Fast charging system based on ST72 microcontroller

Traditional conventional battery charging methods all use small current constant voltage or constant current charging, and the charging time is as long as 10 to 20 hours. In order to shorten the charging time, the charging current of the charging process must be controlled. Many manufacturers mistakenly believe that blindly increasing the charging current is fast charging. In fact, this not only cannot improve the charging efficiency, but also aggravates the polarization of the battery, increases the battery temperature, wastes electricity, and seriously damages the battery. According to the three laws of Maas proposed by American scientist Maas, pulse charging and pulse discharge can remove battery polarization, reduce the internal pressure, temperature and internal resistance of the battery, speed up the charging speed, and realize the fast charging of the battery. The charge-discharge-charge cycle method is required. This mode of fast charging system generally includes a control circuit, a charging circuit, and a discharge circuit. According to the parameters measured during the charging process, the (charge and discharge) amplitude of the pulse voltage and the width ratio of charging and discharging are properly adjusted by the single-chip microcomputer to ensure that the constant value can be obtained. The best result of charging efficiency.

1 System Structure

1.1 Control loop

The ST72 series MCU is a simple, flexible, low-cost 8-bit MCU newly produced by ST Microelectronics of France. It adopts unique manufacturing process and technology, which greatly improves the anti-interference ability. It is designed for embedded control applications, especially for control systems with high noise resistance and high overall cost performance. It has the following main performances:

(1) The operating power supply voltage has a wide adaptability range, from 2.4V to 5.5V; low power consumption, internal digital watchdog.

(2) Rich I/O functions, with 3 internal timers/counters, some with up to 16 channels of 10-bit A/D converters, two pulse width modulation, multiple communication interfaces, and up to 12 levels of interrupt sources.

(3) A variety of internal program memories, in addition to the cheap OTP (One Time Programming) one-time programming and writing, there is also a very convenient FLASH program memory that can be programmed online and easily modify program code. The memory capacity ranges from 1.5KB to 60KB, which can meet various programming needs.

According to the needs of system functions, the ST72 series ST72264G1 single-chip microcomputer is selected as the control chip. The chip has 4KB of FLASH internal memory, 2-way PWM output, maximum clock frequency of 8MHz, 20 I/O pins, 6 multiplexed 10-bit A/D converters, and three communication interfaces: SPI, SCI, and I2C. The control circuit uses ST72264G1 plus LCD display and keyboard interface, etc. It uses the A/D port of the single-chip microcomputer itself to form voltage, current, and temperature detection circuits.

1.2 Charging and discharging circuit

The charging circuit is a power circuit composed of a new type of power chip TOP225Y. This chip is a three-terminal isolated pulse width modulation monolithic switching power supply integrated circuit produced by Powergraph Corporation of the United States. It integrates the PWM integrated circuit and the MOSFET power device in the same chip, and has all the functions required for the PWM type switching voltage stabilized power supply. The output end is completely isolated from the power grid through the high-frequency transformer, realizing the integration of the switching power supply isolated by the reactive frequency transformer. The whole circuit has the advantages of large output power range, low cost, high degree of integration, and simple circuit design.

The discharge circuit is composed of a battery and a resistor, and the MOS tube is used as a switch to control the shutdown of the discharge circuit. The shutdown of the MOS tube is controlled by a control signal given by the microcontroller.

The principle block diagram of the intelligent charging system is shown in Figure 1.

The microcontroller converts the output digital voltage into analog voltage based on voltage feedback and current feedback. The analog voltage is then isolated by an optocoupler, and then the current of the power chip TOP225Y control pin is adjusted. The chip adjusts the duty cycle of the control signal to adjust the amplitude of the pulse voltage or the size of the trickle current. Generally, a D/A converter is required to convert the digital voltage of a microcontroller into an analog voltage. Since the ST72 series microcontroller has a PWM output, the PWM output can be directly used to convert the digital voltage into an analog voltage, which can ensure accuracy and save the D/A converter. [page]

2 System software design

The system software is written in C language and burned into the internal program memory of the microcontroller after assembly, simulation debugging. The software design is carried out in accordance with the specifications of software engineering, and the structure is hierarchical and the functions are modularized, which not only enhances the readability, maintainability and scalability of the software, but also improves the reliability of the software. The overall structure diagram of the program is shown in Figure 2.

The program first detects whether the battery is fully charged and whether the battery is good or bad. Whether the battery is fully charged can be determined from the voltage at both ends. The quality of the battery needs to be determined through open circuit, short circuit, temperature measurement and other test measures. A relatively small charging current (about 1/5 of the fast charging current) can be applied to the battery to make the battery voltage reach a certain value within a certain period of time to determine the quality of the battery. After starting pulse charging, the program automatically tracks and detects various parameters of the battery. When the parameters reach the pulse charging termination condition, the system terminates the fast charging state and switches to the trickle charging state to replenish the power lost by the battery due to self-discharge. In the trickle charging state, the charger will replenish the battery at a certain charging rate, so that the battery is always fully charged.

In order to ensure that the battery charging state can be accurately and reliably controlled under any circumstances, this system adopts a comprehensive control method of voltage control, temperature control and timing control.

(1) Voltage control uses the inflection point control method. This charging termination method consists of detecting the inflection point of the voltage curve and is applicable to both nickel-metal hydride and nickel-cadmium batteries. Using a single-chip microcomputer to detect the voltage curve requires finding the first derivative of the voltage curve with respect to time and detecting its peak value. The A/D converter of the ST72264 single-chip microcomputer measures the battery voltage once every 4 seconds. In order to reduce the measurement error caused by high-frequency interference (such as interference caused by pulse charging current), the battery voltage is measured repeatedly to obtain an average measurement value. Then, a rolling average is made for the current and previous average measurement results to remove the low-frequency interference caused by the change in the electrochemical cell voltage. The derivative of the voltage with respect to time is obtained by calculating several measurement data. Because the voltage is a digital quantity obtained by the A/D converter, the curve of the first derivative of the voltage with respect to time is not smooth, so the curve must be smoothed. The inflection point (point B in Figure 3) is identified by detecting the first highest peak value encountered on the smooth derivative curve. When this peak value is detected, the fast charging is terminated.

(2) Temperature control uses the temperature change rate method. After NiMH and NiCd batteries are fully charged, the battery temperature rises rapidly, and the rate of increase ΔT/Δt is basically the same. When the battery temperature rises by 1°C per minute, fast charging should be terminated immediately. It should be noted that: since the relationship between the resistance value and temperature of the thermistor is nonlinear, in order to improve the detection accuracy, efforts should be made to reduce the impact of the nonlinearity of the thermistor.

(3) Timing control is to set a time limit, and stop fast charging when the time is exceeded.

3. System performance evaluation

The charging system can be used to charge nickel-hydrogen batteries and nickel-cadmium batteries. For batteries with a capacity of 1000 mAh, the full charging time is 25 minutes; for batteries with a capacity of 1300 mAh, the full charging time is 33 minutes; for batteries with a capacity of 1600 mAh, the full charging time is 40 minutes, which is much faster than conventional charging systems. In addition, the system has a simple structure, easy debugging, safe and reliable charging, and will not damage the battery or shorten the battery life.