Design of multi-mode lithium 18650 li ion batterycharging circuit based on microcontroller

　　[Power Network Technology Sharing] This article introduces an intelligent charging software control method for a universal lithium 18650 li ion batterycharger. This charging method is used to convert different charging modes according to the charging characteristics of the battery. The AT89C51 microcontroller chip is used as the control chip for the charging process for circuit design to implement A/D conversion and display circuits. The correct charging mode is selected based on the lithium 18650 li ion batteryvoltage information detected by the external circuit to achieve trickle current, constant current, constant voltage and Intelligent switching between float charging modes.

　　introduction

　　A good charger plays a decisive role in the service life of the battery. Smart chargers have the advantages of simple operation, high reliability and strong versatility. They are an important part of the charger family and are also the basis for future charger development. The main direction. The so-called smart charger refers to an intelligent charger that can independently select the charging method according to the user's needs, charge different types of rechargeable batteries, and protect the charged lithium 18650 li ion batteryduring the charging process to prevent over-voltage and excessive temperature. charger.

　　The charge controller requires long-term control and voltage detection. If it is implemented with traditional circuits, the circuit will be complicated. Using single-chip microcomputer control can greatly simplify the circuit and reduce costs. This charger uses AT89C51 microcontroller for charging timing control. During scheduled charging, if the lithium 18650 li ion batteryvoltage is higher than another value, charging will be stopped. The method from trickle charging, constant current charging, constant voltage charging to float charging is adopted. After charging is completed, it automatically switches to float charging to prevent lithium 18650 li ion batterydischarge and has a display circuit. Suitable for charging nickel-cadmium and nickel-metal hydride batteries. The charger is controlled by a microcontroller and has better charging effect.

　　Smart charger hardware design

　　The schematic block diagram of a universal charger composed of a microcontroller and a charger chip is shown in Figure 1. In the figure, AT89C51 and ADC0809 together form the core of the charger. ADC0809 detects the voltage at both ends of the rechargeable lithium 18650 li ion batteryat any time. When the lithium 18650 li ion batteryis placed, ADC0809 transmits the detected voltage information at both ends of the lithium 18650 li ion batteryto AT89C51 through the data port. After data processing, AT89C51 selects the appropriate charging mode based on the received voltage information. . Since the chip can only receive voltage information from 0V to 5V, two series resistors (equal and large resistance values) are connected in parallel at both ends of the rechargeable battery. The detected voltage is half of the lithium 18650 li ion batteryvoltage, which can be multiplied by two. Find the lithium 18650 li ion batteryvoltage. If the lithium 18650 li ion batteryvoltage is lower than 4.7V, stop control is enabled and the charger does not work. If the lithium 18650 li ion batteryvoltage is greater than 7.3V or less than 5.2V, it means that the lithium 18650 li ion batteryhas been over-discharged. In order to avoid excessive charging current of the battery, causing thermal runaway, the lithium 18650 li ion batteryshould be charged with a stable small current trickle to activate the battery. At this time, the P1.2 port of the microcontroller sends a high-level signal to start the trickle charging mode. Similarly, when the voltage is between 5.2V and 6V, the P1.0 port is set to high level, and pulse control is called at the same time to control the closing of the switch to charge the lithium 18650 li ion batteryin constant current charging mode. When the voltage is between 6V and 7.3V, the P1.1 port is set to high level to start the constant voltage charging mode. The entire charging process takes 3 hours. When the circuit works for three hours, the system automatically powers down and terminates the charging process.

　　This design uses 8155 as the I/O expansion interface of AT89C51 to connect four digital tube display circuits. The LED display uses a common cathode, the segment selection code is provided by the 8155PB port, and the bit selection code is provided by the PA port. When the trickle charging mode is enabled (P1.2 is set high), the first digital tube displays; when the constant current charging mode is enabled When the constant voltage charging mode is enabled (P1.0 is set high), the bit selection word moves to the right, and the second-digit digital tube displays. When the constant voltage charging mode is enabled (P1.1 is set high), the bit selection word moves to the right, and the third-digit digital tube displays. When charging stops, P1.0, P1.1, and P1.2 ports are all set to low level, and the fourth digit digital tube displays. If the program is slightly modified, the display circuit can also display the current charging voltage value, thereby charging the lithium 18650 li ion batterymore effectively.

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