Simple Constant Voltage, Constant Current Battery Charger Circuit

This article shows you how to build a smart battery charger for a 12V battery!

This charger uses a common chip called the LM317 and keeps two things steady: voltage and current.

Voltage: This makes sure the battery does not get overloaded.

Current: This keeps the battery charging at a safe speed.

What is a Constant Voltage, Constant Current Battery Charger:

A Constant Voltage, Constant Current CVCC battery charger is a type of battery charging circuit that maintains a stable voltage across the battery terminals during the charging process.

Additionally, it regulates the charging current to ensure that the battery receives a consistent current throughout the charging cycle.

This dual mode operation helps optimize the charging process and ensures that the battery is charged efficiently and safely.

Circuit Working:

Parts List:

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

The resulting rectified DC is smoothed using a 1000uF capacitor.

The filtered DC, now 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.

It can be used to set the exact full charging voltage for the 12V battery which is around 14V.

Operation:

The LM317 maintains a constant voltage across the battery terminals as set by the potentiometer.

The BC547 transistor helps maintain a constant charging current.

It adjusts the current flow through the LM317 based on the batteries need.

The LED connected in series with the BC547 transistor provides a visual indication of the charging state.

It remains illuminated during the charging process and turns off when the battery is fully charged.

The potentiometer allows fine tuning of the charging voltage to meet the specific requirements of the 12V battery.

This circuit provides a reliable and adjustable charging solution for a 12V battery ensuring both constant voltage and constant current during the charging process.

The LED indicator adds a visual cue for the charging status.

Circuit Construction:

Building the constant current constant voltage battery charger circuit using the LM317 IC with the specified connections involves the following steps.

Bridge Rectifier:

• Connect the AC terminals of the 12V 1A transformer to the bridge rectifier.
• Connect the DC terminals of the bridge rectifier to the 1000uF capacitor.
• Connect the positive and negative terminals of the capacitor to the positive and ground of the PCB.

LM317 Configuration:

• Connect the output of the bridge rectifier to the input pin of the LM317.
• Connect the ADJ pin of the LM317 to one end of the 240 ohm resistor.
• Connect the other end of the 240 ohm resistor to the OUT pin of the LM317.
• Connect the middle terminal of the potentiometer to the ADJ pin of the LM317.
• Connect one end of the potentiometer to the ground of the PCB.

Current Control using BC547 Transistor:

• Connect the collector of the BC547 to the ADJ pin of the LM317 through the LED.
• Connect the emitter of the BC547 to the ground.
• Connect the base of the BC547 through the current limiting resistor RC to the ground.

Charging Indicator LED:

• Connect the anode of the LED to the collector of the BC547.
• Connect the cathode of the LED to the ground.

Final Connections:

• Connect the positive output of the circuit to the positive terminal of the battery you want to charge.
• Connect the negative output of the circuit to the negative terminal of the battery.

Testing:

• Apply power to the circuit by connecting the transformer to the mains.
• Use a multimeter to verify that the output voltage from the LM317 is in the expected range.

Charging LED:

• Connect a discharged battery to the circuit and observe if the charging LED lights up indicating that the battery is charging.

Note:

• Ensure that the components are correctly placed on the PCB and there are no short circuits.
• Adjust the potentiometer carefully to avoid overcharging the battery.
• If you encounter any issues double check the connections and component values.
• Always exercise caution when working with electrical circuits and follow safety guidelines.

Conclusion:

To conclude, this CCCV battery charger circuit ensures efficient and controlled charging of 12V batteries, with the LED indicator signaling the charging status.

The adjustable voltage feature adds flexibility to accommodate different battery types and requirements.

References

Constant current charging

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