Design of an intelligent fast charger for electric vehicle batteries

1 Introduction

The battery is the key power output unit of electric vehicles. Among the common batteries such as lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, lithium batteries and fuel cells, lead-acid batteries and nickel-metal hydride batteries are widely used in electric vehicles because of their high energy ratio, light weight, good temperature characteristics, low pollution, and insignificant memory effect. However, due to incorrect charging methods, the service life of rechargeable batteries is far less than the specified life. In other words, many batteries are not damaged by use but by charging. It can be seen that the quality of the charger has a great impact on the battery life. Based on this. This paper proposes a design of an intelligent charger using the C805lF040 single-chip microcomputer intelligent charging control solution, which can effectively improve the charging efficiency and extend the battery life.

2 Hardware Design

2.1 System Block Diagram

Figure 1 Block diagram of electric vehicle intelligent charger system

The electric vehicle intelligent charger uses the c8051040F single-chip microcomputer as the control core, and mainly includes AC/DC converter, IGBT power module, high-frequency transformer, rectifier filter circuit, single-chip microcomputer control circuit, pulse width modulation circuit and status display circuit, etc. Figure 1 is its system block diagram.

The maximum output power of the switching power supply in this scheme is 2.6KW, and the AC input range is 170V-270V. The charger circuit mainly includes two parts: the main charging circuit and the single-chip microcomputer control circuit. The working process of the whole circuit is: 220V single-phase AC is filtered by the capacitor after full-bridge rectification. A DC of about 300V is obtained. After passing through the inverter bridge composed of 4 IGBTs, high-frequency AC is obtained, which is coupled to the secondary side through a high-frequency transformer. Then it is rectified by rectifier tubes D6 and D7. Finally, a stable DC output is obtained after filtering by inductor L3 and capacitor C7. Due to the use of intelligent charging, the charging voltage and charging current are different in each stage according to different batteries. Therefore, the C8051040F single-chip microcomputer of Cygnal Company is used as the charging process control device. During charging, the single-chip microcomputer detects the charging current, charging voltage, and battery temperature of the rechargeable battery to prevent overvoltage and overcurrent of the circuit. If the battery temperature is too high, it can also be determined whether to switch to the next charging stage by detecting the battery voltage and current values. At the same time, the single chip microcomputer gives the voltage or current value of each stage of charging, and compares it with the corresponding voltage and current value obtained by sampling. The phase shift control chip uCC3895 changes the PWM value to change the conduction time of the power tube, so as to achieve the purpose of obtaining different stable output values ​​at different stages of the battery.

2.2 Introduction to single chip microcomputer control circuit

The charging control circuit uses the C8051F040 microcontroller for data acquisition and control. The chip is a fully integrated mixed-signal system-on-chip (SOC). It has a CIP-51 core that is fully compatible with the 805l instruction set. It integrates almost all analog and digital peripherals and other functional components required to form a single-chip data acquisition or control system in one chip. These peripherals or functional components include: ADC, programmable gain amplifier, DAC, temperature sensor, I2C bus, UART, SPI, timer, programmable counter, timer array, etc. The C805lF040 microcontroller adopts a pipeline structure, and the machine cycle is reduced from the standard 12 system clock cycles to 1 system clock cycle, which greatly improves the processing power and the peak performance can reach 25MIPS. It has a built-in 64K bytes of Flash program memory, 256B of internal RAM and 4KB of XRAM located in the external data memory space. The C805lF040 has an on-chip JTAG debugging circuit. Non-intrusive, full-speed in-system debugging can be performed through the 4-pin JTAG interface and using the device installed in the final application system. Because it has up to 8 12-bit ADCs and 8 8-bit ADCs, it can sample the single-ended input voltage and current from port PORTC. 6-channel PWM, on-chip programmable watchdog timer, can greatly simplify the peripheral design of the microcontroller control circuit and ensure the safe operation of the program. ADC is responsible for the voltage and current during charging, J2C is responsible for the collection of temperature data, PWM outputs the reference value of the voltage and current during charging to the comparison circuit, and the microcontroller controls the switch power supply control module UCC3895.

Voltage detection circuit: The voltage sampling circuit is composed of precision resistors and adjustable resistors. Since the maximum setting range of the AD measurement of the single-chip microcomputer is 5V, the battery pack voltage should be proportionally reduced within the 5V range. Then the AD conversion function inside the C805lF040 is used for conversion. The single-chip microcomputer calculates the battery voltage internally. The circuit uses the 12-bit AD conversion inside the single-chip microcomputer. It reduces the complexity of the circuit design and improves reliability and accuracy. In order to resist electrical interference and high-voltage electric shock, the circuit uses high-speed isolation optocoupler PC8l7 isolation.

Current detection circuit: Generally, a sampling resistor with a small resistance is connected in series in the circuit when current is collected. The voltage on the sampling resistor is input into the conversion channel of the microcontroller for A-to-V conversion. The voltage value is then converted into the current value through calculation. However, since the charging current in this scheme is relatively large, using resistor sampling will consume a little more power. Therefore, this scheme uses a current transformer for current sampling.

Temperature detection circuit: Temperature sampling uses temperature sensor LM92. LM92 is a single-chip high-precision digital temperature sensor produced by National Semiconductor Corporation. At room temperature, the temperature measurement accuracy can reach plus or minus 0.33 degrees. And it can be compared with the temperature point set by the user. The internal register of the sensor can be read and written through the I2C bus interface. It is easy to program and easy to use, and is widely used in high-precision temperature measurement and temperature control.

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In the pre-processing stage before charging begins, the software selects the corresponding charging algorithm according to different batteries. The channel selection control word is written into the mode register PCAOCPMn of the C805lF040 microcontroller. The counter, timer register PCA0 and module capture/compare register PCAOCPn are initialized. The frequency of the PWM output signal depends on the time base of the PCA0 counter/timer. Changing the value of the module capture/compare register PCA0CPn can change the duty cycle of the PWM output pulse.

After charging starts, the software collects and samples the voltage value on the battery voltage divider resistor at a fixed time. At the same time, the current transformer circuit detects the charging current in real time. After calculation, the output parameters of the PCAOCPn microcontroller PWM are set to achieve the best intelligent charging control.

2.3 Status LCD display module circuit

LCDl286A dot matrix LCD display is selected as the status display. The LCD display module circuit can be directly connected to P5 and P3 of the I/O port of the single-chip C8051F040, with P5 as the data port (D0.D7); P3.0, P3.1, P3.2, P3.3, P3.4 and P3.5 are connected to the 6 signal lines LCDD/1, LCDR/W, LCDE, LCDCS1, LCDCS2 and LCDRST of the LCD module to control the reading and writing operations of the LCD. There is a status display at each stage of charging, such as: the battery is in the charging state, the battery enters the temperature control state due to high temperature, the battery fast charging ends the charging state, etc.

3 Software Design

This software mainly consists of system initialization, preprocessing, and selection of pulse fast charging module and algorithm or constant current, constant voltage, floating charging module and algorithm according to different battery types and states. The process is shown in Figure 2.

Figure 2 Main program flowchart

3.1 Initialization

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At the initial stage of the program, the C8051FU40 microcontroller should be initialized first. The input and output states of the I/O ports are set by setting the I/O port coding cross switch, determining the chip pin function, setting interrupts, TIM timer parameters, etc.

3.2 Preprocessing

The pretreatment stage is the preparation before entering fast charging.

After the program is initialized, the internal temperature sensor of the C805lF040 microcontroller is used to detect the ambient temperature. When the ambient temperature is too low or too high, the battery cannot be charged, otherwise it will be damaged.

Then, set the A/D conversion parameters and channels to detect the terminal voltage of the battery. Compare the detection data with the theoretical experience value to determine the type of battery and whether it is connected correctly. For batteries with low terminal voltage, use short-term pulsating current charging, which is conducive to activating the chemical reaction substances in the battery and partially restore the damaged battery cells. For batteries with terminal voltage within the nominal range, select the corresponding charging control module and algorithm, and for batteries with terminal voltage outside the nominal range, the software automatically removes them.

3.3 Fast charging

According to the predetermined charging control module and algorithm, set the C805lF040 single-chip microcomputer PWM control register PCAOCN, mode register PCAOMD and 16-bit capture and compare register PCAOCPn. Turn on the interrupt enable bit and start fast charging.

During fast charging, the C8051F04J0 microcontroller must continuously detect the following key technical indicators: whether the circuit is open, whether the battery is unbalanced, whether the battery reaches the specified safety voltage, whether the battery temperature is too high, and whether the battery meets the -△v or △T/△t conditions.

The battery circuit break is mainly judged by detecting the current size on the sampling resistor. In order to avoid misjudgment, the detection should be repeated. When a circuit break occurs, the pre-processing stage should be returned. The judgment of the circuit break should be made when the battery terminal voltage has reached the predetermined value. Otherwise, when the battery terminal voltage has not reached the predetermined value, the charging current is relatively small, which may lead to misjudgment.

The battery terminal voltage detection uses the on-chip 12-bit high-precision A/D module of the C8051F040 microcontroller. It adopts interrupt control mode. This can save the waiting time of the C8051F040 microcontroller during the conversion period. The terminal voltage detection data is used to calculate the battery voltage negative growth -△V through the charging algorithm to see whether it meets the fast charging termination condition, and the output parameters of the C8051F040 microcontroller PWM are modified in real time to control the size of the charging current.

The battery temperature detection is performed after the terminal voltage detection. The C8051F microcontroller accesses the corresponding digital temperature sensor LM92 by setting different address codes to read the temperature data. The charging algorithm is used to calculate whether the battery temperature change rate △T/△t meets the fast charging termination condition, and the output parameters of the C8051F040 microcontroller PWM are modified in real time to control the size of the charging current.

To prevent the battery from being damaged, charging should be stopped immediately when the battery voltage reaches the maximum voltage Vmax or the maximum temperature Tmax, otherwise the battery will be damaged.

4 Conclusion

The experimental results show that the intelligent fast charger with C8051F040 single chip as the control core can work normally. Since C8051F040 has a good performance-price ratio, its unique analog circuit module, high-precision A/D conversion, I2C bus interface and high-speed PWM functions are applied to charging control. The internal and external functions of C8051FD40 are effectively used to increase the intelligence and practicality of the product. It saves product development time and cost, reduces production costs, and also improves product consistency and reliability, which has a good promotion value.

The author's innovation: This design uses the SoC microcontroller C805lF040 as the main body to build a hardware design platform and software design method for the electric vehicle battery charging system. The μC/OS II real-time operating system is embedded in the C805lF040, which can greatly improve the system's stability and real-time response capabilities, and enhance the system's reliability, scalability, and portability.