Common rechargeable batteries include nickel-metal hydride batteries, nickel-cadmium batteries, lithium batteries, and polymer batteries. Among them, lithium batteries are widely used for their high energy density, stable discharge characteristics, no memory effect and long service life. At present, most mobile phones and digital cameras use lithium batteries. Battery life and single cycle time are closely related to the charger maintenance process and usage. A good charger not only can fully charge the battery in a short period of time, but also can play a certain role in maintaining the battery, repairing the memory effect caused by improper use, that is, the battery activity decline phenomenon.
Single chip circuit
The MCU chip is AT89C52 MCU of Atmel Company, B1 is the buzzer, and the P2.0 port output of the MCU controls the optocoupler device, which can turn off the charging power supply when needed.
Figure 2 52 single chip circuit schematic
Charging circuit control module
The charge status output pin /CHG is connected to the P3.2 port of the microcontroller via the inverter 74LS04 to trigger an external interrupt. The PNP is a P-channel field effect transistor or a triode. D1 is a green LED that lights up when it is energized; D2 is a red LED that illuminates when the power is turned on. R1 sets the resistance of the charging current, the resistance is 2.8 kΩ, and the maximum charging current is set to 500 mA; C2 is the capacitance for setting the charging time, the capacitance is 100 μF, and the maximum charging time is set to 3 hours.
Figure 3 charging circuit control section
Lithium battery smart charger function
Need to complete pre-charge, fast charge, full charge, power off and alarm and other functions. These functions mainly rely on the built-in charging state control of the intelligent charging management chip MAX1898 and the control of the peripheral microcontroller AT89C52.
Precharge: Turn on the input DC power after installing the battery. When the charger detects the battery, reset the timer to enter the precharge process. During this time, the charger charges the battery with 10% of the fast charge flow. Restore the battery voltage and temperature to a normal state. The precharge time is controlled by an external capacitor. Here, 100nF is selected, that is, the precharge time is 45 minutes. If the battery voltage reaches 2.5V or above within the specified charging time, and the lithium battery temperature is normal, the fast charging process is entered; if the lithium battery voltage is still lower than 2.5V during the pre-charging time, the lithium battery is considered to be unchargeable, / The LED connected to the CHG pin flashes to indicate a charging failure.
Fast charge: When the precharge is over, the lithium battery voltage rises above 2.5V, and the MAX1898 quickly charges the lithium battery with a constant circuit. Fast charge is also called cross current charging. 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 fast charge process, the lithium battery voltage reaches the set termination voltage, the fast charge ends, the charge current decreases rapidly, and the charge enters the full charge process. At this time, when the charging rate falls below the set value or the full charge time expires, the top end of the charging process is entered. At the top turn-off charge, the MAX1898 replenishes the battery with minimal charge current. Under normal circumstances, full charge and top cut-off charging process can extend the use time of lithium batteries by 5%-10%.
Power-off: When the battery is fully charged, the pulse level sent by the second pin/CHG of the MAX1898 will be low to high, which will be detected by the MCU, causing the interrupt of the MCU, and the status of the charging is judged in the interrupt. At this point, the MCU will control 6N137 through the P2.0 port, cut off the power supply of the LM7805 to the MAX1898, thus ensuring the safety of the chip and the lithium battery, and also reducing the loss.
Alarm: When the battery is fully charged, the MAX1898 chip itself will extinguish the external LED green light. However, for the sake of safety, after detecting the pulse of full state, the MCU 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 the charging error occurs, the MAX1898 itself will control the LED green light to flash at a frequency of about 1.5Hz. Do not turn off the power supply of the MAX1898 at this time, let the user see this prompt.
Electronic enthusiast network technology editors comment analysis:
The design of the charger for the AT89C52 single-chip microcomputer and the charging integrated circuit used in this design can not only realize the charging of the lithium battery, but also realize the corresponding overvoltage and temperature protection, so that the performance of the lithium battery can be fully utilized and avoided. The charger may cause damage to the battery during charging, and has certain intelligent functions. The program effectively protects the battery, shortens the charging time and maximizes the service life of the lithium battery, in line with the current environmental protection trend.
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