Smart battery fast charger circuit design using a single smart circuit

　　Circuit Principle: The battery automatic charger shown in the figure uses a single transistor as the simplest window comparator. When the battery voltage is lower than the preset value, the battery starts to be charged, and when the battery voltage exceeds the preset value, the power is automatically cut off. Because the circuit has a precise variable power supply voltage, the upper and lower limits of the battery voltage can be accurately set. A 15V DC voltage source is used to power the circuit, but the voltage source is separated from the NC pin of the relay to prevent the voltage from passing through the battery pin. First, the variable power supply is fixed at 13.3V and connected to both ends of the battery to be charged in the circuit. The slider of VR1 is pushed to the end in the direction of the pin connected to the positive pole of the battery. The slider of VR2 is pushed to the end in the direction of the pin connected to VR1. Bias VR1 is generated, and the transistor is turned on. Then the slider of VR1 is pushed to the other end in the direction connected to the VR2 pin. Now set the voltage source used for the test to 11.8V. Adjust VR2 to cut off the transistor again. When the test voltage rises to 13.3V dc again, adjust VR1 to turn on the transistor. After setting the upper and lower limit voltages, connect the NC pin to the circuit. At this point, the battery charger can work normally.