Low-cost multi-series lithium-ion battery charging circuit

The working principle of the circuit is explained below:

The circuit shown in the figure is essentially a power supply circuit with constant current/constant voltage output function. It has two feedback circuits, current feedback and voltage feedback, where the positive and negative poles of the current feedback correspond to the 1st and 2nd pins of KA7500B. The output current generates a voltage drop on resistors R12 and R20, which is fed back through resistors R9, R10, R14, and R15. When the voltage of the 1st pin of KA7500B is greater than the voltage of the 2nd pin, KA7500B will reduce the output pulse width (pins 8 and 11) to reduce the current, otherwise increase the pulse width to keep the output current constant at the preset value, and its constant current value conforms to the following formula:

In the formula, R is the resistance value of R12 and R20 in parallel, so the theoretical calculated value of the constant current is 735mA.

The positive and negative poles of the voltage feedback in the circuit correspond to the 16th and 15th pins of KA7500B. After power-on, the 14th pin of KA7500B outputs a stable 5V voltage, which makes the LED light up as a power indication. At the same time, the 5V voltage is used as a reference voltage and provided to the 15th pin of KA7500B as a voltage reference. After the output voltage is divided by R19, R10, VR1 and R17, it is compared with the voltage reference. When the voltage is too large, the pulse width is reduced, and when it is too small, the pulse width is increased to maintain a constant output voltage value. The output voltage value conforms to the following formula:

Since the two feedbacks of KA7500B are controlled after being "ANDed" inside, when the output voltage is lower than the constant voltage value, the current feedback plays a control role. When the output voltage reaches 8V4, the voltage feedback plays a control role. In this way, the circuit completes the constant current/constant voltage control function. Its principle is exactly the same as the working principle of the voltage-stabilized power supply, except that this circuit is a switching power supply control method, so it has high efficiency and low temperature rise.

In the figure, D1 is a diode to prevent polarity reversal input, D2 is a freewheeling diode for the switching power supply, and T1 works in the switching state.

After understanding its working principle, it is much easier to expand its application, which is described as follows:

1. In normal use, adjust the VR1 potentiometer to get the corresponding output voltage.

2. Charging lithium-ion batteries of different capacities : The resistance values ​​of R12 and R20 can be changed, and the required current value can be calculated and set according to the formula described above. If the current is too large, a heat sink should be added to T1, and the freewheeling diode should be replaced with 1N5822 to withstand a larger current.

3. Charging high voltage lithium battery: Just change R10 or R17. The specific required output voltage value can be set after calculation according to the formula.

4. As a high-precision voltage-stabilized power supply: The output voltage principle is the same as above, but attention should be paid to the ripple. If possible, the inductance of the inductor and the filter capacitor in the circuit should be increased to reduce the ripple.