Note: * is required.

[Please fill in during the registration stage↓]

* 1. Self/team introduction

The team members are from Xi'an Jiaotong University and are all current students. The team’s main research direction is key technologies for electric vehicles. Mainly engaged in the field of electric vehicle power battery - lithium battery characteristic analysis and its control technology. The team members have a relatively deep theoretical foundation in the field of lithium batteries, and at the same time have rich experience in hardware project development. Today, with the rapid development of electric vehicles, in-depth research on lithium battery control systems is in line with the needs of the national development strategy and is of great significance to the development of electric vehicles. During the scientific research process, it was discovered that this field requires the collection of group lithium battery information, so the team participated in this competition with the aim of developing a status parameter collection board for the field of lithium battery control systems. After the design and development is completed, the project will be open sourced. Provide assistance to other technical personnel in related fields. The hardware single boards, single board programs, host computer programs and other related materials involved in this project are all open source, ensuring that relevant personnel can quickly develop the required products through these materials.

* 2. Briefly introduce the project

Lithium batteries are widely used as power batteries in electric vehicles due to their advantages such as long cycle life, high energy density, low self-discharge rate, no memory effect, and being clean and environmentally friendly. However, improper operation of lithium batteries can lead to serious consequences, such as reducing battery life, reducing battery capacity, and even causing thermal runaway, fire and explosion, etc. At the same time, in the fields of lithium battery state of charge SOC prediction, health life SOH estimation, battery fault diagnosis and battery thermal management technology, it is necessary to collect the two parameters of lithium battery pack voltage and temperature in real time, so it is necessary to design and manufacture a device with high Accurate lithium battery status parameter acquisition board. Due to the high battery life requirements of electric vehicles, a large number of lithium battery cells are integrated into the battery pack of electric vehicles, which requires that the designed state parameter acquisition board has high reliability and can meet the requirements of vehicle regulations. The lithium battery status parameter acquisition board developed in this project is used to collect the voltage information of each single lithium battery during the operation of the battery pack. It has the following characteristics:

(1) It can collect up to 12 voltage signals at the same time, and the accuracy can reach 2mV;

(2) The main control and sampling front-end chips are automotive grade chips;

(3) High and low voltages are isolated;

(4) This project not only developed the data acquisition board, but also developed a matching host computer, which can be used immediately and greatly reduce the hardware development time. In addition, all the materials of this project have been open sourced, and we hope to provide some help to relevant developers.

[Please fill in during the competition stage↓]

* 1. Describe project details

The acquisition board can collect voltage signals of up to 12 batteries at a time, and the accuracy can reach 2mV. The sampling front-end uses an integrated chip developed by ADI specifically for the acquisition of voltage signals from lithium batteries in electric vehicles. Since there are a large number of battery cells in electric vehicles, using traditional discrete components to collect voltage signals will lead to an increase in hardware costs and the number of components. The problem of reduced reliability caused by the increase. The main control of the acquisition board uses NXP's S12 series microcontroller, and the isolated SPI communication method is used between the microcontroller and the sampling front-end chip. The power supply design of the acquisition board is a dual power supply with an input voltage of 12V. After being stepped down by the primary 7805LDO, it supplies power to the low-voltage side chip. At the same time, the DCDC isolation power supply is used to generate a 9V input voltage signal on the high-voltage side. The physical connection between the power supplies is realized through DCDC. isolation. The 9V input voltage of DCDC is converted to 5V by the high-voltage side 7805LDO to power the chip on the high-voltage side. The communication between the acquisition board and the outside uses the CAN interface. The CAN driver chip is a 1050T chip produced by NXP, and the CAN controller is a peripheral that comes with the microcontroller. In order to facilitate debugging, a serial port is left, and the host computer and the acquisition board communicate through the serial port. The host computer is written using Labview software, and the collected data is intuitively displayed on the interface for reference by relevant personnel. At the same time, the program can be modified for different tasks to meet other needs.

* 2. Describe the challenges faced by the project and the problems solved

1. Difficulty in collecting multiple voltages

Lithium battery testing equipment used in laboratories generally has eight channels, the number of channels is limited and the price is expensive. The actual battery pack may be composed of hundreds of batteries connected in series. If you study the battery pack, you will need more voltage acquisition channels. This acquisition board can collect 12 channels of voltage information on a single board, and multiple acquisition boards can be used later. To form a distributed application, if the sampling frequency is 1Hz, it can support up to 15 acquisition boards and voltage platforms up to 600V.

2. Serial communication data synchronization problem

When the lower computer and the upper computer use serial port communication, there is a problem of data out-of-synchronization. The data read by the upper computer may be one frame of data spliced ​​into two adjacent frames of data in the cache. This will cause the data collected by the upper computer to be Does not match the lower computer. In order to solve this problem, a communication protocol between the host computer and the acquisition board was developed, using the response mode. The host computer sends information such as the acquisition board address, sampling frequency, command type and frame tail to the acquisition board at a certain acquisition frequency. The acquisition board receives the corresponding information and verifies it, and returns the data after the verification is correct. In this way, the buffer There is always only one frame of data, which avoids the problem of data being out of sync. In addition, this protocol can support communication conflicts between multiple acquisition boards and a main control board. In addition to solving the problem of serial port communication being out of sync, you can also pack the data into data with frame headers and frame tails, and intercept the complete data by identifying the frame headers or frame tails. However, this method requires special attention that the frame headers and frame tails cannot match the frame headers and frame tails. Valid data must be repeated, otherwise it may be easily misidentified. In addition, this communication method is a passive method, and serial communication conflicts are likely to occur in the communication between one master and multiple slaves.

The format of the command sent is shown in the figure below. The command sent includes the acquisition board address, sampling frequency, command type and frame end. Each acquisition board has its own address, and will respond only if the address of the command is consistent with its own address. Therefore, communication conflicts between multiple acquisition boards can be avoided. Since the response communication mode is adopted, the sampling frequency is also the frequency of sending commands.

The acquisition board will return different data according to different command types, so the host computer needs to determine the number of bytes of the serial port buffer read each time according to the command type, and judge whether the number of bytes in the buffer is equal to the number required by the command. Whether the number of bytes in the reply is consistent can preliminarily determine whether the communication is normal, as shown in the figure below. If the communication is abnormal, a corresponding prompt will be given.

3. SPI communication timing issues between the sampling front-end and the main control chip

The chip used in the sampling front-end is ADI's LTC6811 chip. This series of chips has been used in many mass-produced electric vehicles such as Tesla, and its excellent performance has been verified. The chip communicates with the main control chip on the acquisition board through the SPI interface. In order to reduce power consumption, the core of this chip has multiple working modes, and its communication interface Yeju has different working modes, as shown in the figure below. The sampling chip core needs to switch between different modes when working, and also switches between different communication modes when communicating. At the same time, the response times of different command types are also different, so during development, extra attention needs to be paid to the communication timing issues between the sampling front-end chip and the main control chip.

And the time to complete different types of commands is also different, so after sending the command, you need to wait accordingly according to the completion time of the command.

4. High and low voltage isolation of sampling plate

The sampling front-end chip is responsible for the collection of voltage and temperature. The sampling front-end is directly connected to the battery. The voltage of the battery pack of an electric vehicle can reach hundreds of volts through series connection. In order to ensure the reliability and safety of the acquisition board, high and low voltages must be isolated. Since SPI communication is used, a digital isolation chip is used. The schematic diagram of the digital isolation chip is shown in the figure below. The isolation chip requires two isolated power supplies for power supply, namely VCC and VCC_5V.

The low-voltage side VCC power supply of the digital isolation chip is supplied through the 5V power supply output by the 7805LDO buck chip, as shown in the figure below:

The high-voltage side VCC-5V of the digital isolation chip is boosted and isolated by DCDC and then regulated to 5V by LDO to power the high-voltage side of the digital isolation chip. Through actual testing, it was found that the output voltage of the DCDC isolation power supply is unstable and the output voltage ripple noise is too large, which may affect the communication between the sampling chip and the main control chip of the acquisition board. Therefore, the method of first boosting the voltage and then using the LDO to step down the voltage is used to improve the quality of the output power supply. Such a solution will also slightly increase the cost, and users can also modify this part according to their own needs. (Just modify the hardware, the acquisition board program and the host computer do not need to be changed)

* 3. Describe the key points involved in the hardware and software parts of the project

During the project development process, we encountered many problems, ranging from problems with the welding process of simple hardware (small size) to problems with the working principle of the hardware involved. Now I will share with you the problems encountered during the development process. I hope it will be helpful to future learners and will not make the same mistakes again.

Question 1: The output voltage of the 7805LDO on the high-voltage side is about 7V, which is much higher than 5V. After analysis, it was found that the cause of this phenomenon was improper selection of diodes. Since the output voltage of the LDO is sometimes higher than the input voltage when the LDO is powered off, in order to protect the LDO, the input and output antiparallel diodes are generally used for voltage clamping. Schottky diodes are used in this design. This diode has the characteristics of voltage drop and fast switching speed, but it also has the disadvantage of large leakage current. Since the LDO used is SE8550, the static power consumption of this LDO is extremely low. Therefore, the resistance of the resistor used for feedback is relatively large, and the leakage current of the Schottky diode cannot be discharged when no load is used, causing the input point to output capacitance. Continue charging until the output voltage is higher than 5V.

Problem 2: The voltage at both ends of the battery is inconsistent with the pin voltage of the sampling chip. After analysis, it was found that due to the loop current formed between the chip pins, the loop current produced a voltage drop on the filter resistor, resulting in inconsistent voltage between the battery terminal and the chip pins. Through further testing, it was found that the residual solder paste when soldering the chip has weak conductivity, causing a loop to form between the pins. After cleaning, the collected voltage returned to consistency.

In terms of hardware, the sampling front-end uses ADI's LTC6811 chip, which can collect 12 voltage signals at the same time, with high acquisition accuracy and fast speed. At the same time, the chip meets the ISO26262 functional safety standard. The main control chip of the acquisition board uses Freescale's 16-bit automotive-grade microcontroller, which has high reliability while meeting performance requirements. In terms of communication, a CAN communication interface is used, and a serial port is also left for easy debugging. This collection version also implements isolation between high and low voltages, and uses a dual-power supply digital isolation chip between the sampling front-end and the microcontroller to further improve safety and reliability.

In terms of software, the development environment of the microcontroller adopts the CodeWarrior5.1 development environment officially provided by NXP, and the development software of the microcontroller program is carried out in this development environment. Labview software is used to develop the host computer and the collected voltage signals are intuitively displayed to the user. The communication between the acquisition board and the host computer adopts serial communication. The acquisition board collects 12 voltage signals and two temperature signals and sends them to the host computer. If the acquisition board sends the data to the host computer at a fixed frequency, the data received by the host computer may be out of sync (the host computer cannot recognize the first byte ). In order to solve this problem, a certain communication protocol was developed, and the question and answer communication mode was adopted between the host computer and the acquisition board. The host computer sends data in the specified format to the acquisition board (a total of 5 bytes, which are the acquisition board address, sampling frequency, command type and two-byte frame tail). After receiving the command from the host computer, the acquisition board first compares whether the addresses are consistent, then extracts the command type, and responds accordingly after receiving the two-byte frame tail of 0xFAFA. In this way, the problem of asynchronous serial communication between the host computer and the acquisition board is avoided.

* 4. Project material list display

(1) See the attachment for the BOM list of electronic components used in the sampling board;

(2) The lithium battery pack for testing is composed of twelve 18650 lithium batteries connected in series.

* 5. Upload project pictures

5.1. If the contest logo is not printed on the PCB, it will be deemed as giving up the participation.

PCB and silk screen

The front side of the sampling plate is as shown in the figure:

The back of the sampling board with the competition LOGO

12 18650 lithium batteries connected in series and collection board for testing

Host computer interface

5.2. Other pictures of the project

Simply compare the voltage data collected by the acquisition board with the multimeter, and compare the voltage value measured by the acquisition board with the DC 4V voltage range of the LCSC530+ model. The results are as shown in the table below:

The voltage of the acquisition board is collected through the 7MHz standard mode, and other voltage ADC conversion methods can be used to further improve the voltage accuracy. It can be seen from the above table that the voltage difference between the two is very small, which can meet the accuracy requirements required for research on key technologies of electric vehicle lithium batteries.

* 6. Demonstrate your project and record it as a video for uploading

6.1. Video upload contest official website

6.2. Video title and link at Station B

(1) The 5th Lichuang Electronic Design Competition: "Lithium Battery Voltage and Temperature Acquisition Board" Project - Self-introduction.

Video link: https://www.bilibili.com/video/BV1QT4y1w7fD/See  video for details

(2) The 5th Lichuang Electronic Design Competition: "Lithium Battery Voltage and Temperature Acquisition Board" Project - Project Introduction.

Video link: https://www.bilibili.com/video/BV1da4y1L7R7/See video for details

(3) The 5th Lichuang Electronic Design Competition: "Lithium Battery Voltage and Temperature Acquisition Board" Project - Function Demonstration & Performance Test.

Video link: https://www.bilibili.com/video/BV1fT4y1w7H2/See video for details

Notes: ①: Video requirements: Please shoot horizontally, the resolution is no less than 1280×720, the format is Mp4/Mov, the size of a single video is limited to 100M; ②: Video upload: please upload to the official website of the competition and Station B simultaneously ( www .bilibili.com ), the top 10 most popular projects at Station B will receive 1,000-5,000 yuan in cash rewards, and other uploaded projects will receive 100 yuan Lichuang Mall no-threshold coupons; ③: Video title: The 5th Lichuang Electronic Design Competition: {Project Name }-{Video module name}; such as the 5th Lichuang Electronic Design Competition: "Autonomous Driving" project - team introduction.

* 7. Whether it is released to the public for the first time

7.1. Please indicate whether the project has been published or won awards before.

This is the first time the project has been entered, has not been published before and has not won any awards.

7.2. If the project is optimized on the original basis, please explain the optimization part

The project is a new development.

* 8. Open source documents

(1) Sampling plate schematic diagram

For the schematic diagram of the sampling board, please see the attachment "Lithium Battery Voltage Collection Board.SchDoc". The schematic diagram is made using AD15 software.

(2) Sampling board PCB

For the schematic diagram of the sampling board, please see the attachment "Lithium Battery Voltage Collection Board.PcbDoc". The schematic diagram is made using AD15 software.

(3) Acquisition board program

The sampling board program is developed using the NXP official development tool CodeWarrior5.1 development environment. The microcontroller model is MC9S12G128. For the microcontroller program, please see the attachment "Collection Board Program.rar".

(4) Host computer program

The host computer is developed using Labview software, and the VI program of the host computer is provided, and users can make changes according to their own needs. For the host computer program, see the attachment "Host Computer Program.rar".

(5) Acquisition board BOM table

The BOM list of the electronic components used in the collection board is shown in the attachment "Component BOM List.xlsx".

* 9. References

Click the link for more details: https://www.szlcsc.com/go/dsxq