In this article we study a simple 3.7V li-ion battery charger circuit with auto-cut off, current control features.

How to Charge using USB Port or 5V Mobile Charger

Once the circuit is assembled and set up, the below shown design can be used for charging any spare Li-Ion Battery through the 5V Mobile Charger or USB port.

First connect the battery across the indicated points, and then plug in the USB connector with your Mobile Charger or computer’s USB socket. The green LED should instant become ON indicating the battery is being charged.

You can attach a voltmeter across the battery to monitor its charging, and check whether the circuit cuts off the supply correctly or not at the specified limit.

Also Read This:- Battery Level Indicator

Li-ion Battery Charger Circuit Diagram

Li-ion Battery Charger, 3.7 Volt battery charger

Parts List:-

  1. Transistor Q1 BC547, Q2 BD140
  2. LED D1-D2 Red & Green
  3. Resistor R1-R5 1k 1/4watt 5pc , R6 220 ohm 1/4watt
  4. IC LM 358
  5. Zener Diode D4 3V 1/4watt
  6. Capacitor 1uf/10v or 16v
  7. Diode D3 IN5408
  8. Connector 2pc
  9. PCB 

Click Here To:- Buy This Kit

How to Set up the above Li-ion Battery Charger Circuit

  1. First, make sure the preset is moved at the ground side fully. Meaning, the pin#2 should be at ground level through the preset initially.
  2. Next, without any battery connected, apply an Variable Power supply across the +/- supply lines of the circuit, 
  3. You will see the Green Led coming on instantly
  4. Now check the voltage at output and set output voltage at 4.2V With Variable Power Supply
  5. Now, slowly rotate the preset, until the green LED just shuts OFF, and the RED LED switches ON.
  6. That’s all! The circuit is now all set to cut off at 4.2 V when the actual Li-Ion cell reaches this level.
  7. For the final testing, connect a discharged battery to the shown position, plug-in the input power through Mobile Charger , and have fun watching the cell getting charged and cut-off at the stipulated 4.2 V threshold.

PCB Layout:

Li-Ion Battery Charger Auto Cut-off
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