure of Panasonic's 18650 lithium-ion battery. The system can perform real-time monitoring of each battery and each line. If an emergency occurs, the line where the fault is located can be fused within milliseconds [7]. Professor Jiang Jiuchun from Beijing Jiaotong University has conducted in-depth research on the mathematical models of power batteries and battery state estimation methods. The prediction accuracy of the SoC can reach ±3%, and for the first time proposed a method for non-destructive fast charging of lithium-ion power batteries; in terms of control, he proposed the main Passive balance control technology improves the utilization of pack capacity; the communication protocol studied by it has risen to the national standard [8].

Although traditional wired monitoring systems can effectively complete tasks such as battery SoC prediction, battery safety management, battery pack energy balance, and battery thermal management. However, for lithium-ion battery packs used in electric vehicles and industrial mobile robot systems, it is usually necessary to obtain the battery pack status in real time through a host computer or a mobile terminal. Traditional wired monitoring methods can no longer meet the needs of today's users, and there is an urgent need to develop a low-cost, high-reliability wireless monitoring battery management system. This research combines WiFi communication technology and proposes a remote battery pack state measurement system design scheme, which is verified through an experimental system. The experimental results show that the battery pack state measurement system runs stably, the state measurement accuracy reaches 0.5 level, and it has a certain degree of anti-electromagnetic interference capability.

2 hardware design

In the proposed wireless monitoring system for embedded battery pack status, the monitored object is the power supply circuit of a 7.5 AH 24 V DC 18650 lithium battery pack built in the laboratory to a single DC motor. The load selects a LX44WG single-shaft worm gear motor with a DC speed regulator. The physical connection of the system is shown in Figure 1.

Select STM32F103VET6 MCU as the master chip. The onboard MCU (Micro Controller Unit) is an LQFP package, 100-pin STM32F103VET6. It has 512 kB of Flash and 64 kB of SRAM. STM32F103 VET6 uses the Cortex-M3 core designed by ARM, with a main frequency of 72 MHz[9,10].

The battery voltage and current are measured by the DC voltage transmitter SIN-DZU-30V and the DC current transmitter SIN-DZI-20A respectively. After being collected by the ADC interface circuit and internally converted by the MCU, the floating-point values of the voltage and current are obtained. The DC voltage (current) transmitter linearly converts a certain range of input terminal voltage (current) into a certain range of analog output voltage. The input current range of the current transmitter is 0~20 A, and the output voltage range is 0~3.3 V. The STM32 development board can collect the analog voltage output by the DC current transmitter and the voltage transmitter through the ADC. In order to prevent the transmitter from outputting AC component signals and affecting the measurement accuracy of the system, a tantalum capacitor must be connected in parallel with the analog signal output end of the voltage transmitter. Based on experience, choose a 16V 10 μF tantalum capacitor as the filter capacitor of the transmitter.

The DHT11 temperature sensor is used to measure the temperature of the battery pack, and the measured data is sent to the MCU via a single-bus communication method. The data packet of DHT11 sensor temperature measurement data is generally 40 Bit, and it needs to be verified during the data transmission process to ensure that there is no misinformation or incompleteness during the data transmission process.

The measured battery voltage, current, temperature, motor status and other data are temporarily stored in the I2C EEPROM memory, and the program and Chinese character library are burned in the Flash program memory. And when needed, it can be displayed on a 3.2-inch resistive touch LCD screen, or sent to the mobile APP for display by using the ESP8266 serial WiFi module. The ESP8266 module is mainly used to transmit small amounts of data, and cannot be used to transmit large data files such as images, audio, and video. When upgrading the system, consider using a wireless image transmission module to replace ESP8266.

The wireless transmission circuit design is shown in Figure 2 [11,12].

3 software design

The STM32F103VET6 microcontroller is based on the ARM Cortex-M3 embedded core of the ARMv7 architecture, and can be developed using the general ARM embedded system development environment. It adopts the Harvard structure in which the instruction bus and the data bus are separated, and has a faster processing speed than the von Neumann structure. The battery pack state measurement system software program was developed in the Keil uVision5 development environment. The software program is programmed in C language. The compiled .hex file can be interfaced to the program memory through the DAP emulator via the JTAG interface.

The software design of the battery pack state measurement system can be divided into the software design of the lower computer and the software design of the upper computer. The software design of the lower computer includes voltage and current reading, temperature reading, waveform display, filtering, safety management, battery pack SoC, data storage, wireless display and control, button detection, relay control, motor control and delay management, etc. Subroutine design. The software system process is shown in Figure 3.

When the program detects that ESP8266 has a new message, the software enters the message detection program. Through this program, the new user's connection and the old user's exit connection can be detected in real time, and the control commands from the upper computer can be obtained. The TCP/IP communication between the lower computer and the upper computer can be realized by using the socket (Socket) interface. The lower computer starts the TCP Sever server first. After the upper computer opens the TCP Client client, it needs to connect to the TCP Sever server of the lower computer.

4 Experimental results

The system voltage measurement accuracy can be obtained by comparing the voltage value measured by the system with the voltage value measured by a digital multimeter. The current measurement accuracy of the system can be obtained by connecting the ammeter in series with the wire passing through the current transmitter to obtain the current value measured by the meter, and then comparing it with the current value measured by the battery monitoring system. The model of the multimeter used for the measurement is VICTOR VC890D, its basic accuracy is ±(3+0.5%), and the detection frequency is 3 Hz. When the voltage value given by VC890D is 22.3 V, the test voltage value of this system is 22.30136 V; when the voltage value given by VC890D is 22.2 V, the test voltage value of this system is 22.29590 V. The test results show that: the data measurement frequency of this system can reach 5 Hz, and the measurement accuracy can reach an accuracy of 0.5. The interface of the Android APP is shown in Figure 4.

5 Conclusion

When there is no external device connected to the Socket service of the STM32 development board, the system default delay balance time is 250 ms, that is, the detection frequency of the battery pack status measurement system is 4 Hz. When the voltage, current and temperature waveform display function of the battery pack status measurement system is removed, the delay balance time of the system can be reduced to 200 ms, that is, the detection frequency of the system reaches 5 Hz.

After system testing, the system software has a good filtering effect, which can effectively filter out the AC component and retain the DC component. The data measurement frequency of this system can reach as fast as 5 Hz, and the measurement accuracy can reach an accuracy of 0.5. Therefore, the entire program meets the system design requirements and has a strong engineering application prospect. After industrial redesign and upgrading, this system can be expanded into a management system for grid-level lithium battery energy storage systems.