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Thread: Two XM-L direct driven with one 18650: Is it dangerous?

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  1. #1

    Default Two XM-L direct driven with one 18650: Is it dangerous?

    I'm building a new bicycle light and obviously killed the cheap driver I bought for it. Now I wandered if I could direct drive the two Cree XM-L T6 wired in parallel from my AW 2900mAh protected cell without risking the cell to vent? The LED get very hot of course, but I think I could get rid of the heat while riding the bicycle.
    As I used a small clicky switch (rated only 1,5 A, 220 V) and never learned to solder properly I think there is probably quite a voltage drop in my circuit. Am I right to assume, that from the fully charged cell the LEDs would only see about 3.7 V under load, so the parallel LED would share perhaps 4 A? Would that mean it is safe to use the AW 2900 in this light?

    Thank you for your help!

  2. #2
    Flashaholic
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    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    If its a protected cell the protection circuit will prevent it from over current. When the current draw from the XM-Ls is too high your light will not turn on.

  3. #3

    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    The battery will be slightly below 4.2 Volt fully charged, not 3.7V.

  4. #4

    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    Quote Originally Posted by mrradlos View Post
    I'm building a new bicycle light and obviously killed the cheap driver I bought for it. Now I wandered if I could direct drive the two Cree XM-L T6 wired in parallel from my AW 2900mAh protected cell without risking the cell to vent? The LED get very hot of course, but I think I could get rid of the heat while riding the bicycle.
    As I used a small clicky switch (rated only 1,5 A, 220 V) and never learned to solder properly I think there is probably quite a voltage drop in my circuit. Am I right to assume, that from the fully charged cell the LEDs would only see about 3.7 V under load, so the parallel LED would share perhaps 4 A? Would that mean it is safe to use the AW 2900 in this light?

    Thank you for your help!
    you mean little clicky you got there might be loaded with 4A??? that is your voltage loss right there, those things made for 1,5A load, i would imagine 5 min none stop work will melt the switch. even if it is rated for 1,5A at 220v it doesn't mean it will handle higher current at lower voltage.
    you really should mesure voltage, current on your particular set up, instead of asking us to guess. cuz this is all we could do, we don't see your set up, even guage of wires will make big diffeence. mesure, don't guess.
    Last edited by alpg88; 12-07-2011 at 07:18 PM.

  5. #5

    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    Yes, there will probably be a big voltage drop on that clicky from what I read. The switch is the one build in the ultrafire 980L, bought at manafont.com. People measured current draws well over 4A in this torch with that switch.
    My light has been on very short intervals as the lighthead gets burning hot within about 30 sec. without airflow and because I am concerned about the protected AW cell (there was no stop from the protection circuit).
    I do have a cheep digital multimeter with thick leads. I know how to measure the voltage of the cell and that seems to be very accurate. I tried to measure the current in a torch, but I seem not to do that right as the current readings are not steady and the light output of the light drodped dramatically when I used the multimeter. Can anybody tell me where to put the pointy red and the black leads of the multimeter?
    1. Current: Would the (-) end of the AW cell be the right place for the black lead and the red lead just where the wire comes out from the switch going to the (-) end of the cell?
    2. Voltage: Red lead to (+) end of the cell, black lead to (-) end of the cell ?

  6. #6

    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    O.K. I just used my multimeter:
    1. AW 2900 on the Table without anything connected: 4.14V difference between (+) and (-) side of the cell
    2. AW 2900 in the light, light switched on: 3.44V difference between (+) and (-) side of the cell
    3. I didn't measure the current as described before (number 2). I put the leads of the multimeter on each side of the switch, witch is in the red (+) wire between cell (+) and LED (+), the light turned on. The multimeter read 2.09A.
    I don't know if the current measurement means anything. At 3.44V a single XM-L should draw a lot more current. And I presume, with the multimeter in place I even got rid of the Voltage-Drop of the switch when I checked the current, even though the light didn't seem to get brighter? So I presume a single XM-L would draw 4.18A at 3.44V and my two LED wired in parallel half this?
    By the way I placed the leads of the multimeter on each end of the installed cell, the multimeter shot up to over 9A and I quickly took them away again - the protection didn't prevent that draw, it probably needs even more?

    Therfore it seems to be no problem to direct drive two parallel wired XM-L from one 18650?!

    I might try a AW IMR next to get a more light ....

  7. #7
    HKJ's Avatar
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    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    Quote Originally Posted by mrradlos View Post
    By the way I placed the leads of the multimeter on each end of the installed cell, the multimeter shot up to over 9A and I quickly took them away again - the protection didn't prevent that draw, it probably needs even more?
    According to my measurements the PCB overload current protection is around 10 ampere on the AW2600 and AW2900 cells. If you run the cell for a few minutes at that current the PTC might kick in and reduce the current.
    My website with flashlight, battery and charger information: lygte-info.
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  8. #8

    Default Re: Two XM-L direct driven with one 18650: Is it dangerous?

    Quote Originally Posted by HKJ View Post
    According to my measurements the PCB overload current protection is around 10 ampere on the AW2600 and AW2900 cells. If you run the cell for a few minutes at that current the PTC might kick in and reduce the current.
    Thank you HKJ for your information. 10A is a lot! I would not be prepared to use my cell with more then 2C (or about 6A) current - even if it is only for the first few minutes with the voltage above 4V.

    Still have to try the IMR ...

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