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Thread: Quick question

  1. #1
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    Quick question

    Hey, guys
    I have an XML board just lying around, but no driver. Was wondering if I could straight-drive it from an 18650

  2. #2
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    quick answer

    no.

  3. #3
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    Yes, with a resistor.

  4. #4
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    Both answers are correct.

    Your 18650 will come off charger around 4.2 volts. The XML operates around 3.2 volts. Leds do not do well over volted. It will get hotter brighter and burn up quickly.
    Resistor would increase circuit resistance to offset the voltage of battery.

    In general do not direct drive XML with 18650. Most likely it will burn up first time you run it.

  5. #5
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    I have been direct driving pairs of LED's from 7.4v packs for years and have never had one blow, melt or fail yet....

    Have I just been lucky?

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    Quote Originally Posted by tanner72 View Post
    I have been direct driving pairs of LED's from 7.4v packs for years and have never had one blow, melt or fail yet....

    Have I just been lucky?
    depends on the LED's and some other stuff. you can overvolt some LED's a little more than others, if the amperage is kept low enough.

    Since you didn't mention what LED's you were direct driving - it's hard to answer that question.

    But - if you were running a pair of something like XR-E in series directly off a 7.4V pack, with something like a 2.2h pack, with no driver or resistor - I can't see how you were keeping them from popping.

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    Would be interesting to know what Led it is.
    Older ones like the p7 have a higher resistance so did fine direct driving.

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    You can direct drive an XML with an 18650 as long as the 18650 has enough internal resistance to limit its discharge current to somewhere in the neighborhood of 4A (you'll need a housing with good heatsinking). Lots of the ....fire cells fall into this range, though I have seen a couple that would go 5A. I have not tested any of the high capacity panasonic cells, they probably can deliver enough current to kill the LED. Add in wiring and a switch, each with their own resistance, and you gain a bit of margin. Do not add cells in parallel though. A parallel pack can deliver more current and the magic smoke will leak.

    All this being said, IMO it is better to use a driver. The ability to use multiple cells, have different brightness levels, maybe temp sensing and voltage sensing is just too good to ignore.

  9. #9
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    Quote Originally Posted by Vancbiker View Post
    ......and the magic smoke will leak.
    LOL. Good one.

    I used to work in the car audio industry, I've had to explain the "magic smoke" principle to a lot of people that didn't know what I was talking about.

    Sent from my DROID RAZR using Tapatalk 2

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    I also depends on the internal resistance of the battery too.

    I have been doing it on P4's and more recently XPG and XPG2's using Tenergy cells with no magic smoke yet.

    But I remember Troutie and I have this discussion a few years back - he tried it with another type of cell and had to stop pretty shapish.

    There is more in this thread.

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