smps based 12v battery charger

Discussion in 'The Projects Forum' started by onlyvinod56, Jul 29, 2013.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008

    I have designed many push pull inverters using 3524, 3525. But i dont know how to design a battery charger. I found few charging circuits on internet. But, I am afraid of making those as i dont know the basic concept in charging a battery. I believe that if something goes wrong, the battery may explode. Is that true?

    Anyway, I am attaching the picture and schematic of the charger. check those.
    I have few questions about smps and a battery charger.

    1. What actually happens in the feedback loop (optocoupler and FB pin of IC)?
    does it control the duty cycle or shutdowns the ic?
    2. How the smps controls the charging current?
    3. What happens if the battery is left connected to the charging circuit for a long time?
    4. A pot is given at the output section of smps (connected in series with regulator KA431). what is the use of it?
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    OK two ways to charge a battery,

    1) constant voltage
    2) constant current

    only one will do for the battery, most Lead acids are constant voltage, lithium and nicads usually constant current.

    Your circuit is a constant voltage device,
    The feedback path for the opto coupler is used to trigger the chip shutdown pin, by setting the voltage on the reference Zener usually a TL431, 3 pin device, this has a precision voltage set by the two resistors from gate to cathode, gate to anode,

    The pot is used to alter the output voltage.

    when the zener strikes it conducts and fires the opto which shuts the ic down, this happens fast so the output voltage shuts down , then everything repeats over again, and maintains a regulated voltage.


    The charger has no intelligence and so wont be able to stop charging when the battery gets full, you could use a window comparator, or timer , to stop charging after a period.
    Last edited: Jul 29, 2013
  3. MaxHeadRoom


    Jul 18, 2013
  4. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    Thank you very much Dodgydave.

    I have one more doubt.
    To disconnect the battery from charger, can I use a current sense resistor and a comparator to drive a relay. I think, when the battery gets fully charged, the current goes zero. So, I'll compare the current with zero and if the result is positive, opens the circuit.

    How about this?
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Yes that should work, if you monitor the voltage across the battery, or put a series resistor and measure the current, and cut it off at your set point.

    You could use a simple comparator current sensor like this to trip your relay to off.

    set the desired current by altering R3, or R2 to set the ref voltage on Pin3.
    Last edited: Jul 30, 2013