lithium 3400mah 3.7v 18650 battery

　　Solution for intelligent control of lithium 3400mah 3.7v 18650 battery charging process

　　In recent years, with the expansion of cities and towns, traffic has become more and more congested, and exhaust gas emitted by motor vehicle fuel has become one of the main causes of air pollution. Electric bicycles have become an ideal means of transportation because of their lightness, safety, labor saving and environmental protection.

　　The lead-acid battery currently used in electric bicycles is a chemical power source widely used in the world. This product has the advantages of good reversibility, stable voltage characteristics, long service life, wide application range, abundant raw materials and low cost. And has been widely used. However, due to the unreasonable charging methods of ordinary batteries, their lifespan is greatly shortened. Nowadays, many ordinary batteries have a service life of only 3 to 5 years, which is far lower than the design specification of 10 to 15 years. Research has found that the battery charging process has a greater impact on battery life. In other words, the vast majority of batteries are not worn out, but charged. It can be seen that a charger with good performance plays a decisive role in the service life of the battery, and also reduces the economic investment of electric bicycle users.

　　Since the single-chip microcomputer has good control functions and is cheap, applying the single-chip microcomputer as a control component in the design of lead-acid batteries can realize intelligent control of the battery charging process to achieve the best charging effect.

　　1Circuit structure of charger

　　The circuit structure of the charger is shown in Figure 1. The four parts of the mains access circuit, high-frequency conversion circuit, 3842 voltage stabilizing circuit and secondary rectifier and filter circuit constitute the high-frequency switching charging power supply. The single-chip microcomputer control circuit, sampling circuit and charging output circuit constitute the output control circuit.

　　1.1 High frequency switching charging power supply

　　The mains access circuit converts 220V AC power into 300V DC power through the RS508 single-phase bridge integrated rectifier circuit. The high-frequency conversion circuit is mainly composed of a high-frequency transformer T and a MOSFET tube. This article uses a single-ended flyback circuit. A very important feature of the single-ended flyback converter is that the transformer acts as an inductor, that is, the transformer stores energy when the primary switch (MOSFET tube) is turned on, and the transformer releases energy to the secondary side when the switch is turned off. The high-frequency transformer T has 3 secondary windings and 1 feedback winding. The output of secondary winding 1 provides 15V power for the photocoupler (TLp250), the output of secondary winding 2 provides 5V power for the microcontroller, the output of secondary winding 3 forms the main charging circuit, and the output of the feedback winding provides power for 3842. The 3842 regulates the output of the high-frequency conversion circuit by adjusting the pulse width of the output pWM waveform and thereby adjusting the turn-on and turn-off time of the MOSFET tube. The main charging circuit outputs very pure DC after being filtered by LC-π type.

　　1.2pIC16C73B microcontroller control strategy

　　pIC16C73B is a control component in the charging circuit. It is mainly used to realize charging state conversion, temperature compensation control, charging stop control and output drive waveform. It is an 8-bit high-performance EpROM-based microcontroller produced by Microchip. It offers significant improvements in execution speed and code compression compared to other similarly priced microcontrollers. Since the required OpT (one-time programming) products can be purchased at any time, the cycle of product design and development using pIC16C73 is shortened. The superior performance of the pIC16C73 microcontroller is mainly due to its reduced instruction set (RISC) and the Harvard architecture. It can use two-level pipeline instructions to fetch and execute, except for the jump instruction which requires two cycle, all other instructions can be executed in a single cycle.

　　1.2.1 Control of state transition

　　When the battery is charging, the battery voltage sampling circuit immediately performs a cyclic scan of the battery's terminal voltage, and sends the sampled terminal voltage signal to an AD port of pIC16C73B for conversion. The microcontroller decides which method to use based on the current status of the battery. Charge the battery. The control process block diagram is shown in Figure 2.

　　1.2.2 Control of temperature compensation

　　During the charging process, the recombination reaction in the battery generates a large amount of heat. Since the sealed structure of the battery prevents heat from dissipating easily, the temperature of the battery rises and the electrolyte dries up, causing the battery's thermal runaway. And without temperature compensation, the battery may be undercharged or overcharged, shortening the battery life. The battery temperature sampling circuit collects the current battery temperature, uses the pIC16C73B microcontroller to perform A/D conversion, and performs temperature compensation. After activating the temperature compensation function, the charging voltage can be corrected according to the following equation:

　　Vtc=Vn-TcN(T-20)

　　In the formula, Vtc is the voltage after temperature compensation, Vn is the uncompensated film; Tc is the temperature compensation coefficient, mV/℃, the compensation coefficient is generally -2~4mV/℃; N is the value of each group of batteries; T is The temperature collected by the temperature sensor.

　　1.2.3 Charging stop control

　　When the battery temperature is controlled during normal charging, the temperature change of the battery is not obvious. However, when the battery is overcharged, its internal gas pressure will increase rapidly. The oxidation reaction on the negative plate will cause internal heat and the temperature will rise rapidly (it can be as high as one minute). several degrees Celsius). Therefore, the pIC16C73B microcontroller can determine whether the battery is full based on the temperature change value of the battery, thereby issuing a charge stop control signal.

　　1.2.4 Output of driving waveform

　　The pIC16C73B microcontroller is used to output a rectangular wave with a frequency of 120Hz and a duty cycle of 0.8. This rectangular wave directly drives the MOS tube (RpF540) after being amplified by the optical coupling to charge the battery.

　　2Charger performance test and testing

　　The charging test resulted in a charging waveform as shown in Figure 4.

　　The charging pulse can fully restore the waste materials in the battery, allowing the battery to be fully charged quickly. The intermittent period allows the battery to have sufficient reaction time, reduces the amount of gas evolution, and improves the battery's charging current acceptance rate. Multiple test results show that the battery can generally be fully charged in 5 to 6 hours.

　　3 Conclusion

　　The high-frequency switching charging power supply is the hardware foundation of the charger circuit, and the pIC16C73B microcontroller is the control component, which can accurately monitor the battery status, charge control, stop charging control, and temperature compensation control. According to the charging waveform, the current slowly decreases during the charging process, which is very similar to the optimal charging curve proposed by American scientist Maas, which improves the battery charging acceptance rate and effectively extends the battery life.

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