Design of a fast charger for electric bicycle based on AT89C51

1. Introduction

Electric bicycles are favored by many users because of their pollution-free, noise-free, light and beautiful features. However, their limitations are also exposed in use, such as the battery running out halfway, and the battery life will gradually shorten as the use time increases. This paper aims to study and develop an electric bicycle fast charger (battery specification 36V, 12A) that can intelligently change the charging mode according to the degree of battery saturation and can charge the battery in a short time (four hours).

2. Pulse fast charging method

The pulse charging method is to charge the battery intermittently with pulse current. The charging pulse makes the battery fully charged, and the intermittent period allows the oxygen and hydrogen produced by the chemical reaction of the battery to have time to recombine and be absorbed, eliminating polarization and reducing the internal pressure of the battery, so that the constant current charging of the next pulse can proceed smoothly, so that the battery can absorb more electricity. Intermittent pulses give the battery a sufficient reaction time, reduce the amount of gas evolution, and improve the charging current acceptance rate of the battery.

3. System composition

The main circuit of the charger adopts a half-bridge conversion high-frequency switching voltage regulator, and the control circuit is implemented by a single-chip microcomputer. After the common mode signal is removed by the EMI filter, the AC power of the power grid is bridge rectified, and then connected to the two switch tubes V1 and V2 after voltage division by two capacitors, so as to transform the sinusoidal AC voltage into a pulse voltage approximately higher than the charging voltage. After passing through the half-bridge filter and LC filter circuit, the voltage reaches a relatively stable value.

The control circuit is composed of a single-chip microcomputer AT89C51. The power supply is provided by the AC power of the power grid after being transformed by a transformer, rectified by a full-bridge, and stabilized by a voltage regulator. The single-chip microcomputer controls the output pulse by software by detecting the voltage signal of the temperature sensor, thereby controlling the on and off of the switch tube. In addition, by detecting the charging voltage and current value, the output pulse width of the single-chip microcomputer is controlled to enter different charging stages.

(I) Half-bridge conversion high-frequency switching voltage regulator power supply

The half-bridge conversion circuit is shown in Figure 1(a), and the output voltage waveforms at each point are shown in Figure 1(b). The main advantages of the half-bridge conversion circuit are its anti-unbalance performance, and the power switch tube used has a low withstand voltage that will not exceed the peak value of the input voltage; the saturation voltage of the transistor is also reduced to the minimum; the withstand voltage of the input filter capacitor can also be reduced.

(II) Single chip microcomputer control

The single-chip circuit design uses the P1 port of the AT89C51 single-chip computer as the input and output port. The temperature signal detected by the temperature sensor is input through the P3.2 port of the single-chip computer, and the voltage signal is input through the P3.1 port. The output signal is provided by P1.1~P1.5 of the single-chip computer. The specific distribution is shown in the table below.

Overall circuit design #e#

(III) Overall circuit design

The electric bicycle fast charger circuit is mainly divided into three parts: main circuit, control circuit and detection circuit.

The main circuit consists of a bridge rectifier circuit and a half-bridge inverter circuit.

The control circuit is implemented by the single-chip microcomputer AT89C51, which takes corresponding actions according to the detected voltage signal value: outputs pulse voltages of different widths and gives different instructions.

The detection circuit includes temperature detection circuit and voltage detection circuit. Temperature is realized by temperature sensor, and voltage detection is realized by voltage divider resistor.

The main circuit diagram is shown in Figure 2.

IV. Conclusion

This paper introduces the circuit working principle and implementation method of an electric bicycle fast charger based on AT89C51 single-chip microcomputer control. The charger has the advantages of high automation, low operating cost and reliable operation. After testing, the prototype has high charging efficiency, fast charging speed, low loss and low cost.