Automatic Lead Acid Battery Charger Circuit

This circuit shows you how to build a smart charger for car batteries lead acid batteries!

Unlike a regular charger this one automatically stops charging the battery when it is full so you do not have to worry about overcharging it.

This is handy for cars backup power supplies UPS, and other things that use car batteries.

Circuit Working:

Parts List:

This automatic lead acid battery charger circuit provides a cost effective alternative to pricey battery chargers.

The key to reducing expenses is to procure an inexpensive battery charger and enhance it with an automatic charge breaker circuit, such as the one demonstrated here.

At the core of this automatic lead acid battery charger lies a comparator.

It assesses the voltage of the lead acid battery against a reference voltage.

When the lead acid batteries voltage surpasses a predetermined maximum level the circuit interrupts the charging process via the relay.

Conversely, when the lead acid batteries voltage drops below the preset minimum level the circuit closes the relay, allowing charging to resume.

As the battery voltage increases so does the voltage at the non inverting input.

Once it reaches a certain threshold adjusted with P1, the non inverting input surpasses the level of the inverting input.

To prevent the circuit from oscillating constantly toggling the relay on or off due to minor fluctuations in battery voltage the comparator is designed with hysteresis which is set using P2.

This also determines the minimum battery voltage level at which charging restarts.

Calibrating the Automatic Battery Charger Circuit:

The optimal method for calibration involves using a variable voltage regulated power supply in place of the lead acid battery.

Set the regulated power supply to 14.5V and adjust P1 until the relay switches open.

Next, set the regulated power supply to 12.4V and adjust P2 until the relay closes again.

It may be necessary to repeat this process several times as adjustments to P1 and P2 affect each other.

Formula:

Using the IC 741 operational amplifier to design an automated lead acid battery charger entails building a circuit that continuously measures the battery voltage and regulates the charging current accordingly.

The following are some essential equations and factors to take into account while creating an automatic lead acid battery charger that uses an IC 741 operational amplifier:

Voltage Regulation and Control:

Voltage Reference (Zener Diode Setup):

To get a steady reference voltage V_ref, use a zener diode.
Usually, a voltage across the zener diode V_Z is given (e.g. 6.2V or 9.1V)

V_ref​ = V_Z​

Feedback Voltage Divider:

The operational amplifier IC 741 in this example will compare the reference voltage Vr_ef established by the feedback voltage divider with the battery voltage V_bat

Lets assume the following:

V_cc: The operating amplifiers supply voltage IC 741.
V_ref: The battery chargers preferred reference voltage.

The resistor values for the feedback voltage divider R1 and R2 are determined as follows:

Voltage Divider Equation:

The output voltage V_out of the voltage divider is given below:

V_out​ = V_cc​ [R2​​ / (R1​+R2)]​

In our circuit diagram R1 = P1+R8 and R2 = R9, therefore substituting these figures in the above equations we get:

V_out​ = V_cc​ [R9​​ /(P1​+R8+R9)]​

With the IC 741 operational amplifier, this formula offers a simple way to figure out the resistor values for the feedback voltage divider in your circuit for an automated lead acid battery charger.

Based on your unique voltage needs as well as the features of your power supply and operational amplifier, adjust the resistor values.

How to Build:

Here is the steps, how to build the Automatic Lead Acid Battery Charger Circuit:

Prepare the Comparator Circuit:

• Connect the comparator IC according to its datasheet.
• Typically, it has two inputs inverting and non inverting and one output.
• Set up the voltage divider using resistors to provide a reference voltage to one of the comparator inputs.
• Connect the other input of the comparator to the lead acid battery being charged.

Add Hysteresis:

• Introduce hysteresis by adding feedback resistors between the output and one of the inputs of the comparator.
• Adjust the hysteresis using preset P2 to prevent oscillations in the circuit.

Integrate Relay Circuit:

• Connect the output of the comparator to the base of a transistor usually NPN.
• Connect the collector of the transistor to the coil of the relay and the emitter to the ground.
• Ensure the relay is connected in such a way that it breaks the charging circuit when activated.

Power Supply and Protection:

• Integrate a voltage regulator to ensure stable voltage supply to the circuit.
• Add protection diodes to prevent reverse current flow and protect sensitive components.

Calibration:

• Follow the calibration procedure mentioned earlier:
• Use a variable voltage, regulated power supply in place of the lead acid battery.
• Adjust preset P1 until the relay switches open at the desired maximum voltage.
• Adjust preset P2 until the relay closes at the desired minimum voltage.

Testing:

• Connect the charger to a power source and test the circuit with a lead acid battery.
• Ensure the relay behaves as expected breaking the charging circuit when the battery reaches the maximum voltage and resuming charging when it drops below the minimum voltage.

Finalization:

• Once satisfied with the circuits performance finalize the connections on a PCB or breadboard and secure components in place.

Note:

Remember to follow proper safety precautions while working with electronic circuits and ensure correct polarity and component orientation to avoid damage.

Conclusion:

The automatic lead acid battery charger circuit continuously monitors the batteries voltage and adjusts the charging current and voltage accordingly.

It typically follows a multi stage charging profile, including bulk charging absorption charging and float charging to ensure optimal charging efficiency and battery health.

Overall, an automatic lead acid battery charger circuit provides a convenient and efficient solution for charging lead acid batteries while ensuring their longevity and performance.

References:

Simple_Switchmode_Lead-Acid_Battery_Charger

Lead–acid battery