In this post, we’ll explore a budget-friendly solution for an automatic cutoff battery charger circuit, ideal for extending battery life and preventing overcharging. This circuit is designed to automatically disconnect the battery from the charger once it reaches a preset voltage, ensuring safe and efficient charging.
The design is straightforward, requiring only a relay switch, a transistor, a few resistors, capacitors, and LEDs. Whether you’re charging 3.7V lithium-ion batteries, 5V batteries, or 12V lead-acid batteries, this circuit is versatile enough to handle a variety of applications.
How the Automatic Cutoff Battery Charger Works
The Core Idea
An automatic cutoff charger monitors the battery's voltage during charging. When the voltage reaches the predefined threshold, the charger disconnects the battery. This feature protects the battery from overcharging, which can reduce its lifespan or even cause damage.
Circuit Overview
Below is the circuit diagram for an automatic cutoff battery charger that supports 3.7V, 5V, 9V, and 12V batteries.
Adjusting the Cutoff Voltage
Setting the cutoff voltage is simple:
Select Battery Voltage:
Decide on the type of battery you want to charge.- For a 3.7V lithium-ion battery, use a 5V relay and a 220Ω resistor for R3.
- For a 5V battery, use a 6V relay and a 470Ω resistor.
- For a 12V battery, use a 12V relay and a 1kΩ resistor.
Tune the Cutoff Voltage:
- Connect a DC supply (4V to 15V) to the circuit’s power input.
- Adjust the potentiometer (RV1) until LED D2 lights up, signaling that the cutoff voltage is set.
Step-by-Step Circuit Operation
Charging Phase:
- When the battery is below the cutoff voltage, the 2N2222 transistor is in the off state.
- The power supply charges the battery, with current flowing through resistor R2, the potentiometer RV1, and resistor R1, while capacitor C1 stabilizes the circuit.
- During this phase, LED D2 remains lit, indicating the charging process.
Cutoff Phase:
- Once the preset voltage is reached, the transistor turns on.
- Current flows through the relay switch, activating it and disconnecting the battery from the charger.
Reset:
- After cutoff, the circuit remains in the disconnected state until manually reset or the battery voltage drops below the threshold.
Key Components and Alternatives
- Transistor: The circuit uses a 2N2222 transistor, but you can substitute it with similar BJTs like the 2N3904 or BC547. For improved efficiency, consider using a MOSFET.
- Relay Switch: The relay voltage rating should match the battery voltage for optimal performance.
- Resistors and Capacitors: Ensure you use the correct values based on your battery’s voltage requirements.
Applications of the Auto Cutoff Battery Charger
This versatile circuit is ideal for applications such as:
- RC Cars and Drones: Protect the rechargeable batteries in your hobby projects.
- Robotics: Use it in wifi controlled NodeMCU car or autonomous robot car to ensure safe charging.
- DIY Electronics: Power your Arduino or ESP32-based projects without worrying about overcharging.
Conclusion
Building an automatic cutoff battery charger is a cost-effective way to enhance battery safety and longevity. With just a handful of components, you can create a reliable charging solution for various battery types, making it perfect for robotics, RC applications, and other DIY projects.
By following this guide, you can customize the circuit for different voltage levels and ensure efficient charging for your batteries. This simple yet effective design is a must-have for anyone working with rechargeable batteries.
