car battery charger circuit schematic

Car Battery Charger with Transistors

This car battery charger circuit can be used to charge 12V and 6V batteries. If it is used a transformer that can deliver 4A to 5A at a voltage between 12.6V and 16V then we can get rid of the switch for 6V or 12V batteries.

12V car battery charger circuit schematic

car battery charger circuit schematic

The car battery charging current is automatically limited to 4.2A. If there is a 600mV voltage on R1 (4A thru it), then the T1 transistor starts to conduct. Excessive charging current is avoided because the current value on T3’s base is limited. The difference between applied load current (at T4’s collector) and real voltage of the battery is balanced thru T4’s collector-emitter junction.

The power input of T4 (2N3055) is the product of load current and voltage difference already mentioned. When charging 6V car battery this power reaches a maximum of 40W. The rectifier diodes must be able to deliver 4A at 40V. T4 2N3055 must be mounted on a good heatsink in order to dissipate the heat.


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  • ghanal warner

    I was just wondering from a simple car sub charger. Is it possible to use a 5v trailer charger and convert it to a functional wall charger?

  • Ramesh

    I want battery charging circuit in petrol engine vehicles by using electrical solenoid coil, connected to Motor with propeller shaft of engine & alternator.

  • Thomas

    Thank you for the circuit.I want a slow charger, say around 2A max. so what changes i need to do in this circuit.


  • Hesham Zakaria

    Hello Jim,

    First of all thanks very much for sharing this, I bet I will try it but I need to know about that byx21-40 ?

    Is it like a diode rectifier ??

    and what is 21-40 means like between 21-40 or just it’s datasheet code (because i hardly found it on google)

    and if it’s being sold just as it is and what if I didn’t find it in the market ?

    sorry for all the questions but I’m a mechanical engineer and electronics is a hobby for me so im not professional :))

    Thanks again and hope to hear the answers soon

  • Mihai

    Sorry Keith, I didn’t see that you already suggested swapping base and emitter.

  • Mihai

    Hi Keith,
    Unfortunately, a PNP type will not work since the base should be more negative than emitter in order to turn it on. See the voltage drop on R1(0.35Ω). As a matter of fact, there’s something wrong with the entire diagram. Conceptually. It’s a piece of s….

  • Jim Keith

    T1 should be PNP (not NPN BC107) and if so, the base and emitter inputs must be swapped to make it work. Also BC 107 is obsolete –use 2N3906 (PNP)

  • Mihai

    There is something wrong with T1(BC107).
    The collector potential of T1 is more negative than emiter potential. This transistor doesn’t work right.

  • arya

    does the charge will stop itself if the battery is full?

  • fitifiti

    why don’t we use resistor instead of transformer?

    • Jim Keith

      Reasons why we use a transformer:

      1. Charging current will substantially exceed line current due to voltage matching provided by transformer turns ratio.

      2. Transformer provides isolation –eliminates shock hazard.

      3. Transformer provides much higher efficiency than the suggested resistor e.g. 70% vs 10%

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