Distributed battery management system based on CAN bus

Abstract: This paper mainly discusses the digital technology of automobiles and electric vehicles, the structure of computer control systems, and related issues of field communication. Taking the battery energy system as the application background, a management system with dual CAN buses as internal and external communication methods and a multi-module distributed structure is studied and designed. The system is divided into several modules, each of which realizes its own independent functions, including data acquisition, measurement of multiple voltages, currents and temperatures, power estimation and communication management, and large LCD display.

In order to meet the requirements of high performance, safety and scalability required by system development, a management system structure idea of ​​dual CAN bus communication and distributed processing is proposed. The technical issues of CAN bus design, circuit and application are introduced in detail. Keywords: battery management system; CAN bus; distributed structure; electric vehicle

Distributed Battery Management System Based on CAN Bus

LIAN Zi-feng, ZHENG Hang-bo, QI Guo-guang

Abstract:The digital technology of vehicle and electronic vehicle (EV), with the emphasis on the structure of the system controlled by computer and something about the communication are mainly discussed.The management system which has been studied and designed uses double CAN(Controller Area Network) bus as its interior and exterior communication method. The system adopts distributed structure and consists of several modules, each of which realizes its special function, such as data collection, the measure of multi loop voltage,current and temperature, the computing of the SOC (State of Charge), the management of communication and liquid crystal display.

In order to fulfill the need of high performance, security and extend for the system, some ideas, including communication with double CAN bus and distributed process are putted forward. The CAN bus design, the circuit and the technological problem in the application of CAN bus are also discussed.

Keywords:Battery management system (BMS); CAN bus; Distributed structure; EV

1 Introduction

With the rapid development of high technology and its industry, battery energy systems with large storage capacity have been increasingly valued by people and have been widely used in many fields. For example, in the new direction and new hot spot of the development of the automobile industry - the research and industrialization of electric vehicles and hybrid vehicles, they will serve as the main supplier of on-board energy.

The battery pack is composed of a certain number of single cells connected in series, and it can be charged and discharged hundreds to thousands of times. During use, it is necessary to pay attention to the various characteristics of each single cell, battery temperature, remaining battery power and total current parameters, because these parameters directly affect the service life of the battery. It is necessary to optimize the operation and effectively monitor to prevent the battery from overcharging, over-discharging and over-temperature, so as to extend the service life of the battery and reduce costs, especially to improve the reliability of the battery. The electronic, control and digital technologies supporting the battery pack can be called digital "battery electronic technology". Similarly, in the electronic and digital technologies of automobiles, multiple CPUs have been used to complete the control of various parameters and functions. Considering the safety of the car, the operation must be very reliable, so a parallel independent multiple system structure has been developed, and then connected by a field bus to form a unified large system.

2 Distributed Management System

2.1 System Structure

The system needs to realize different types of multiple functions. The centralized or central processing method cannot meet the security requirements, so a distributed structure must be adopted. The system has a harsh working environment and is often subject to strong electromagnetic interference and pulse current interference. In order to ensure reliability, the high-performance CAN field bus is considered to be adopted and developed as the communication system. Moreover, the CAN bus has been used in automobiles for a long time and has strong anti-interference. At the same time, the technology is relatively mature and has become the standard for automobile communication. Therefore, the CAN bus is used to realize the internal communication and external communication of the system.

This distributed system is designed with CPU80C552 as the common module platform. Due to the limited storage space and operation of the CPU, multiple CPUs must be used to realize the various functions required by the management system. The completed basic system consists of four modules in parallel: data acquisition, balanced charging, power estimation and communication display; each module realizes its function respectively, and communicates data through the CAN bus, which can realize the collection and measurement of single battery voltage, total voltage, charge and discharge current, temperature, and power estimation. At the same time, the system also has strong scalability, and can carry out research and development of specific battery diagnosis and battery safety performance protection functions. In the lithium battery management system, 108 batteries use 9 measurement main boards, plus 4 basic boards, a total of 13 boards.

Figure 1 Overall structure of the battery management system

2.2 Design of the main module of the management system

The main functions of the system include data acquisition, power estimation and display diagnosis. Since 80C552 has 8-channel 10-bit A/D conversion function, the acquisition module first uses the linear optocoupler method to measure the voltage of the single battery, and converts the analog quantity into digital quantity through its 4 A/D ports and stores it in the memory. The temperature measurement adopts single bus technology and uses Dallas digital chip to measure the temperature. The chip has a 12-bit accuracy level and can measure the temperature of the system very accurately. The total voltage and current signals are converted into 0-10V signals through special sensors, and converted into digital quantities through 14-bit A/D conversion devices and stored in the system.

The communication and display module provides dual CAN communication interfaces, which can transmit data with various modules in the system and external vehicle systems through CAN; at the same time, the system provides RS232 interface, which can realize communication with PC; the module also provides 5.5-inch LCD display drive function and buttons for human-machine friendly operation; the module also has upper and lower limit alarms and self-test functions for voltage, power, current and temperature to ensure the safety of the system.

The basic structural block diagram of each system module is shown in Figure 2.

Figure 2 Module structure diagram

2.3 Power Estimation

The power estimation uses the ampere-hour method of real-time current integration for basic estimation, and then corrects various parameters that affect the battery power, such as temperature, self-discharge and aging, and takes into account the inconsistency between single batteries, so as to obtain an accurate battery pack power.

Figure 3. Battery capacity estimation block diagram

3 CAN bus system

3.1 Introduction to CAN

CAN bus is a kind of field bus. It is a serial high-speed data communication bus developed by Bosch in Germany in 1986 to solve the data exchange between numerous control and test instruments in modern automobiles. It adopts the physical layer and data link layer of the seven-layer structure of the ISO/OSI model, and has high reliability, real-time and flexibility.

CAN bus has the following unique advantages:

1) CAN can work in a multi-master mode. Any node on the network can send information to other nodes on the network at any time, regardless of master or slave, and the communication mode is flexible.

2) CAN can transmit and receive data in point-to-point, point-to-multipoint and global broadcast modes. The communication medium uses twisted pair, coaxial cable or optical fiber, which is flexible to choose. The maximum communication distance can reach 10km/5kb/s, and the communication rate can reach up to 1Mb/s/40m. The number of nodes on CAN depends on the bus drive circuit, and can actually reach 110;

3) In the case of serious errors, the CAN node has the function of automatically shutting down the output, cutting off its connection with the bus so that other operations on the bus are not affected. It adopts NRZ encoding/decoding and bit filling technology. The user interface is simple, programming is convenient, and it is easy to form a user system;

4) CAN uses non-destructive arbitration technology. When two nodes transmit information to the network at the same time, the node with lower priority will actively stop sending data, while the node with higher priority can continue to transmit data without being affected, effectively avoiding bus conflicts.

5) CAN uses a short frame structure, each frame is 8 bits, the transmission time is short, the probability of interference is low, and each frame of information has CRC check and other error detection measures to ensure that the data error rate is extremely low.

3.2 CAN bus design

The overall structure of the CAN bus is shown in Figure 4. Two 120Ω resistors are configured at both ends of the bus. Their function is to match the bus impedance, which can increase the stability and anti-interference ability of the bus transmission and reduce the error rate in data transmission. The CAN bus node structure is generally divided into two categories: one is connected to the PC using a CAN adapter card to realize the communication between the host computer and the CAN bus; the other is composed of a single-chip microcomputer, a CAN controller and a CAN driver, as a node to transmit data with the CAN bus. In this system, the CAN controller uses the SJA1000 and 82C200 produced by Philips. It acts as a sending and receiving buffer to realize data transmission between the main controller and the bus; the CAN transceiver uses the PCA82C250 chip, which is the interface between the CAN controller and the physical bus. It can mainly provide differential sending capabilities for the bus and differential receiving capabilities for the CAN controller.

Figure 4 CAN bus system structure diagram

4 CAN bus software design

The three-layer structure model of CAN bus is: physical layer, data link layer and application layer. The functions of physical layer and data link layer are completed by SJA1000. The development of the system is mainly in the design of application layer software, which mainly consists of three subroutines: initialization subroutine, data sending and data receiving program. At the same time, it also includes some data overflow interrupt and frame error processing.

After the SJA1000 is powered on and reset, it must be initialized by software before data communication can be carried out. The initialization process mainly includes configuring the clock frequency division register CDR, bus timing registers BTR0 and BTR1, acceptance code register ACR, acceptance mask register AMR and output control register OCR in its reset mode, so as to define the bus rate, acceptance mask code, output pin drive mode, bus mode and clock frequency division. The specific process is shown in Figure 5. The following is the process of SJA1000 sending and receiving data. The basic process is that the main controller saves the data to the SJA1000 send buffer, and then sets the send request TR flag of the command register to start sending; the receiving process is that SJA1000 stores the data received from the bus into the receive buffer, notifies the main controller to process the received information through its interrupt flag, clears the buffer after receiving, and waits for the next reception. The specific process is shown in Figures 6 and 7.

Figure 5 CAN bus initialization

Figure 6 CAN data transmission process

Figure 7 CAN data receiving process

For example, the format of the total voltage sent by the battery management system to the vehicle system is shown in Table 1.

Table 1 BCU_VCU_VOLTAGE (0x08) sends the current voltage of the battery pack back to the VCU

Among them, ID is the address of the receiving node bus, the voltage value is multiplied by 10 and rounded before sending, and 0x08 means that the content of the sent frame is the voltage of the battery pack.

5 CAN bus application issues

In terms of hardware, reasonable power supply must be considered, and attention must be paid to the filtering between the power supply and ground of each CAN device, as well as the design of the reset circuit; at the same time, when actually designing the printed circuit board, reasonable wiring should be carried out, and the ground wire should be strengthened to enhance the system's anti-interference ability.

In software design, the setting of CAN bus timer is very critical. BTR0 determines the propagation time period, phase buffer segment 1 and phase buffer segment 2; BTR1 determines the synchronization jump width and frequency division value. In the bit timing register, the values ​​set for TSEG1, TSEG2, SJW and BRP are 1 less than their functional values, so the setting range is [0.....N-1] instead of [1.....N]. Therefore, the bit time can be obtained by [TSEG1+TSEG2+3]tq or [synchronization segment+propagation segment+phase buffer segment 1+phase buffer segment 2]tq, where tq is determined by the system clock tSCL and the baud rate pre-division value BRP: tq=BRP/tSCL. At the same time, it should be noted that since the CAN system clocks of different nodes are provided by different oscillators, the actual CAN system clock frequency of each node has a tolerance with the actual bit time. The change of ambient temperature and oscillator aging affect the initial tolerance. To ensure accurate data transmission, it is necessary to ensure that the CAN system clock frequency of each node is within the specific frequency tolerance limit. Therefore, when selecting an oscillator, the node with the highest oscillator tolerance range should be used as the standard. In addition, in an expandable bus structure, the maximum node delay and the maximum bus length must be considered. In general, the delay is 5.5ns/m.

In actual operation, the CAN bus is often blocked or suddenly closed. The main reason is that frames are lost during data transmission, causing errors. When the error counter reaches a certain value, the bus will be automatically closed. Therefore, the error status ES bit must be identified in time during the software design process. When an error occurs, the SJA1000 needs to be reset by software to restore communication.

6 Conclusion

In the development of the battery management module for electric vehicles under the "863 Major Project", a distributed structure of CAN bus communication is adopted. The bench test results of nickel-hydrogen battery packs and lithium battery packs show the advanced nature of the system structure, realize the independent functions of each module, and work normally and reliably. The number of nodes of the CAN bus of the lithium battery pack system has increased to 12, and it can still work normally under strong electromagnetic interference, and the line connection is very simple and practical.

The parameters, measurement methods, number of batteries, safety requirements and groupings of the two battery packs are different, but the system can adapt effectively, reflecting its good adaptability and great flexibility.