Design of intelligent management system for electric vehicle battery based on ATmega8

Abstract : Based on the ATmega8 microcontroller, this paper designs a distributed, modular electric vehicle battery management system that communicates via the LIN bus and has intelligent charging functions, which realizes the effective monitoring and management of multiple battery groups.
Keywords : battery management system; intelligent charging; LIN bus

Introduction
The advantages of electric vehicles with no (low) pollution make them the main direction of contemporary automobile development. The development of electric vehicles needs to solve two major problems, namely energy storage and power drive. Since the problem of insufficient energy storage of power batteries is difficult to solve in the short term, energy management technology has become the key to the development of electric vehicles. In traditional charging technology, the commonly used constant voltage charging, constant voltage current limiting charging, constant current charging and other modes are all manually controlled charging processes, and most of them have serious overcharging phenomena. The quality of charging directly affects the service life of the battery. The design of the new battery intelligent management system is to detect the status of the power battery online, improve the charging quality and efficiency, and make the operator only perform auxiliary work.

Figure 1 System block diagram

Composition and hardware design of the management system
The intelligent management system designed in this paper is a distributed and modular vehicle battery monitoring system, which is mainly composed of a main control module, a controllable charging system module, a voltage acquisition submodule, a temperature acquisition submodule, a current measurement submodule and a display module, and communicates with each other through the LIN bus. The principle block diagram of the management system is shown in Figure 1.

Figure 2 LIN bus communication circuit diagram

LIN bus communication circuit
LIN bus communication is simple and convenient, making the intelligent power management system and the various systems of the car both interconnected and relatively independent, thus overcoming the current loopholes in battery management and greatly improving the safety and controllability of cars and car batteries. Figure 2 shows its specific circuit. Each module in this design contains this circuit to achieve information sharing and transmission. The actual communication baud rate in this design is 1200bps. Among them, pc817 plays an isolation role, and max1487 ensures that the sending and receiving signals are staggered in time.

Voltage detection circuit design
There are three main methods for measuring the voltage of multiple batteries in series: variable resistance voltage division, relay switch switching, and distributed voltage measurement. The detection object of this design is a battery pack with 4 groups in parallel, each group has 40 batteries in series, and the terminal voltage is 48V. The nominal voltage of a single battery is 1.2V. It is mainly used to detect the battery status to avoid the use of a single bad battery. The required accuracy is not very high. Therefore, each pressure measurement module measures a group of batteries, that is, every 8 batteries are measured as a unit. Taking the process and cost into consideration, the pressure measuring circuit adopts a circuit structure combining variable resistance voltage division and relay switch.

Figure 3 Voltage measurement circuit diagram

As shown in Figure 3, U1~U5 are the voltage levels after voltage division, which are respectively connected to the PC0-PC4 ports with A/D conversion function of the single-chip computer to complete voltage sampling. When selecting the parameters of the adjustable resistor R1 and the fixed resistor R2, the voltage division should ensure that Ui≤5V, that is, sampling the i-th channel,

where Umax is the maximum voltage of the unit battery pack. This design uses a relay switch to detect whether the module is completely disconnected from the battery pack when it is not working to avoid small current discharge of the battery; using adjustable resistors, a unified variable design can be used in the program processing after A/D conversion, simplifying the program and facilitating actual debugging.

Temperature detection design
The DS18B20 digital temperature sensor is mainly used in the temperature measurement module. The main features of this device are: the unique single-wire interface only requires one interface pin for communication; multi-point capability simplifies distributed temperature detection applications; no external components are required; data line power supply is available; no backup power supply is required; the measurement range is -55℃~+125℃, and the increment value is 0.5℃; the temperature is read in 9-bit digital value mode; it has a user-definable, non-volatile temperature alarm device. In addition, since each DS18B20 has a unique serial number, multiple DS18B20s can exist on the same single-wire bus, which brings great convenience to the application.

Figure 4 Main circuit of controllable charging module

Design of controllable charging module
This module (see Figure 4) is the difficulty in the actual design. It is connected to the external power grid to charge the on-board battery. It can charge the battery in stages and with different currents according to the instructions or flags issued by the control circuit, and has the function of automatic power off to achieve intelligent charging. According to the actual high power and high voltage characteristics, its main circuit adopts a full-bridge topology structure, and the output circuit adopts a full-bridge rectifier. At the same time, in order to improve the working state of the power switch device, the main circuit adopts soft switching technology.

Figure 5 Main control and LCD display module circuit diagram

Main control and LCD display module
The main program module is the core of the whole system. It collects data from each module as needed, judges and analyzes the data, and displays the relevant information on the LCD screen. When in the charging state, the charging control is carried out according to the information sent by the current acquisition submodule, combined with the battery voltage parameters and temperature measurement values. According to the power value calculated by the current measurement module, the charging mode is judged and changed. When the battery is fully charged, the microcontroller will set a mark for the data, so that the controllable charging module disconnects the relay and the charging circuit is disconnected from the battery pack.

This design uses the MGLS240128T dot matrix LCD display module with built-in T6963C. The circuit of the display and main control module is shown in Figure 5. Among them, VCC is a 5V power supply, D0~D7 is connected to an 8-bit data port of MC68HC912D60A, pins 5, 6, and 8 are control ports, which are used to control the read and write operations of the LCD display module, and RST (pin 10) is the hardware reset pin of the LCD display module. The V0 port inputs the LCD display drive voltage, and the sliding rheostat is used to adjust the LCD display brightness.

Software programming and testing
The core software of this management system is implemented in C language programming under the ICCAVR compilation environment. According to the modular design in hardware design, each module has an ATmega8 chip, so when programming, it is programmed separately according to the module task. The main program of the submodule basically includes module initialization and data processing, as well as serial port receiving and sending interrupt programs. In the serial port interrupt program, the main control module sends a data request, and each submodule sends the corresponding data to the main control module in the interrupt program according to the relevant data string received. In addition, each submodule performs actual device operations based on the data returned by the main control module, and the management system communicates and operates through the LIN bus. In the software debugging of the communication, by using the serial port debugging software and connecting the bus data to the PC through the serial port, it is convenient to monitor the data of each module, find problems and debug.

Conclusion
This design uses ATmega8 microcontroller, making full use of its multiple peripheral interfaces and powerful functions, easy operation and low cost. The whole system has been used on an electric engineering vehicle and the system is stable.