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Thread: Direct drive 3x XR-E with 8x AA NiMH?

  1. #1
    Enlightened
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    Default Direct drive 3x XR-E with 8x AA NiMH?

    I'm considering doing direct drive of 3x Cree XR-E LEDs in series with 8x AA NiMH batteries. Is this reasonable?

    How I came to think this might work...

    1) Looking at SilverFox's NiMH Shootout http://www.candlepowerforums.com/vb/...ad.php?t=79302 it seems that that at 1 amp, the voltage on a NiMH AA drops from 1.4v to ~1.25 in 15 minutes. 75 minutes later it drops to ~1.2 volts.

    2) Looking at Newbie's Production CREE XR-E Testing http://candlepowerforums.com/vb/showthread.php?t=138503 the Vf vs current chart shows Vf increasing from ~3.2V at 500 mA to almost 3.4V at 1000 mA. My understanding is that Vf varies widely on the XR-E (3.0 to 3.5) but I'm assuming that if I have 3 in series, I can assume they "average out" to about 3.3V at ~800 mA.

    So... 8x 1.25V = 10V battery =~ 3x 3.3Vf. I'd be using Artic Alumina to attach the LED to an aluminum tube similar to what Nightrider did http://candlepowerforums.com/vb/showthread.php?t=142791 so the heat sinking should at least be "pretty good". Maybe add a 1/2 Ohm resister just for good luck? (This would also help me determine the current)

    I don't mind if the LEDs "only" last ~100 hours under these conditions with ~90% lumen maintenance since I'm sure I'll want to install new LEDs in a year or two.

    The alternative is to use the awesome looking bFlex http://www.taskled.com/bflex.html with a 14.8V Li-Ion pack http://www.batteryspace.com/index.as...&Category=1073. I'm a little leary of Li-Ion, but running at approximately 700 mA discharge current probably isn't in the "danger zone"

    Any advice on either the NiMH idea or Li-Ion alternative is appreciated.

    Thanks,
    Dale

  2. #2
    Flashaholic*
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    The Cree XR-E can vary a lot more than 3.0 to 3.5 volts in thier forward voltage. You would have to check what the vf happens to be on the LED`s you have to figure out exactly what battery set up and what resistor you would need if any to run a series of them direct drive.

  3. #3
    Flashaholic* Long John's Avatar
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    I would prefer a setup with a Maxflex or Fatman

    Best regards

    ____
    Tom
    It's cheaper to get a high quality light than spending money for many cheap lights.

  4. #4
    Flashaholic*
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    Quote Originally Posted by Long John
    I would prefer a setup with a Maxflex or Fatman

    Best regards

    ____
    Tom
    MaxFlex sounds like it would be awesome! Hope it is a reality some day!

  5. #5
    Enlightened
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    I have a quick question, What is a resistor?

  6. #6
    Unenlightened
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    I put together a light with exactly this configuration: three Cree xr-e, direct drive by eight AA's in series. No driver, no resistors. Mine draws about 600 ma with freshly charged batts(energizer 2500's). I have not tried it with alkalines. I plan on checking the current occasionally to make sure the vf doesn't drop during break-in like luxeons do. If the vf drops and the current goes over 700ma, then I'll add a small resistor.

  7. #7
    Enlightened
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    Thanks 1idjack.

    I appreciate you sharing your experience. That sounds like a great setup to me.

    msm1018 - A resistor is a electronic device that causes a voltage drop as you run current through it. The amount of voltage drop is
    V (voltage drop) = I (current in amps) x R (resistance in ohms)

    Dale

  8. #8
    Enlightened
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    Default Re: Direct drive 3x XR-E with 8x AA NiMH?

    Quote Originally Posted by daq


    msm1018 - A resistor is a electronic device that causes a voltage drop as you run current through it. The amount of voltage drop is
    V (voltage drop) = I (current in amps) x R (resistance in ohms)

    Dale
    would that be built into a diver? I appreciate the help!

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