Simple Constant Voltage, Constant Current Battery Charger Circuit

This post discusses how to construct a clever 12V battery charger!

Using a common chip known as the LM317 this charger provides two constants: voltage and current.

Voltage reduces excessive load on the battery

And current maintains a safe rate of battery charge.

What is a Constant Voltage, Constant Current Battery Charger:

A battery charging circuit known as a Constant Voltage Constant Current (CVCC) charger keeps the voltage across the battery terminals constant while charging.

Also it controls the charging current to ensure that the battery gets a steady current during the charging cycle.

The battery is charged safely and effectively because of to this dual mode performance which also helps to improve the charging process.

Circuit Working:

Parts List:

The 12V AC from the transformer is converted to DC using a bridge rectifier made of 1N5402 diodes.

Further 1000uF capacitor is used to smooth the resulting rectified DC.

The filtered DC which is around 16V is applied to the LM317 IC.

The LM317 is configured in a constant voltage constant current mode.

The ADJ pin is connected to a 240 ohm resistor and the other end of the resistor is connected to the OUT terminal.

The BC547 transistor is used for current control to supply a constant current to the battery.

The collector of BC547 is connected to the ADJ pin of LM317 through a series LED which indicates the charging state.

A current limiting resistor RC is connected between the base and emitter of BC547.

This resistor is also in series with the ground supply of the LM317 circuit.

Formula for calculating RC value:-

Rc = 0.6 / 10% of Battery Ah

And For Li-ion the Rc formula will be:

Rc = 0.6 / 50% of Battery Ah

As long as the battery is consuming charging current the LED connected in series with the collector of BC547 remains illuminated indicating that the battery is charging.

When the battery is fully charged and stops consuming current the BC547 switches off turning off the LED indicating that the battery is fully charged.

The potentiometer is connected between the ADJ pin of LM317 and ground.

For the 12V battery it can be used to accurately change the full charging voltage which is around 14V.

Operation:

The potentiometer controls the voltage across the battery terminals which is maintained by the IC LM317.

A constant charging current stays constant with the help of the BC547 transistor.

According to the needs of the batteries it modifies the current flowing through the LM317.

The LED connected in series with the BC547 transistor provides a clear indication of the charging status.

It remains lit during charging and it switches off when the battery is fully charged.

It is possible to finely adjust the charging voltage to the requirements of the 12V batteries by using the potentiometer.

This circuit provides a stable and flexible charging solution for a 12V battery maintaining a constant voltage and current during the charging process.

The LED indication makes it clear that the device is charging.

How to Build:

To build a Simple Constant Voltage, Constant Current Battery Charger Circuit follow the below mentioned steps for connections.

Bridge Rectifier:

• The 12V 1A transformer must be connected the bridge rectifiers AC terminals .
• Join the 1000uF capacitor to the bridge rectifiers DC terminals.
• The capacitors positive and negative terminals should be connected to the PCBs positive and ground.

LM317 Configuration:

• Connect the bridge rectifiers output to the LM317s input pin.
• Wire the LM317s ADJ pin to one end of the 240 ohm resistor.
• Use the LM317s OUT pin to connect the opposite end of the 240 ohm resistor.
• The ADJ pin of the LM317 should be connected to the potentiometers middle terminal.
• Connect one end of the potentiometer to the PCBs ground.
• Controlling current with a BC547 transistor:
• Through the LED connect the BC547s collector to the LM317s ADJ pin.
• The BC547s emitter should be connected to ground.
• Use the current limiting resistor (RC) to connect the BC547s base to the ground.

Charging Indicator LED:

• The LEDs anode should be connected to the BC547s collector.
• Make sure the LEDs cathode is grounded.

Final Connections:

• The battery to charge should have its positive terminal connected to the circuits positive output.
• The circuits negative output should be connected to the batteries negative connector.

Testing:

• Once the transformer is connected to the mains power the circuit.
• Check if the LM317s output voltage falls within the required range using a multimeter.

Charging LED:

• If the charging LED illuminates when a discharged battery is connected to the circuit it means that the battery is charging.

Note:

• One should confirm the components have been placed correctly on the PCB and check for any short circuits.
• The potentiometer must be carefully adjusted to avoid battery overcharging
• Check the component values and connections twice before looking for any issues.
• Take care and follow all safety precautions while working with electrical circuits.

Conclusion:

In conclude the LED indication indicates the charging condition of the 12V batteries and the Simple Constant Voltage, Constant Current Battery Charger Circuit (CCCV) ensures effective and regulated charging.

To meet various battery kinds and needs the adjustable voltage feature gives versatility.

References:

Constant current charging

Using constant current and constant voltage charging to fully charge a battery