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    This Automatic Battery charger turns On only when the battery demands charging current. It can be used to charge 12 Volt Lead Acid or Tubular batteries .The automatic switching helps to keep the battery always in top condition. The overcharge and over discharge cut off facilities are included so that the charger can be left unattended for long periods.

    The switching action through the relay is achieved by by sensing the terminal voltage of the battery under charge. Relay contacts break the AC supply to the charger transformer when the battery voltage rises above 14 volts. When the terminal voltage drops below 11.5 volts, relay contacts complete the Neutral path of AC supply to the charger transformer.

    Operational Amplifier IC1 is used as a precision voltage comparator to monitor the voltage level of the battery. Its Inverting input gets a reference voltage of 1.8 V from the junction of Red LED and R3 while the Non inverting input gets slightly higher voltage of 2 volts ( as set by VR1). This makes output of IC1 high. PNP transistor then remains off to keep the relay de-energized. Since the Neutral line is connected through the Comm and NO contacts of the relay, AC path remains cut off and no charging takes place.

    When the battery voltage drops below 11.5 volts, voltage at the Non inverting input (pin3) drops below that of the Inverting input (pin2) and the output of IC1 turns low. T1 then conducts to actuate the relay. Relay contacts completes the neutral path and charging process starts. Green LED indicates the charging process.

    Charger On Demand Circuit

    Settings

    1. Discharge the battery using a 12 volt Car bulb till the battery voltage drops to 11.5 volts.
    2. Measure the terminal voltage using a multimeter. It should be around 11.5 volts. Connect the battery to the outputs of the circuit.
    3. Adjust VR2 till the relay clicks and Green LED turns On.
    4. Charging processes starts and measure the terminal voltage at an interval of 15 minutes.
    5. When the battery voltage increases to 14 volts, adjust VR1 till relay turns off. At this point, Green LED also turns off.
    6. Careful setting of VR1 and VR2 gives 3 volts difference between the On and Off states.
    7. The Charger is now ready to use.

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    7 Responses to "On Demand Charger"

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    1. Pentaconto says: on October 21, 2010 at 1:51 pm

      You need a diode in antiparallel with the green LED. In this circuit, the green LED is not protected against reverse voltage of AC supply, which can reach 300 volts peak, damaging the LED. It is best to put a small neon lamp. Please check and correct the circuit if necessary.

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      • if i put relay 6volt to 75R instead of 6VOLT TO 300R in this circuits howcome?
        OCHU FROM TANZANIA I NEED HELP!

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    2. DEAR D.Mohankumar,

      Hi! THIS CIRCUIT IS VERY GOOD, BUT CAN YOU PLEASE GUIDE ME IF I WANT TO USE THIS CHARGER TO CHARGE THE BATTERY FROM SOLAR PANEL ONLY, AND NOT BY AC??? KEEPING ALL THE FEATURES OF OVERCHARGEAND OVER DISCHARGE CUT OFF FACILITYS IN THIS CIRCUIT. BY CONNECTING SOLAR TO THE CIRCUIT REMOVING THE BRIDGE, WILL IT CHARGE THE BATTERY IN ONE DAY, AND WHAT WATTAGE OF PANEL I WILL HAVE TO USE?

      REGARDS.

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    3. Ahmad Hassan says: on July 14, 2012 at 7:13 am

      Very Very Good Circuit

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    4. mark krawczuk says: on August 18, 2012 at 8:21 am

      hi, can i replace the bd140 transistor with a differant amp rating so i can charge at say, 6 amps or even 10 amps ?

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    5. Naveen Naveen says: on June 25, 2013 at 2:21 pm
        member

      This circuit is not working properly, pls dont try this

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    6. This is a super circuit for the constructor that like to do his own thing and not just paint by numbers sort of thing. The only current limit is controlled by the relay contacts and the transformer / rectifier limits. I would recommend that a high wattage lamp connected in series old headlamp globe that has one filament blown comes to mind and at motor car service places can be obtained for free.100 watt lamps can carry 8 amps and will then only burn bright.

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