Lithium battery fast charging circuit based on single chip microcomputer

　　Common rechargeable batteries include nickel-metal hydride batteries, nickel-cadmium batteries, lithium batteries and polymer batteries. Among them, lithium batteries are widely used due to their high energy density, stable discharge characteristics, no memory effect and long service life. At present, most mobile phones, digital cameras, etc. use lithium batteries. The service life of the battery and the single cycle time are closely related to the maintenance process and use of the charger. A good charger can not only fully charge the battery in a short time, but also play a certain maintenance role on the battery, repairing the memory effect caused by improper use, that is, the phenomenon of battery activity decline.

　　Single chip microcomputer circuit

　　The single-chip microcomputer chip is the AT89C52 single-chip microcomputer of Atmel Company, B1 is a buzzer, and the P2.0 port of the single-chip microcomputer output controls the optocoupler device, which can shut down the charging power supply in time when needed.

　　Figure 2 52 MCU circuit schematic

　　Charging circuit control module

　　The charging status output pin /CHG is connected to the P3.2 port of the microcontroller through the inverter 74LS04 to trigger an external interrupt. PNP is a P-channel field effect transistor or triode. D1 is a green light-emitting diode, which lights up when it is powered on; D2 is a red light-emitting diode, which lights up when the power is turned on. R1 sets the resistor for charging current, with a resistance of 2.8 kilo-ohms and a maximum charging current of 500mA; C2 is a capacitor for setting charging time, with a capacitance of 100μF and a maximum charging time of 3 hours.

　　Figure 3 Charging circuit control part

　　Functions of Lithium Battery Intelligent Charger

　　It needs to complete functions such as pre-charge, fast charge, full charge, power off and alarm. These functions are mainly realized by the built-in charging state control of the intelligent charging management chip MAX1898 and the peripheral single-chip microcomputer AT89C52.

　　Pre-charge: After installing the battery, connect the input DC power supply. When the charger detects the battery, it resets the timer and enters the pre-charge process. During this period, the charger charges the battery at 10% of the fast charge current to restore the battery voltage and temperature to normal. The pre-charge time is controlled by an external capacitor. Here, 100nF is selected, that is, the pre-charge time is 45 minutes. If the battery voltage reaches above 2.5V within the specified charging time and the lithium battery temperature is normal, the fast charge process will begin; if the lithium battery voltage is still lower than 2.5V during the pre-charge time, it is considered that the lithium battery is not chargeable, and the LED connected to the /CHG pin flashes, indicating a charging failure.

　　Fast charge: When the pre-charge is completed, the lithium battery voltage rises to above 2.5V, and the MAX1898 uses a constant circuit to fast charge the lithium battery. Fast charge is also called cross-current charge. During the fast charge process, the lithium battery voltage gradually rises until the lithium battery voltage reaches the set termination voltage.

　　Full charge: When the lithium battery voltage reaches the set termination voltage during fast charge, fast charge ends, the charging current decreases rapidly, and the charging enters the full charge process. At this time, when the charging rate drops below the set value or the full charge time exceeds the time limit, the top cut-off charging process begins. During the top cut-off charging, the MAX1898 replenishes energy for the battery with a very small charging current. In general, the full charge and top cut-off charging processes can extend the use time of the lithium battery by 5%-10%.

　　Power off: When the battery is fully charged, the pulse level sent by the 2nd pin /CHG of MAX1898 will change from low to high, which will be detected by the microcontroller, causing the interrupt of the microcontroller, and judging the state of charging completion in the interrupt. At this time, the microcontroller will control 6N137 through the P2.0 port to cut off the power supply from LM7805 to MAX1898, thereby ensuring the safety of the chip and lithium battery, and also reducing the loss.

　　Alarm: When the battery is fully charged, the MAX1898 chip will turn off the external green LED. However, for safety reasons, after the microcontroller detects the pulse of the full state, it will not only automatically cut off the power supply of the MAX1898, but also alarm through the buzzer to remind the user to remove the battery in time. When charging errors occur, the MAX1898 itself will control the green LED to flash at a frequency of about 1.5Hz. At this time, do not cut off the power supply of the MAX1898, and let the user see this prompt.

　　The AT89C52 microcontroller and charging integrated circuit used in this design are used to design the charger, which can not only charge the lithium battery, but also realize the corresponding overvoltage and temperature protection, so as to give full play to the performance of the lithium battery and avoid the situation that the charger may cause damage to the battery during charging. It has certain intelligent functions. This solution effectively protects the battery, shortens the charging time and extends the service life of the lithium battery as much as possible, which is in line with the current trend of environmental protection.