12V Battery to Power 12V leds

Discussion in 'The Projects Forum' started by chope1, Sep 21, 2010.

  1. chope1

    Thread Starter New Member

    Mar 10, 2010
    Hello, I said i'd help out a frind of mine with a project lighting up some leds, which i have worked with before but powering 12v leds from a battery supply has thrown up some unexpected problems. any help is gratefully received and much appreciated...

    This is what i am trying to do; power 15 meters of SMT led strip rated at 12v. There will not be access to mains electric and they need to run for at least 12 hours so i an going to power them from a 12v deep cycle leisure battery rated at 125Ah. The leds take 300mA per meter so the total power consumption is 4.4A This should give me more than enough running time. The supplier of the leds said that they must receive a regulated 12v.

    The problem i am encountering is how to supply the leds with regulated 12v dc supply from a nominally 12v battery. As far as i understand the battery will range from about 14v at full charge to around 11.5v fully discharged. I have looked in a lot of places online and not found an off the shelf solution that is not expensive and inefficient. (though if anyone can suggest one!)

    I was thinking of using a linear fixed voltage regulator outputting 12v, such as this:

    with capacitors to smooth output in an arrangement like this:

    The other consideration i have is power dissipation, i borrowed a calculation from another post to help with this:
    (Vin - Vout) x Iload
    with a full battery that would give me an input of around 14V
    so using that calculation i could have a power dissipation of around 9W. I guess it would be good practice to use a heat sink on the regulator?

    THE biggest problem i can see with this is voltage dropout, i am not sure how long the battery will last for before the battery voltage drops to level where the voltage regulator stops producing 12V. with the Vin and the Vout being so close I guess there is not much room to play with here. in fact so little will it really work at all?

    i know it may be less efficient but there are two other designs i thought may provide a longer run time, though it seems a shame to be so wasteful:

    Use a DC to DC converter to provide 15V dc (this is the lowest voltage output i have found a 12V converter will do) then use the 12V fixed regulator as above.

    Really inefficient: use an inverter to get 240v AC then use a standard mains 12v regulator to run the leds.

    Does anyone have any comment/criticism on what i propose on trying out?
    Am i going about this the wrong way?

    Thanks in advance for any help, Charlie.
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010

    DC/DC converters in the 3-5 amp output range are not cheap. I found one that does .9 amp @ $23 but you would need four of them. Of course I have only spent 20 minutes at Allied, so it would be wise to look at several other place. The converter linked does 9-18 volt input to 12 volt out. If you went with two batteries then your options would expand greatly. Lots more 24 to 12 converters out there than 12 to 12.

    Good googling and may the bargain gods smile down on you. :)
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    Are you sure the LED strip doesn't have an operable range from, say, 11v to 15v ?
  4. chope1

    Thread Starter New Member

    Mar 10, 2010
    After more looking around online it seems that as long as the battery is not being charged, raising the voltage, the LEDs are work fine on the 12.6V the battery puts out. I tried hooking up the LEDs and they seem ok after more that two days on. Hope this helps anyone else avoid this problem.
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    When you're charging the battery there's naturally going to be a higher voltage present - or it wouldn't charge.

    Your simple options are either to turn off the LEDs when not needed and charge then, or get a trickle charger that just barely raises the voltage when charging.

    The LEDs should do just fine with an extra 5% or even more voltage without undue stress.

    Alternately you could rig up some sort of relay that kicks some diode drop into the circuit running to the LEDs when the charger was energized.
  6. knowone

    New Member

    Aug 24, 2009
    Nice one Chope1 and retched.
    Try it and see.
    I thought maybe 20 hrs, but you seemed to get 2 days. Nice.
    Operating range from 15 to 10V.
    All that voltage pumping just wastes energy.
    Semiconductor systems are pretty resilient to voltage
    fluctuations and tough conditions.
    Did your party go off well, chope?
    Take care.:D
  7. Wendy


    Mar 24, 2008
    You have practiced the arcane art of necromancy with this thread, it is almost two years old, and 2 of the 4 people are gone, likely never to return.