How to Build a LiPo Battery Charger Circuit

Lithium Polymer (LiPo) batteries are becoming more common in todays electronic gadgets because they are light have a lot of energy packed into them and can be recharged.

However, charging these batteries needs to be done carefully and accurately.

It is important to use a special circuit to avoid overcharging which can harm the battery or even cause fires in serious situations.

Building a LiPo Battery Charger Circuit is simple that uses an IC 741 operational amplifier along with some basic parts.

This design is made specifically for charging a 3.7V, 360mAh LiPo cell.

Circuit Working:

Parts List:

In the circuit diagram provided above you can see how the different parts work together to make a LiPo battery charger.

The circuit takes in a 12V DC input and uses a IC2 7809 voltage regulator to lower it to 9V.

This step is really important because it ensures that the operational amplifier 741 IC1 and the other parts of the circuit get a steady voltage supply.

The IC1 is set up to act as a comparator.

Resistors R1 and R2 create a voltage divider that sets a reference voltage at the inverting input which is pin 3 of the op-amp.

The non inverting input at pin 3 gets a feedback voltage from the battery through resistor R5.

The output from the operational amplifier is at pin 6 and it controls the base of the transistor Q1 which is using resistor R3.

If the battery voltage drops below a certain level the op-amps output turns on Q1 allowing current to flow into the battery to charge it.

When the battery voltage reaches the right level the feedback voltage at pin 3 matches the reference voltage causing the op-amp to change its output.

This turns off Q1 and stops the charging.

The LED1 shows whether the battery is charging and diode D1 is placed to stop any reverse current from going back into the circuit keeping everything safe and working properly.

Formulas with Calculations:

The formulas with calculations are mentioned below for building a LiPo Battery Charger Circuit:

Reference Voltage (Vref):

Vref = (R2 / (R1 + R2)) * VCC

where,

VCC = 9V, R1 = 4.7k, R2 = 7.2k:
Vref = (7.2 / (4.7 + 7.2)) * 9
Vref = (7.2 / 11.9) * 9
Vref = 5.04V

Battery Voltage Feedback (Vfeedback):

Vfeedback = (R5 / (R4 + R5)) * Vbattery

where,

Vbattery = 3.7V, R4 = 100Ω, R5 = 10k:
Vfeedback = (10000 / (100 + 10000)) * 3.7
Vfeedback = (10000 / 10100) * 3.7
Vfeedback = 3.66V

Charging Current (Icharge):

Icharge = VBE(Q1) / R3
Assume VBE(Q1) = 0.7V, R3 = 100Ω:
Icharge = 0.7 / 100
Icharge = 7mA

How to Build:

To build a a LiPo Battery Charger Circuit following steps should be followed for connections:

• Assemble all the components parts as mentioned in the above circuit diagram:
• Connect the input pin of IC2 to 12V DC supply of the circuit.
• Connect the GND pin of IC2 to GND.
• Connect the output pin of IC2 to a resistor R2 and other end of resistor R2 to pin 3 of IC1.
• Connect resistor R2 and resistor R5 in series from output of IC2 to GND.
• Connect pin 2 of IC1 to one end of resistor R5 and other end of resistor R5 connect to GND.
• Connect pin 3 of IC1 between resistor R2 and R1.
• Connect pin 4 of IC1 to GND.
• Connect pin 6 of IC1 to anode of LED1 and connect the cathode of LED1 to one end of resistor R6 and other end of R6 to GND.
• Connect resistor R3 from pin 6 and after LED1 to base of transistor Q1.
• Connect emitter of transistor Q1 to positive of 3.7V Li-po battery.
• Connect collector of transistor Q1 to output pin of IC2 through resistor R4.
• Connect anode of diode D1 from emitter of transistor Q1 and cathode of D1 between pin 2 of IC1 and resistor R5.
• Connect the GND of the circuit to negative supply of 3.7V Li-po battery

Conclusion:

This LiPo battery charger circuit is easy to make, affordable and works well for charging 3.7V LiPo cells safely.

It uses an IC 741 op-amp to keep the voltage just right and the 7809 regulator makes sure the power supply is steady.

You can also add extra safety features like overcurrent protection or an automatic shutoff.

By looking at the circuit diagram and doing the math both hobbyists and engineers can create a dependable charger for LiPo batteries.

References:

Circuit for Lipo 7.2 Volt