Fast Charger Circuit

Here is a High current charger for Fast charging sealed lead acid batteries used in Automobiles, Inverters etc. This charger can give 5 Ampere current for quick rejuvenation of the battery. It uses an Adjustable voltage regulator so that 6 volt and 12 volt batteries can be charged.

LM338K is a high current variable voltage regulator that can provide 2 to 25 volts DC with high current output. It’s important features are 7 Ampere peak output current, adjustable output down to 1.2 volts, thermal regulation etc. It is easy to use and require only 2 external resistors to set the output voltage. The time dependent current limiting ability of the device allows peak current of up to 12 ampere to be drawn for short time. So that it is useful in heavy transient loads and speed start up.

LM 338 Variable Regulator

Design considerations

Use of external resistors

LM338 K is available in metal can package TO3. K suffix metal can. It has only two pins – Vin and Adjust. The body of the device acts as the output pin. On the bottom side, the first pin is adjust and the second pin is Vin. LM338 requires heat sink to dissipate heat since it is draining high current. Two resistors are required to set the output voltage. Resistor R1 (220 Ohms) act as the programme resistor. Usually 1.25 volts reference voltage (Vref) develops between the output and adjusts pins. The reference voltage is impressed across the programme resistor R1. The voltage across R1 is constant so that constant current flows through the output set resistor VR1.Therefore output voltage is

Vout = Vref (1 + VR1 / R1) + I Adj VR1

Use of capacitors

Two capacitors are usually soldered close to the input and output terminals. C2 is the input bypass capacitor. Output capacitor C3 gives stability for the output current.

Circuit operation

Input voltage is obtained from a >15-0-15 volt 5 Ampere secondary transformer with rectifying diodes D1 and D2. Capacitor C1 makes the DC ripples free and around 16 volt DC enters into the Vin input of the regulator. VR1 can be used to set the output voltage from 2 volts to 25 volts. A digital panel meter is used for measuring the output voltage. Ready made LED or LCD panel meters are available at reasonable cost. This can be directly connected to the output. So by adjusting VR1, it is easy to see the output voltage in the meter. If the output put voltage reduces due to line voltage drop, it can be easily detected through the panel meter. The panel meter also can be used to measure the terminal voltage of the battery. If the power supply is switched off, meter shows the terminal voltage. If the voltage remains steady, it indicates that the battery is fully charged and holding charge. Easily available IN 5402 diodes are rated at 3 Amps so if the device become too hot, use a 10 A KBPC 1000 Bridge rectifier.

Fast Charger Circuit diagram

Setting the output voltage

A fully charged 12 volt battery should have a terminal voltage of 13.5 volts and that of a 6 volt battery 6.5 volts. To charge the battery, set the output voltage for charging as 14 volts for 12 volt battery and 8 volts for 6 volt battery.

LM338 on Heat sink

Use heavy gauge Red and Black wires in the output and use crocodile clips for connecting the battery. Before the charger is connected to mains, connect the clips to the battery terminals with correct polarity and see the terminal voltage. If it is less than 13.5 / 6.5, switch on the charger and charge up to one hour. Wait for 10 minutes and again check the terminal voltage. If the voltage remains steady, the battery can be used.

LCD and LED volt Meter


Join the conversation!

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  • sanjeevi

    how to change the mah value of the charger

  • patrick vaz

    Dear Sir,

    Can you add a volt and ammeter in the circuit, instead on the LCD,

    thanks in advance

  • siddu


    i want to design a battery charger for Li ion and NiCd/NiMH cells,which has output voltage of 4.5V to 8V and current rating of 700mAH to 10000mAH. how i hav to approach. i am new to battery design field.

  • joynal

    sir, i need circuit low volt cut off.
    can you give me this circuit…..


    Am a student(DEPARTMENT OF ELECTRICAL/ELECTRONIC ENGINEERING) at Kumasi Polytechnic,Ghana.I want to build battery charger with car alternater, separately from car with D.C MOTOR to drive the alternater,so help with an idea, circuit.THANK U

  • Colin Mitchell

    This is a very poorly design circuit.
    It is far too complex.

    The diodes are only 3 amp and will be destroyed at 5 amp.

    The author says “If the terminal voltage rises to 13.5 volts it indicates full charged condition. Do not keep the charger on for a long time.”

    The regulator transistor is pointless.
    You just need a bridge and wire-wound resistor so a battery takes 3 amps when it is flat and gradually reduces to about 1 amp when it is fully charged.

    All the rest of the circuit does NOTHING.

  • ani

    sir m also very much interested in this project..
    kindly tell me if i can do tha same project using FPGA ? if possible then let ma know as soon so possible..

  • kaluya

    sir am so much interested about the fast charger circuit but i request you also to include the auto part of it please you can send it on my mail.

  • Ralph

    Sir, what is VR1 stands for?

    • sanjeevi

      VR means variable register. you. can. adjust the resistance to control the out put voltage

  • D.Mohankumar

    Check the input and output voltage and current of LM338 after removing the LED,D3 and Panel meter. Adjust VR to get the desired voltage.Give a large heat sink to reduce power loss.

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