Auto Cutoff Lithium Ion Battery (3.7V) Charger Circuit Diagram

Here, we are going to see a 3.7V Lithium Ion (Li-Ion) Battery Charger circuit diagram. You can power up this circuit with your smartphone charger. Basically, it takes the 5V DC input and provides an output voltage of up to 4V to charge a 3.7V Li-Ion battery. The special feature of this circuit is it will automatically cut off the battery from the power supply when the battery is fully charged. Also, it provides an indication of whether the battery is charging or it is fully charged.

Circuit Diagram

Here, you can see the circuit diagram and connection between each component. Even here we have shown how the battery is to be connected for charging.

List of Components Used

• 2 Pcs 1 kilo ohm resistors
• 1 Pcs 10 kilo ohm resistor
• 1 Pcs 10uF, 50V electrolytic capacitor
• 1 Pcs 10K Preset
• 1 Pcs BC547 Transistor IC
• 2 Pcs 1N4007 PN Junction Diode
• 1 Pcs 5V DC Relay
• 1 Pcs Red LED
• 1 Pcs Green LED

Connection Procedure

1. Connect a 1N4007 PN Junction diode across the relay coil in reverse bias.
2. Connect another 1N4007 PN Junction diode to the NC terminal of the relay. The anode side should be connected to the relay and the anode side is to be left as the positive output terminal for connecting the battery.
3. Connect the positive terminal of the green LED to the NO terminal of the relay and connect the positive terminal of the red LED to the NC terminal of the relay. Now connect the negative terminals of both LEDs together and finally it is to be connected to the negative terminal of the circuit in series with a 1 Kilo Ohm resistor.
4. Connect the positive terminal of the relay coil to the positive terminal of the 5V DC input.
5. Connect the negative terminal of the relay coil to the collector terminal of the transistor BC547
6. Connect the emitter terminal of the transistor BC547 to the negative terminal of the circuit
7. Connect a 10K resistor to any one side terminal(1) of the preset. Another end of this 10K resistor is to be connected to the positive terminal of the 5V DC input.
8. Connect another side terminal(2) of the preset to the negative terminal of the circuit.
9. Connect the middle terminal of the preset to the base terminal of the transistor BC547 through a series resistor of 1 Kilo Ohm.
10. Connect the 10uF, 50V electrolytic capacitor across the base and emitter terminal of the transistor BC547. The positive terminal of the capacitor should be connected to the base terminal and the negative terminal of the capacitor should be connected to the emitter terminal that is already connected to the negative terminal of the circuit.
11. When you want to charge a battery you should connect the positive terminal of the battery to the positive output terminal of the circuit that comes from the NC terminal of the Relay through a PN junction diode. And connect the negative terminal of the battery to the negative output terminal of the circuit. Here, both the input and output negative terminal of the circuit is common.

Operation and Working Principle

Connect the battery to the output of the circuit to be charged. Connect a 5V DC power supply to the input of the circuit. As I told you can connect your 5V smartphone charger or any other 5V DC power source. Even you can apply up to 6V DC to the circuit. So you can use a 6V battery as a power source.

Now, when the battery needs to charge the Relay will transfer the power supply through its NC terminal, and the red LED will be in the ON condition which will indicate that the battery is charging. Once the battery is fully charged, the relay will change its NC contact into NO and NO contact into NC. So the battery will be disconnected from the power supply and the green LED will turn On which will indicate that the battery is fully charged.

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