bFlex 14.8v -> 7.4v Configuration - Bad Idea?- Mtbr.com
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  1. #1
    Austin, TX
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    bFlex 14.8v -> 7.4v Configuration - Bad Idea?

    I searched the TaskLED.com forums to find an answer for this question but everything there addressed minimum voltage requirements.

    How bad of an idea would it be to use a 14.8v battery to drive 7.4vf of LEDs using the TaskLED bFlex? Is thermal management or driver inefficiency a big problem?

    My friends and I are building a few sets of lights using 3 XR-Es on the bars and 2 XR-Es on the helmet. We would like to be able to share 14.8v packs among the lights if possible, rather than have 14.8v packs and 11.1v packs dedicated for each. I read on an old post in this forum that the TaskLED drivers may disperse the 7.4v differential voltage as heat (7.4v x 1A = 7.4W) - is this still true? Is there a measurable, quotable percentage of efficiency for the bFlex when stepping down voltage by 50%?

    Thanks

  2. #2
    Randomhead
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    they are using switching, so there should be no problem. There should be an efficiency chart on the page. I don't think there actually is an effect of battery voltage, but the output voltage changes the efficiency. I could be wrong, look on the technical page for your driver to see if you can find that chart.

  3. #3
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    No problem, as it's a buck driver it will just be doing it's job and lowering the voltage for you. It's when it's a boost driver you have to worry about driver heat.

  4. #4
    Austin, TX
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    Thanks for the replies!

    The bFlex documentation doesn't have an efficiency chart, only claiming that it runs at 85% or greater efficiency. Assuming a safe effeciency of 85%, how would I calculate and compare run times for an 8-cell 14.8v 4400mah pack and a 9-cell 11.1v 6600mah pack? The light circuit consists of two XR-E R2 LEDs driven in series at 1A.

    Sorry, my electronics knowledge is basic and rusty right now. I just can't wrap my head around factoring in the buck driver. Obviously the 11.1v pack has one more cell than the 14.8v pack, but 30% more mAh - does that mean 30% more run time as well?

  5. #5
    Randomhead
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    the current drawn with the 14.8v battery will be 11.1/14.8 * the current drawn using the 11.1v battery.

  6. #6
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    Reputation: HEY HEY ITS HENDO's Avatar
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    the (3s3p)11.1v pack has one more cell than the (4s2p)14.8v pack, but 30% more mAh - does that mean 30% more run time as well?
    no, one pack has 8 batteries, the other pack has 9 batteries
    the difference in runtime is, simply put,1 battery worth
    ...Scun.thorpe, UK

  7. #7
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    Convert the rating of each battery to Wh capacity for comparison purposes.

    Volts x Amps = Watts

    14.8 x 4.4 = 65.12
    11.1 x 6.6 = 73.26

  8. #8
    Austin, TX
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    All three answers helped a lot - thanks!

    Regarding the watts calculation, am I on target to figure the consumption as Vf * I / efficiency:

    7.4v * 1A / 0.85 = 8.7W

    Therefore, the batteries should run for nearly 7.5hr and 8.4hr respectively?

  9. #9
    Randomhead
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    sounds right

  10. #10
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    yep
    Last edited by HEY HEY ITS HENDO; 05-22-2009 at 04:16 AM.
    ...Scun.thorpe, UK

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