Design scheme of protection circuit based on lithium battery pack of electric vehicle

　　0 Introduction

　　With the popularity of electric vehicles, lithium batteries have also become the focus of everyone's attention. Lithium batteries are different from nickel-cadmium and nickel-metal hydride batteries. Because of their high energy density, lithium batteries have very high requirements for charging and discharging. When overcharging, over-discharging, overcurrent and short-circuit protection occur, the pressure and heat in the lithium battery increase greatly, which is prone to explosion. Therefore, protection circuits are usually added to the battery pack to increase the service life of lithium batteries. At present, most of the protection circuits used in lithium battery packs of electric vehicles are composed of discrete components, which have the characteristics of insufficient control accuracy, low technical indicators, and inability to effectively protect lithium battery packs. This paper proposes a protection circuit design scheme for 36V lithium battery packs of electric vehicles (composed of 10 3.6 V lithium batteries connected in series) based on single-chip microcomputer. The high-performance, low-power ATmega16L single-chip microcomputer is used as the detection and control core, and the DC/DC conversion control circuit composed of MC34063 is used to provide a regulated power supply for the entire protection circuit. LM60 temperature measurement and MOS tube IRF530N are used as charge and discharge control switches to realize the status monitoring and protection functions of the entire battery pack and individual batteries, so as to extend the battery life.

　　1 Overall design of the system

　　This system uses the microcontroller as the core of data processing and control, and decomposes the task design into functional modules such as voltage measurement, current measurement, temperature measurement, switch control, power supply, and balanced charging. The overall block diagram of the system is shown in Figure 1.

　　Figure 1 Overall block diagram of the system

　　The voltage, current, temperature and other information of the battery pack are added to the A/D input of the signal acquisition part through the voltage sampling, current sampling and temperature measurement circuits. The A/D module converts the input analog signal into a digital signal and transmits it to the single-chip microcomputer. As the core of data processing and control, the single-chip microcomputer monitors the various performance indicators and states of the battery pack in real time on the one hand, and controls the driving of the high-power switch according to these state parameters on the other hand. The use of the single-chip microcomputer makes the system very flexible and easy to realize various complex controls, so that the system can be easily expanded in function and improved in performance.

　　2 System Hardware Design

　　2.1ATmega16 L MCU Module

　　From the perspective of low power consumption and low cost design, the MCU module uses the high-performance, low-power ATmega16 L MCU as the detection and control core. ATmega16 L is a low-power 8-bit CMOS microcontroller based on the enhanced AVRR ISC structure, with 16 k bytes of in-system programmable Flash, 512 bytes of EEPROM, 1 k bytes of SRAM, 32 general I/O lines, 32 general working registers (JTAG interface for boundary scan, supporting on-chip debugging and programming), 3 flexible timers/counters (T/C) with compare mode (on-chip/external interrupts), programmable serial USART, universal serial interface with start condition detector, 8-channel 10-bit ADC with optional differential input stage programmable gain (TQFP package), programmable watchdog timer with on-chip oscillator, an SPI serial port, and 6 power saving modes that can be selected by software. Due to its advanced instruction set and single-clock cycle instruction execution time, the data throughput of ATmega16 L is as high as 1M IPS/MHz, which can alleviate the contradiction between system power consumption and processing speed.

　　The input and output design of the single-chip microcomputer is shown in Figure 2. The 3.3 V voltage obtained by stepping down and stabilizing the power supply part provides the working voltage for the single-chip microcomputer through port 10; ports 12 and 13 are the input terminals of the reverse oscillation amplifier and the on-chip clock operation circuit and the output terminals of the reverse oscillation amplifier, providing the single-chip microcomputer with a working crystal oscillator; port 30 is the power supply for port A and the A/D converter, and is connected to the VCC of port 10 through a low-pass filter when using ADC; ADC3 and ADC2 of ports 37 and 38 are the voltage and current values ​​to be detected after conversion; ADC1 and ADC0 of ports 39 and 40 are the temperature-controlled voltage values ​​after conversion by the temperature sensor.

　　Figure 2 MCU peripheral circuit design

　　2.2 Voltage Regulated Power Supply Module

　　The voltage regulator is an important part of the single-chip microcomputer system. It not only provides multiple power supply voltages for the system, but also directly affects the technical indicators and anti-interference performance of the system. The working voltage of the ATmega16 L single-chip microcomputer is 2.7~5.5 V. To ensure the stable working voltage of the single-chip microcomputer, the voltage regulator is a DC/DC conversion control circuit composed of MC34063. The 25 V voltage from the battery pack is stepped down and stabilized by the circuit to output 3.3 V for the protection circuit to work. The circuit is shown in Figure 3. [page]

　　Figure 3: Voltage stabilized power supply module circuit

　　2.3 Charge balancing module

　　An analog circuit solution is used. That is, an overvoltage protection circuit is built outside each battery. When the voltage exceeds the preset value during charging, the protection circuit automatically closes, allowing the battery to discharge through the resistance loop to protect the battery from overcharging. When the battery voltage decreases to the equalization charging action voltage of 4.18 V, the protection circuit automatically disconnects.

　　2.4 Voltage and current measurement module

　　The voltage to be measured is sent to the microcontroller for detection through the integrated operational amplifier LM358. The LM358 includes two independent, high-gain, internal frequency-compensated dual operational amplifiers, which are suitable for single-power use and dual-power operation modes with a wide power supply voltage range. Due to its low power consumption, it is also suitable for batteries. The Hall sensor UGN-3501M is used to detect DC current. UGN-3501M is an integrated Hall sensor that uses differential Hall voltage output and has a detection sensitivity of 1.4 V /0.1T.

　　The design of the voltage and current detection circuit is shown in Figure 4. The BB and AA connected to the 5 and 6 pins of the operational amplifier LM358 are the charging and discharging voltages to be measured. After amplification, they are output from the 7th pin to the microcontroller for detection. When the voltage to be measured reaches the overcharge and over-discharge protection voltage, the microcontroller controls the disconnection of the charging and discharging circuit. The current detection is completed by the Hall sensor. As shown in Figure 4, the Hall voltage uH output from the 1 and 8 pins of UGN-3501M is connected to the 3 and 4 pins of LM358. After amplification, it is output from the 1st pin ADC3 to the microcontroller for overcurrent protection. The 5, 6, and 7 pins of UGN-3501M are connected to the adjustment potentiometer to compensate for unequal potential and improve linearity. By adjusting the external resistor R16 of the 5 and 6 pins, the output Hall voltage uH can have a better linear relationship with the magnetic field strength.

　　Figure 4 Voltage and current detection circuit

　　2.5 Temperature Detection Module

　　The temperature detection and control module uses the voltage output semiconductor temperature sensor LM60. This sensor is a calibrated integrated temperature sensor with an operating temperature range of -40°C to 125°C and an operating voltage range of 2.7 V to 10 V. The signal output is proportional to the temperature and the signal size can reach +6.25 mV/°C.

　　The temperature detection circuit based on LM60 is shown in Figure 5. The 3.3 V power supply output by the voltage stabilization part powers this circuit. The temperature of the detection point is converted into a voltage value through the temperature sensor and output through ADC0 and ADC1. Then ADC0 and ADC1 are sent to the microcontroller for detection. When the voltage value reaches the temperature control requirement, the microcontroller controls the switch on and off.

　　Figure 5 Temperature detection circuit

　　2.6 Switch Module

　　The switch uses MOSFET, and the model is IR530N, a P-channel MOS tube. Working principle: The microcontroller control port outputs a high level, the power triode is turned on, a voltage drop occurs between the gate and drain of the power field effect tube, and the power field effect tube is turned on. [page]

　　3 System software design

　　The system software is written in C language, the processing program adopts modular programming, and the program runs in the ICCAVR development system.

　　When the battery pack is unloaded, the system enters the power-down mode to minimize power consumption; when the battery pack is connected to a load or is being charged, the microcontroller is activated and switches from the low-power power-down mode to the normal working mode and continues to operate. The entire program flow is shown in Figure 6.

　　Figure 6 Program flow

　　According to the module distribution of this system, the MCU program is divided into voltage measurement module, current measurement module and temperature measurement module. Each module calls common A/D conversion function and delay judgment function, etc., to shorten the code length and enhance the readability of the program code. The code of the main function of the program is given below:

　　4 Conclusion

　　This paper proposes a design scheme for a 36V lithium battery pack protection circuit for electric vehicles based on ATmega16L. The high-performance, low-power ATmega16L microcontroller is used as the detection and control core, and the DC/DC conversion control circuit composed of MC34063 is used to provide a regulated power supply for the entire protection circuit. LM60 temperature measurement and MOS tube IRF530N are used as charge and discharge control switches to realize the status monitoring and protection functions of the entire battery pack and a single battery. Through experiments, this protection circuit system realizes all basic functions. Compared with the traditional battery protection system using separate components, the battery protection circuit system based on a single-chip microcomputer proposed in this paper has the characteristics of small system size, multiple functions, low power consumption, and low cost, and can be used in industrial production.