Car battery charger

Description.
Given below, is a very simple circuit that can be used for charging car batteries. In this circuit there is facility for monitoring the charging current and voltage.

The circuit is based on the IC MC78T12ABT from Freescale.The IC is nothing but a 7812 in TO-3 package with 3A capacity. The transformer T1 steps down the mains voltage and diodes D1&D2 does the job of rectification. Capacitor C1 does the filtering and C2 acts as a decoupling capacitor. The ground terminal of IC1 is lifted to 2.1V using the diodes D3 , D4 and D5 . So the output from the IC1 will be a regulated 14.1V (12+2.1).The battery is charged via diode D6.The D6 blocks reverse flow of current from battery to charging circuit when the mains power is not available. Meter M1 shows the charging current and M2 shows the charging voltage.

Circuit diagram with Parts list.

car-battery-charger-circuit

Notes.

  • The transformer T1 can be a 230V primary; 15-0-15V, 3A secondary step down transformer.
  • The meter M1 can be a 3A FSD  ammeter.
  • Meter M2 can be a 20V FSD  volt meter.
  • Fuse F1 can be a 1A fuse.
Show Comments

49 thoughts on “Car battery charger

  1. sooraj ps

    Sir,
    Can we use 12-0-12 transformer insted of 15-0-15 transformer.because my transformer is 12-0-12.

    Reply
    • seetharaman

      If you have only a 12 volt transformer use a rectifier and 2.5 ohms 10 watts resistance in series with the battery to be charged as a current limiting resistance, no filter capacitor is also required.

      Reply
  2. sonickay

    Hello, I find this circuit and its explanation very interesting and explanatory in its entirety. However, you didn’t say the function of the LED1 in the circuit. also I will like to see a video of building this circuit if available because I dont undastand the windings/coils in T1. what do they symboolise? A video will be helpful please. My email is sonickaybid@yahoo.com

    Reply
  3. SJK

    sir, i have a car battery of 12v,(not sure of how many amps it supply),and a 14v 4amps transformer ,can i make use of this circuit to charge the battery? also how will i come to know when the battery is fully charged? please do reply….thanx

    Reply
    • Seetharaman

      You can use. Once the battery terminal reaches around 13.6 Volt, the charger will only start trickling, hence no problem.

      Reply
  4. Seetharaman

    Hi Rene you can use
    Hi Sara Paul that depends on the battery condition. A flat battery may consume 3Amps. A fully charged battery around 30 to 50mA.

    Reply
  5. robinho

    hi sir ..i am student and i want to design a battery charger as my project,i don’t have much knowledge about battery charger,pls i need help and suggestions from you regarding this project

    Reply
  6. Rene

    What if i don’t have a 230V AC but 127 V AC (because this is the most common voltage for electrical circuits in my country) can I get this circuit to work with a 127V to 15-0-15 V 3A transformer?

    Reply
  7. eby

    Hello sir, I need a charger for 12 V 65 Amps car battery, can I use this circuit by upgrading 15-0-15v 5Amps and by changing diodes to 6A4 kindly advice

    Reply
  8. Wasiu

    Pls which diod can we use to represent dat BY127 if is not available and can we use voltmeter for both M1 and M2

    Reply
  9. seetharaman

    Hi Fredz it can charge only one partially discharged 12 volt 40AH battery and not for even a flat battery.

    Reply
  10. Seetharaman

    Hi Vikram yes partially discharged ones but not a flat battery. for a flat battery of 35AH you require 10amps.

    Reply
  11. Seetharaman

    Hi Vikram you can charge partially discharged 40AH battery over night. For 100AH battery you require atleast 20amps peak current capability charger. this charger cannot handle your requirement.

    Reply
  12. Vikram

    Hi Seetharaman,

    Can i charge my 12V 100Ah battery on this ckt?

    And what is charging time for 12v 100ah battery?

    Thanx in advance

    Reply
  13. seetharaman

    Hi Hari 4nos BY127 as bridge is ok but 12 volt may not be sufficient, you require 0-15 volt transformer. you can use 12 volt transformer with 4 BY127s in bridge and connect the output of the bridge rectifer through 2 ohm 20 watts wire wound resistance in series with positive lead to battery to be chargerd is just sufficient, no need for any IC etc.

    Reply
  14. hari

    Hai Seetharaman,
    I have a 0-12v 3Amp step down transformer.I think,In this case ckt will change with Arranging BY127 as bridge order… is’t ok…?

    Reply
  15. Nidesh k g

    sir,in this circuit, if the capactors c2,c3 are ceramic or polyster? and what is the voltage rating of c2,c3. sir its very urgent Pls reply thanks.

    Reply
  16. Andrew.sk

    Hello peter we appreciate all the schooling and guidance but how do we get in touch with u for practical and theory lessons and can somebody study on line and get a certificate or diploma maybe even degree in the electrical and radio telecommunication? If yes please give us more details about this.

    Reply
  17. Seetharaman

    Hi Abhi the DC voltage will become low and as the battery voltage builds up your charging current will become too low and the battery may not get fullt topped up.
    If you have only 12-0-12 2A on hand then use 2nos 1N5404 and a 2.2 ohms 10watts resistance and charge no other electronics is required. This will be the simplest charger watch for the terminal voltage once it reaches 14 volts terminate the charge, as the peak DC voltage can reach 12 X rt2(1.414). (appoximately 17 volts).

    Reply
  18. patrick vaz

    Hi Seetharaman, what ur saying is I could have a slide switch, if slide switch is moved toward ther 12 volt indication, than with the 2 rectifiers and 1to 2 ohms as current limiting resistance, would be able to charge the 12 volts battery.
    when slide switch is moved towards 6 volts then after full wave rectification followed by a thyristor and zener, transistor i would be able to charge the 6 volts battery, pls if u would add a circuit for the above would really appreciate, thanks for the help

    Reply
  19. patrick vaz

    Hi Seetharaman, what ur saying is I could have a slide switch, if slide switch is moved toward ther 12 volt indication, than with the 2 rectifiers and 1to 2 ohms as current limiting resistance, would be able to charge the 12 volts battery.
    when slide switch is moved towards 6 volts then after full wave rectification followed by a thyristor and zener, transistor i would be able to charge the 6 volts battery, pls if u would add a circuit for the above would really appreciate, thanks for the help
    my email id is patrickvaz2002@yahoo.com

    Reply
  20. seetharaman

    Hi Patrick With a 12 0 12 transformer you can charge a 12volt car battery with just 2 rectofiers and one 1 to 2 ohms current limitting resistance. To charge a 6 volt battery you have to add one thyristor after the full wave rectification with zener and transistor to limit charging voltage at 6.9 volts. Current limitting can also be achieved by limitting charging current at 30% of the rated capacity of the battery.
    Hi Sanjay you chage any chargeable battery with appropriate external charger.

    Reply
  21. patrick vaz

    Hi Seetharaman, I have a 12-0-12 2Amps Transformer, will there be any modifications required to the circuit,I want to induce toggle switches/Sliding switch to have 6 and 12 volts and slow & fast charging, Pls reply thanks

    Reply
  22. patrick vaz

    Hi Seetharaman, I have a 12-0-12 2Amps Transformer, will there be any modifications required to the circuit,I want to induce toggle switches/Sliding switch to have 6 and 12 volts and slow & fast charging, Pls reply!!!

    Reply
  23. patrick vaz

    HI Seetharaman, IS there a possibility of adding a toggle switch for 6 and 12 volts charging and toggle switch for slow and fast charging in this circuit.
    thanks a lot

    Reply
  24. Seetharaman

    hi mr peter the above circuit is most suited for your application requirement. by replacing D3 D4 and D5 with a link and also D6 with a link.

    Reply
  25. Seetharaman

    Hi since this is regulated voltage charger with a flat battery at the start of charging the current will be at 3amps, gradually decrease and once the terminal voltage reaches the rated voltage the current will reduce to minimum and it will be just doing the topping up function, hence no additional circuit is required. regarding the charge indication, replace the link between C2 and D5 connect a 0.22 ohms 5 watt resistance. a BC 547 transistor emitter to C2 & 0.22 ohm junction, base through 100 ohms resistance to 0.22 ohm & D5 junction and collector through LED and series with 2.2Kohm resistance to IC1 ‘in’. The LED will glow brighty to start with and slowly it will dim down as the battery is nearing its rated charge limit.

    Reply
  26. faradaY

    PLS. HOW CAN I MAKE THE CIRCUIT SHUT DOWN WHEN THE BATTERY IS FULLY CHARGED AND VICE VERSA . ALSO INCLUDING INDICATING LEDS. PLS.REPLY ME. 1000 THANKS.

    Reply
  27. MARRIJ ALI

    GREETINGS,
    THIS MARRIJ.SIR I NEED A CIRCUIT THAT CONVERTS 12VDC TO 35VDC FOR A AMPLIFIER.
    PLZZZZZZZZZZZZZZZZZZ HELP ME

    Reply
  28. Peter Steenkamp

    I need to build my own power supply for 220 volt ac to 12 volt dc around 30 watt is ok,it must be reliable for LED signage,,can I be assisted? Thanks

    Reply

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