Auto Cut-Off 3.7 Volt Lithium-Ion Battery Charger Circuit

In today’s project, we will build an Auto Cut-Off Lithium-Ion Battery Charger circuit. As you guys know in one of my previous projects, I made a circuit that can charge a lead acid battery, when the indicator LED turns green it indicates the battery is fully charged and the red LED indicates the battery is in charging condition.

Now if you are charging a lithium-ion battery and the circuit has no auto-cut option then it can become very dangerous. Because the circuit only indicates that your battery is fully charged. Now, we have to remove the battery ourselves after full charge because that circuit doesn’t have an auto-cut option. And It will continue to charge the battery even after the green LED is lit up, this can become dangerous.

Also If you forget to turn that off after some time the battery can explode. So that’s why I am building an upgraded version of the previous circuit and it comes with an auto-cut option. In today’s project, the battery charger circuit provides the automatic cut-off system when the battery is fully charged.


Auto Cut-Off 3.7 Volt Lithium-Ion Battery Charger

Circuit Diagram

3.7 Volt Lithium-Ion Battery Charger Circuit Diagram

Components Required

  • BC547 NPN Transistor
  • 10KΩ Resistor
  • 1KΩ Resistor (x2)
  • 10KΩ Preset
  • 10uF/16V Capacitor
  • Red & Green LED
  • 5V Relay
  • Connection Wire
  • Veroboard
  • +5V Power Supply

PCB Design

After designing the schematic diagram of the Digital RPM Meter, the assembled components and wiring are too clumsy and looked very unprofessional. In fact, the wiring also has a chance of loose connection. To give it a clean and professional look and also to compress the size, I decided to build its PCB prototype using EasyEDA software as it is so simple to use. Now come to the main part, where we need to order our PCB prototype. I always prefer PCBWay for their quality assurance, fastest delivery and also for 24/7 customer support.

PCB View

3.7 Volt Lithium-Ion Battery Charger 2D View
2D View
3.7 Volt Lithium-Ion Battery Charger 3D View
3D View

Why I Choose PCBWay as My First Preference?

I’ve done several runs with PCBWay and am happy with the results, it’s good quality. It is one of the most experienced PCB manufacturing companies based in China with an experience of more than a decade in the field of PCB prototype and fabrication. PCBWay has always been committed to technological innovation and meeting the needs of their customers from different industries in terms of quality, delivery, cost-effectiveness and any other demanding requests. The etching, solder mask, and hole sizes are all done well and that is what matters to me. It takes a few hours for a design to get approved and then a couple of days to finish and ship.

With more than a decade in the field of PCB prototypes and fabrication, PCBWay has proved its assurance from time to time. They always look at the customer’s needs from different regions in terms of quality, on-time delivery, cost-effectiveness and any other demanding requests.

PCBWay Gold Finish
PCBWay Thickness

These PCBs were manufactured by PCBWay and the finish quality really impressive, especially with the gold finish path. If you want to finish your product faster you could also ask PCBWay to make a panelized order where you receive multiple PCBs in a single panel. With this, you will also receive SMT Stencil from PCBWay, saving you time and effort. All the orders are high quality and could select a lot of settings such as thickness, flexible PCB, the colour of the solder masks, the number of layers, material, surface finish and more.

PCBWay different types of solder masks

How to Adjust the Cut-Off Point by Varying the Preset?

First, we disconnect any battery supply or any type of input supply and then rotate the preset knob to the ground level. Then we take a variable DC power supply which has its output precisely adjusted to 4.2V which is the optimum full charging level of the standard 3.7V Lithium-Ion battery.

Connect this power supply to the output side of the circuit, across the points where the battery is supposed to be connected.

You will see the green LED illuminating. At this stage, the relay must switch ON, however, it won’t since there’s no 5V supply from the input side of the circuit. No worries, we can still set up the circuit by looking at the LEDs.

After this, slowly adjust the preset until the green LED just switches off and the red LED just switches ON.

That’s all, the auto cut-off set-up is complete for the circuit.

Working Principle of Auto Cut-Off 3.7 Volt Lithium-Ion Battery Charger

In the above-mentioned circuit, the charging process is driven by a BC547 NPN transistor and a relay. While charging the circuit, it takes input from a 5-volt adopter as shown above. After the battery charge reaches a certain point the transistor BC547 activates and it’s driving a relay that turns on and changes the COM pin’s attachment from NC to NO, which is connected to the green LED as it lits up after the relay change over indicates that the battery is fully charged.

That’s how the automatic cut-off lithium-ion battery charger circuit works depending on the NPN transistor and 5-volt relay.

3 thoughts on “Auto Cut-Off 3.7 Volt Lithium-Ion Battery Charger Circuit”

  1. Then you need to change the relay state from LOW to HIGH in the coding section and vice versa.

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