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automatic battery charger circuit schematic

Automatic Battery Charger

Here is a 12 volt lead acid automatic battery charger that shut off the charging process once the battery attains full charge. This prevents overcharging of the battery so that, the charger can be left unattended.

If the terminal voltage of the battery reduces below the set level, say 13.5 volts, the circuit automatically turns on to the charge mode.

Charging current as well as the power to the circuit is obtained from a 0-18 volt 2 Ampere step-down transformer. The low voltage AC is rectified by the bridge rectifier comprising D1 through D4 and made ripple free by the smoothing capacitor C1. For charging purpose, 18 volt DC is used while to power the circuit, 9 volt regulated DC from IC1 is used.

Schematic of the Automatic Battery Charger Circuit

automatic battery charger circuit schematic

IC2 (CA3140) is used as a simple voltage comparator to drive the relay. Its inverting input gets 4.7 volt reference voltage from the Zener ZD, while the non inverting input gets an adjustable voltage through the POT VR1.So normally, the inverting input pin 2 gets higher voltage from the Zener (as adjusted by VR1) and output of IC2 remains low. T1 then remains off keeping the relay off. The charging current passes to the battery through the NC (Normally Connected) contacts of the relay.

When the terminal voltage of the battery increases to 13.5 volts, pin 3 of IC2 gets higher voltage than pin2 and the output of IC2 becomes high. This activates the relay and the contacts break. Charging current to the battery cut off and the relay remains as such since the battery voltage(13.5V or more) keeps the voltage at pin3 of IC2 is higher than that of pin 2.

Charger Settings

Before connecting the battery, set the input voltage to IC2 using a fully charged battery or variable power supply. Turn the switch S1 to the off position and switch on the power. Then connect a fully charged battery/ variable power supply to test points TP observing polarity. Measure the input voltage to pin 3 of IC2.

Slowly adjust VR1 till the input voltage to pin 3 of IC2 raises to 5 volts. At this point, relay should energize and Red LED turns on. Then connect the battery for charging and switch on S1. If the battery takes charge, current to pin 3 of IC2 will be low since most of the current drain occurs into the battery. This keeps the relay off. When the battery voltage increases above 13.5 volts, no more current passes into the battery, so that the voltage at pin3 of IC2 rises and relay turns on.

221 Comments

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  • m.ilyas khan

    sir i am new in electronics .. i have a smester project of smps using ka3525 ic . sir plz help me that how can i generater pwm and 100khz using ka3525 ic …thanks sir .
    waiting for ur kind help …..

  • Bikram tiwari

    Hlw sir.i’m studying engineering.i want to make my robot.give me any idea which make me comfortable in my project.thank u.

  • Ataul

    Sir,is it work well without any hysteresis resistor ?

    • Mihai

      NO, it doesn’t work properly. Don’t make this project. It has multiple flaws. Find another one.

  • 8.4V Li-Ion Battery DC-DC Charger Circuit

    Hi,
    I have power source of 24V and 100ma input but my DC-DC charger should consume only 50mA as input and it should produce 9V and 150-500mA to my 8.4V Li-Ion Battery.
    At any point of time,charger shouldn’t consume more than 50mA from input.
    Please tell me the possibility and schematic of above the requirement.
    I strongly appreciate your help
    Thank you in advance.
    please reply to contact@silicontechlabs.com
    Regards,
    Praveen

  • ilyas khan

    sir please can you tell me how?

    • Mihai

      It’s mandatory to use a step down transformer since it provides galvanic isolation from the mains.

      Google for SMPS battery charger. I’m sure you’ll find what you are looking for.

      Good luck!

  • Colin Mitchell

    “sir i have final year project on switched mode car battery charger, this charger can be made with or without transformer.
    sir i need the advantages and disadvantages of this charger with transformer and without transformer, sir please help me to make the concept clear about the advantages and disadvantages of SM battery charger with and without transformer.
    thanx”

    The circuit will be extremely dangerous without a transformer.

  • ilyas khan

    sir i have final year project on switched mode car battery charger, this charger can be made with or without transformer.
    sir i need the advantages and disadvantages of this charger with transformer and without transformer, sir please help me to make the concept clear about the advantages and disadvantages of SM battery charger with and without transformer.
    thanx

  • raja

    I need a simple circuit for converting 6volt 4.5amps dc supply as 5v 850mamps dc supply for charging my mobile

    • Mihai

      All you need is a LDO(Low Drop Output) regulator like LM2940, BA05T(Rohm),
      LD1117, LT1573, etc.

      LM2940 is a well known type readily available.

  • donwine

    I sell to airplane manufactures like Boeing and Airbus. So, no I am not selling on this forum. Info on a 4.2v smart charger is a simple request – I thought!

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