3.7V Lithium Ion Auto Cutoff Battery Charger Circuit Design and Working Principle

If you are building electronics circuits with batteries such as for remote controlled devices, drones, RC cars or robotics you might be using Lithium Ion battery. If you are one of them then the automatic cutoff battery charger for 3.7V lithium ion battery presented here might be helpful to you. Here a 3.7V Lithium Ion auto cutoff battery charger circuit design and working principle is explained. You will learn how the battery is disconnected from the charging circuit after the battery is full. This charging circuit has advantage since it does not use relay switch as used in the automatic cutoff battery charger circuit with relay.

Circuit Diagram of 3.7V Lithium Ion auto cutoff battery charger

 In the circuit diagram, besides resistors, LEDs, headers and power socket, you can see that the automatic battery charger circuit uses TL431 and IRFZ44N. Let explain what these components are and their role in the battery charger circuit.

The TL431 is a versatile precision shunt regulator widely used in various applications, including as a key component in automatic battery charger circuits. Its ability to maintain a stable voltage reference makes it ideal for controlling charging processes. In a battery charger circuit, the TL431 serves as a voltage reference for the feedback loop. By comparing the battery voltage to a reference voltage set by the TL431, the circuit can adjust the charging current accordingly. This ensures that the battery receives the appropriate charging current, preventing overcharging or undercharging, which could otherwise damage the battery. The TL431's precise voltage regulation capability, coupled with its ease of use, reliability, and cost-effectiveness, makes it a popular choice in designing efficient and safe automatic battery charger circuits.

The TL431 is can be salvaged from old PC power supply unit. In power supply unit, the TL431 acts as a precision shunt regulator, comparing a reference voltage to the feedback voltage and adjusting the output accordingly by controlling a series pass transistor. 

The IRFZ44N is a powerful N-channel MOSFET transistor known for its high-speed switching capabilities and low on-state resistance. In an automatic battery charger circuit, the IRFZ44N can be employed as a key component within the charging circuitry. Its low resistance minimizes power loss during the charging process, allowing for efficient energy transfer from the power source to the battery being charged. By controlling the flow of current between the power supply and the battery, the IRFZ44N can be utilized in conjunction with appropriate control circuitry, such as a microcontroller or a voltage regulator, to regulate the charging process. Its ability to handle high currents and voltage levels makes it suitable for designing robust and efficient automatic battery charger circuits, ensuring safe and reliable charging while protecting the battery from overcharging or damage. 

The IRFZ44N can be salvaged from old PC power supply unit. You can also attach the heat sink that will also be available on the IRFZ44N on the PC power supply unit. 

You can also use other MOSFET like IRF540N. 

Application

This lithium ion automatic cutoff battery charger can be used in robots, RC cars, drones etc. For example see battery charging used in wifi controlled RC car and bluetooth Arduino RC car.

Video

The following video shows simulation of 3.7V Automatic Cutoff Battery Charger Circuit. It shows how 3.7V Automatic Cutoff Battery Charger Circuit works and how the battery is disconnected from the charging circuit after the battery is full. This circuit has advantage because it does not use relay switch.