Design of intelligent charger for lead-acid batteries based on single chip microcomputer

0 Introduction

Batteries are important energy devices widely used in various sectors of the national economy, such as national defense, transportation, communications, and electricity, and are indispensable products in social production and operation activities. The service life of batteries is a common concern of professionals and users. It is determined by many factors, the most important of which is the physical properties of the battery itself. In addition, battery management technology and unreasonable charging and discharging modes are the main reasons for shortening battery life.

How to charge batteries efficiently, quickly and without damage has always been a concern in the battery industry and a very important part of battery use and maintenance. Therefore, research on advanced charging technology and charging devices is an important topic in the battery field, and it is a high-end topic involving power electronics, automatic measurement and automatic control technologies.

1 Smart charger hardware part

The overall structural design block diagram of the charging device hardware is shown in Figure 1, which consists of two parts: the main circuit and the control circuit. The part outside the dotted box is the main circuit, and the part inside the dotted box is the control circuit. The function of the main circuit is to convert the input three-phase AC power into the DC power required by the battery load; the control circuit is used to realize various functions of the power supply.

Figure 1 System block diagram

1) Charging main circuit

The main circuit adopts AC-DC-DC conversion circuit. The function of AC-DC part is to step down and isolate the three-phase AC power supply U, V, w through the rectifier transformer, and obtain the uncontrollable DC voltage U1 after rectification by the three-phase uncontrolled bridge composed of six diodes, as shown in Figure 2. That is to say, for the three-phase voltage of AC380V input, the size of U1 is only related to the transformation ratio of the rectifier transformer. Once the grid voltage and the transformation ratio of the transformer are determined, U1 will remain constant.

Figure 2 Three-phase buck rectifier circuit

[page] The DC-DC part adopts Buck circuit, as shown in Figure 3. When charging the battery, the fully controlled device VT2 is turned off, and the DC voltage U1 passes through the Buck circuit composed of the fully controlled switch device VT1, the freewheeling diode VD2 and the inductor L (which also serves as a filter), and controls the output voltage U2 by controlling the on-off of VT, thereby controlling the charging current and charging voltage of the battery.

Figure 3 Buck-Boost circuit

2) Main circuit protection circuit

(1) Overcurrent protection.

Diodes and thyristors have small thermal capacity, and overloads of more than 5 times are required to be cut off within 0.02 seconds, otherwise the components will be damaged, so fast fuses must be used.

(2) Overvoltage protection.

The ability of thyristors and rectifier diodes to withstand overvoltage is limited. Even if the voltage exceeds the reverse voltage breakdown voltage of the component by a small amount and for a short time (0.5~1us), the component may be reversely broken down and damaged. The overvoltage protection measure is to connect a RC absorption circuit, connect it in parallel on the AC side, DC side, or in parallel with the rectifier element to reduce the voltage change rate at both ends of the RC circuit, thereby playing an overvoltage protection role.

3) Control circuit.

The control circuit includes voltage feedback circuit and current feedback circuit, PI regulator and feedback selection circuit, D/A conversion circuit, CAN bus circuit, single chip microcomputer and its peripheral circuits. The block diagram of each module is shown in Figure 4.

Figure 4 Control circuit block diagram

2 Software Design

The overall block diagram of the software design is shown in Figure 5. The specific process is as follows: First, call the initialization function to initialize all devices. Then determine whether the shutdown is normal. If the last shutdown is a normal shutdown, call various mode selection functions to select the charging mode; if it is abnormal, read the EEPROM to read the status of the charger during the abnormal shutdown. Then enter the loop. Within 5 minutes, the given voltage of the charger in the automatic charging state is allowed to be set; every 10 minutes, the current status of the charger is recorded in the EEPROM. Select the charging method, use the corresponding subroutine to process the data, obtain the output result, and convert it into the required voltage signal through DA; call the timing function once a second to record and display the total charging time and the charging time of the stage at that time; determine whether the conditions for stage conversion are met and control the stage conversion light; determine whether the charging is completed according to the shutdown conditions.

Figure 5 Main function flow chart

In Figure 5, the constant voltage and constant current method includes three charging methods: ① constant current correction constant voltage charging in the initial stage, ② constant current correction constant voltage charging in the final stage, and ③ constant current correction constant voltage charging in the initial and final stages. In actual applications, it can be adjusted in the program according to needs and charging effects.

[page] 3 Experimental Analysis

This intelligent charger has two charging modes: manual mode and automatic mode. The following will verify them respectively.

The manual mode charging waveform is shown in Figure 6, where the first channel is the battery terminal voltage; the second channel is the drive signal of the primary side of the pulse transformer; the third channel is the given reference voltage of the analog PI regulator; and the fourth channel is the actual charging current signal measured by the current clamp.

It can be seen that in manual mode, the microcontroller can normally take out the signal from the potentiometer and convert it into a corresponding voltage signal and send it to the analog PI regulator for reference. The battery terminal voltage fluctuates with the charging current, so normal manual charging can be performed.

The charging waveform in manual mode is shown in Figure 7. The charger switches stages when the battery cell voltage is greater than or equal to 2.35V. After each switch, the charging current is reduced to half of the previous stage charging current.

The program is set to perform three stage conversions, with overcharging in the third stage.

Figure 6 Manual mode charging waveform

Figure 7 Automatic mode charging waveform

The first channel is the battery terminal voltage, the second channel is the signal of the current feedback circuit after the first stage amplification circuit, the third channel is the given reference voltage of the analog PI regulator, and the fourth channel is the actual charging current signal measured by the current clamp. It can be seen from the figure that the charger can normally complete the three-stage constant current charging.

4 Conclusion

The hardware design of the intelligent charging equipment control system with the AT90CAN32 microcontroller as the core. The software flow of the entire system was designed using a modular programming method, and the main program and subroutines of each module were written to achieve data acquisition, event management, charging control algorithm and output control. The intelligent charging device has the following features:

(1) It can complete multi-stage constant current charging. Compared with the previous two-stage constant current charging, it can effectively control the charging current near the battery gasification current.

(2) Not only can the shutdown be completed by judging the charging time, but the automatic shutdown can also be achieved by judging the terminal voltage change rate of the battery.

(3) A variety of feedback can be used to determine the battery status. Voltage feedback and regulation circuits are added to the original charger current feedback circuit.

(4) It can automatically determine the number of battery blocks and change the voltage feedback proportional coefficient.

(5) Record the charging status of the charger in real time to implement power-off protection of the charger.

(6) It has a CAN bus interface for easy expansion.