Simple Fast Charger Circuit with Auto Cut Off

This circuit is a speedy charger for batteries like the ones in cars and backup power systems.

It can juice up your battery extra fast, but do not worry about overcharging.

The circuit has a built in safety feature that automatically stops charging once the battery is full, keeping it healthy and lasting longer.

So you can get back on the road or keep your devices powered up quickly and safely.

Circuit Working:

Parts List:

This circuit designed for fast charging sealed lead acid batteries used in automobiles, inverters, etc. this high current charger can deliver 5 Amp for rapid battery rejuvenation.

It incorporates an adjustable voltage regulator, allowing for the charging of both 6V and 12V batteries.

The charging process automatically stops once the battery reaches full capacity.

The LM338 is a versatile high current variable voltage regulator capable of providing 2 to 25V DC with a high current output.

Noteworthy features include a 7 Amp peak output current, adjustable output down to 1.2V and thermal regulation.

It is user friendly and requires only two external resistors to set the output voltage.

Its time dependent current limiting ability permits a peak current draw of up to 12 amperes for short durations making it suitable for heavy transient loads and fast startups.

Due to its high current consumption the LM338 requires a heat sink for heat dissipation.

Two resistors are needed to set the output voltage with resistor R1 120 ohms acting as the program resistor.

A reference voltage of 1.25V Vref is typically generated between the output and adjust pins.

This reference voltage is applied across the program resistor R1 ensuring a constant current flow through the output set resistor VR1.

Therefore, the output voltage can be calculated using the formula which is mentioned below.

The input voltage is derived from a 0-15V 5 ampere secondary transformer with a 10 ampere KBPC rectifier module.

Capacitor C1 eliminates DC ripples allowing around 16V DC to enter the vin input of the regulator.

VR is used to adjust the output voltage from 2V to 15V.

A digital panel meter is employed to measure the output voltage which can be easily connected directly to the output.

By adjusting VR the output voltage can be monitored on the meter.

Any reduction in output voltage due to line voltage drop can be easily detected using the panel meter which can also measure the terminal voltage of the battery.

When the power supply is turned off the meter displays the terminal voltage.

A steady reading indicates that the battery is fully charged and holding the charge.

The Auto Cut Off circuit features a 5 Ampere darlington NPN transistor BC547.

When the batteries terminal voltage exceeds 12.5V, zener diode conducts preventing the LM338 regulator from operating.

If the battery voltage drops below 12V the zener diode turns off allowing the regulator to resume charging.

To set the output voltage a fully charged 12V battery should have a terminal voltage of 13.5V, and a 6V battery should have 6.5V.

Set the output voltage for charging to 14V for a 12V battery and 8V for a 6V battery.

Use heavy gauge red and black wires for the output and connect using crocodile clips to the battery terminals with the correct polarity.

Before connecting the charger to the mains attach the clips to the battery terminals and check the terminal voltage.

If it is below 13.5 to 6.5V, turn on the charger and charge for up to one hour.

Wait for 10 minutes and recheck the terminal voltage.

If the voltage remains stable the battery is ready for use.

Formulas and Calculations:

The following are the main formulas and factors to take into account while creating a rapid charging circuit with an automatic cut off utilizing an LM338 voltage regulator and related parts:

Voltage Regulation using LM338:

Using external resistors, the LM338 voltage regulator may be tuned to produce a certain voltage.

The output voltage, V_out can be computed as follows:

V_out​ = Vr_ef ​× (1+R2 / R1​)+ I_adj ​× R2

where,

• V_ref is the LM338s reference voltage, which is normally 1.25V.
• R1 and R2 are coupled with external resistors to modify the output voltage.
• I_adj is the adjustment current, which is normally extremely tiny about 50µA.

Current control:

An external transistor BC547 is frequently used for current control, particularly if the charging current exceeds the LM338 alones current handling capacity.

You may calculate the current flowing through the load (battery) by:

I_charge​ = V_out​ − V_{battery​​} / R_sense​

where,

• V_battery is the voltage of the battery.
• R_sense​ a current sensing resistor connected in series with the charging channel.

Calculations:

From our circuit diagram above to create a charger with a 5A charging current for a 12V battery:

Transformer Selection:

To account for voltage dips during rectification and regulation, assume that the transformer secondary voltage V_sec is required for a 15V AC output.

Peak AC voltage V_peak = V_sec × √2

Rectification and Filtering:

Following rectified, the highest DC voltage Vdc = V_peak−V_diodedrop

Notes:

Make sure the parts you choose can withstand the charging circumstances by looking at their voltage and current ratings.

To avoid overheating, factors like heat dissipation and cooling for parts like the LM338 and transistor should be taken into account.

It is important to include safety precautions (such as fuses and current limiters) to safeguard the battery and the charger circuit.

You should be able to construct and calculate the parameters for a rapid charger circuit with auto cut off utilizing an LM338 and other related components with the use of these formulae and considerations.

Depending on the particular battery type and charging needs, adjustments can be required.

How to Build:

To build a Simple Fast Charger Circuit with Auto Cut Off follow the below mentioned connections steps:

Transformer and Rectifier Module:

• Connect the secondary side of the transformer to the KBPC rectifier module to get 0 to 15V DC.

Voltage Regulator LM338K:

• Connect the input Vin of LM338 to the rectified DC voltage.
• Connect the output Vout to the battery.

Adjustable Voltage Divider:

• Use two resistors R1 and VR1 to set the output voltage of the LM338K.
• Refer to the LM338 datasheet for the specific values.

Auto Cut-Off Circuit:

• Connect the Zener diode and Darlington NPN transistor BC547 as per the auto cut off circuit diagram.

Digital Panel Meter:

• Connect the digital panel meter to measure the output voltage.

Steps:

• Assemble the components on a suitable PCB or breadboard.
• Follow the circuit diagram to connect the components as described above.
• Ensure correct polarity and connections.
• Provide power to the circuit using the transformer and rectifier module.
• Use the adjustable voltage divider to set the desired output voltage for charging.
• Connect the battery to be charged using the crocodile clips.
• Monitor the charging process using the digital panel meter.
• The auto cut off circuit should automatically stop the charging process when the battery is fully charged.

Safety Precautions:

• Use appropriate insulation for all connections.
• Use a suitable heat sink for the LM338 regulator.
• Double check all connections before applying power.
• Do not leave the circuit unattended during operation.
• Use caution when working with high voltage and current components.

Note:

• This circuit is for educational purposes.
• Ensure compliance with local regulations when building and using high current charging circuits.
• The values of components such as resistors, capacitors and transformer ratings may need to be adjusted based on specific requirements and component availability.

Conclusion:

A Fast Charger Circuit with Auto Cut Off is a versatile and efficient solution for quickly charging batteries while ensuring safety and preventing overcharging.

By incorporating features such as the LM338 voltage regulator, auto cut off circuit, and adjustable voltage divider this circuit can effectively charge a variety of batteries used in automotive and other applications.

References

Datasheet IC LM338