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  1. #1
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    5x XM-L bike light at DX ...

    ... and for only USD 36.90, but without batteries
    5 x Cree XM-L T6 3000lm 3-Mode White Bicycle Headlamp - Dark Grey (4 x 18650) - Free Shipping - DealExtreme

    I have no personal experience of this light, and I think the light is new at DX.



    /Håkan
    SWEDEN

  2. #2
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    Holy crap!
    I bet that is going to get China Syndrome HOT!!!!
    And I bet that will make the Xeccon battery put up a white flag.
    I might buy one just to see how strong Xeccon's battery is.....

  3. #3
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    I've seen something like this on ebay, but that included the usual suspect batteries and charger. It has the the UI that nobody wants, not enough levels and a strobe mode in the mode rotation.

    I wonder though if there is a tradeoff between the number of leds, the drive current, and the heat generated. For example, do five leds driven at a lower current generate less heat compared to three leds driven at a higher current but with the same lumen output? Does anyone know the answer to this?

    My uninformed opinion is that the 4 xml and 5 xml probably generate more heat than the housing can scrub off. They aren't really increasing the surface area of the cooling fins that much.

    Both the triple xml and 5 xml have less than ideal UI's in my opinion. The 4 xml UI is good.

  4. #4
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    I agree with your confusion.
    Or I join in on your confusion?
    I'm confused.
    Yes...the 3x with it's low/med/high/strobe off modes is what you get with a cheap light.
    The one I have has no thermal step down and it gets HOT fast when you stop...so you have to fumble through the modes to shut off or to get rolling again. That's the price you pay for a cheap light.

    The 5x here says it goes high/lo/strobe???
    Do all LEDs run during the modes?
    And as you said...how hard is it being driven? Is my 3x as bright as the 5x? And what kind of spread and throw does it have? And does it step down for heat?

    Do I buy another cheap light just to test what's out there?

  5. #5
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    Under Mode Arrangement it says
    Hi > Lo > Fast strobe

    Which is garbage.
    If you have one of these lights, it's probably not worth spending another $35 or $40 to buy another very similar model. We have no idea on how long they last or how well they perform on a really hot summer night. You might as well save up for Gloworm X2 or another "real" light.

    I'm pretty sure these Chinese manufacturers are not putting a lot of effort into thermal engineering. There are some guys on the DIY forum who know how to do these types of calculations, so maybe they can give us their informed opinions on these lights. Judging by their DIY designs, with their massive cooling fins, these 4xml and 5ml don't have enough cooling surface.

    All the heat these things are generating probably adversely affects the life of the driver board.

    I know that a there's a lot of people that think of a $40 - $50 light as disposable, but not me. It's a quarter of a way or maybe even half way to a reputable light. So why waste the money? I can however see the point of buying a $30 single xml, because the lighthead generates less heat making it probably longer lasting. The batteries that come with these things are pretty awful and even dangerous.

    I got myself an Magicshine 872 clone lighthead for $35 and I was itching to buy an Ultrafire D99 from DX, so I definitely have light fever. But buying all these lightheads from DX and having to wait four weeks for delivery of a questionable light is probably not the way to go.
    Last edited by varider; 03-05-2013 at 04:02 PM.

  6. #6
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    varider....
    I agree 910 %.
    See my Xeccon Battery review post.
    I thought about buying one of these 5x lights to really put a load on the Xeccon battery.
    But I'm not going to bother.
    I'm going to keep that $36 in my pocket and buy good lights when my cheapo experiment dies.

  7. #7
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    I think the design without deliberateness. It just meet some freshman who superstitious in high lumens.


    Andy

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    I mentioned these a couple weeks ago. Even if these had the same mode arrangement as the quad the trade off ( added lumen output to run time ) really is not worth it. It might be worth buying if it had a 4-level output ( 300-500-1000-Max lumen ( strobe press and hold ). Once again, it will get real hot in the over 1000 lumen mode. Even with a six cell you would only get about 1.5hr on high...then again you wouldn't run high except when at extreme speed. I think the real shame in all of this is that the Chinese don't seem to want to put a decent driver set-up in these. If they had the mode arrangement I mentioned I might actually want one...even if it was just to play with.

    ...With that in mind; I might still want one ( ) just to play with. I could get a "Y" adapter, run my Gloworm most of the time and then turn the BIG light on for that killer downhill. Since I don't do killer downhills much if I got one it wouldn't see much use.....( but sure would be fun when I did. )

  9. #9
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    Quote Originally Posted by Cat-man-do View Post
    I think the real shame in all of this is that the Chinese don't seem to want to put a decent driver set-up in these. If they had the mode arrangement I mentioned I might actually want one...even if it was just to play with.
    I agree. Most cheap bike light brand like Trusfire,ultrafire or other no brand bike light don't have a decent circuit board, of course they don't have a electronics engineer.
    A good circuit board means deliberate‘s design,meticulous's test and consummate's maching. The circuit board should stabilized, constant current and good efficiency.


    Andy

  10. #10
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    If we do the math on a 6600mAh battery with this light - assuming the listing is correct about the drive current being 2.8A then 6.6/2.8 = 2.35 hours or 2 hours 21 mins. Not too shabby. IMO at 2.8A it will get quite toasty inside considering it lacks serious heat sinking.

    [Edit Add] Working Voltage 5.4 ~ 12V??
    Last edited by mtbRevolution; 03-05-2013 at 05:53 PM. Reason: working voltage question mark
    Leonard - All things Xeccon + Beyond
    mtbRevolution.com

  11. #11
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    Quote Originally Posted by Xeccon View Post
    If we do the math on a 6600mAh battery with this light - assuming the listing is correct about the drive current being 2.8A then 6.6/2.8 = 2.35 hours or 2 hours 21 mins. Not too shabby. IMO at 2.8A it will get quite toasty inside considering it lacks serious heat sinking.
    Dear Leonard ,

    As far as i know, is absolutely wrong way to calculate runtime.

    The battery pack is 7.4V, 6600mAh, i think the efficiency of the circuit board no more than 70% at max outout;
    At 2.8A, the driving voltage of single XM-L T6 is around 3.7V, they have 5 XM-L T6 LED;
    Base on law of conservation of energy, it calculate as below:

    (7.4V*6.6A*70% ) / (3.7V*2.8A* 5) = 0.66Hrs

    If it is real 2.8A and not drop down, theoretically, i think it should calculate like this.
    But if it drop down, i need to see discharge curve.


    Andy
    Last edited by Waldens; 03-05-2013 at 06:35 PM.

  12. #12
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    The 2 most likely wiring setups for a 5 LED light is all LEDs in series or all in parallel.

    If this light uses a series configuration the forward voltage requirement is about 18V at 2.8A. A driver that boosts the voltage from 7.4V to 18V at 2.8A is going draw about 8A from the battery, be working hard,and have low efficiency. Low efficiency is realized as heat generated by the driver. That, coupled with the heat of 5 XMLs at 2.8A is in excess of 50W. A conservative bikelight design would put the heatsink area for that at about 100 square inches. Cool air (<45F) and moderate airflow of 6-7 mph would let the housing drop to half that and stay acceptably cool. Stop that air flow or ride on a 70F night and it will cook itself if there is no thermal sensing in the driver. I doubt there is more than 20 sq in area on this housing based on DXs dimensions and a liberal factor for the small fins it has.

    A parallel configuration at 2.8A would only have each LED receiving .56A. That is still fairly bright with XMLs but very noticeably less bright than higher currents. It would make use of a simple buck type driver and only have about 10W of heat to deal with. This seems a much more rational layout though the Lumen output would be less than a good dual series XML at 2.8A.
    Last edited by Vancbiker; 03-05-2013 at 10:03 PM. Reason: Corrected spelling

  13. #13
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    Quote Originally Posted by Vancbiker View Post
    The 2 most likely wiring setups for a 5 LED light is all LEDs in series or all in parallel.

    If this light uses a series configuration the forward voltage requirement is about 18V at 2.8A. A driver that boosts the voltage from 7.4V to 18V at 2.8A is going draw about 8A from the battery, be working hard,and have low efficiency. Low efficiency is realized as heat generated by the driver. That, coupled with the heat of 5 XMLs at 2.8A is in excess of 50W. A conservative bikelight design would put the heatsink area for that at about 100 square inches. Cool air (<45F) and moderate airflow of 6-7 mph would let the housing drop to half that and stay acceptably cool. Stop that air flow or ride on a 70F night and it will cook itself if there is no thermal sensing in the driver. I doubt there is more than 20 sq in area on this housing based on DXs dimensions and a liberal factor for the small fins it has.

    A parallel configuration at 2.8A would only have each LED receiving .56A. That is still fairly bright with XMLs but very noticeably less bright than higher currents. It would make use of a simple buck type driver and only have about 10W of heat to deal with. This seems a much more rational layout though the Lumen output would be less than a good dual series XML at 2.8A.
    Yep, I think you're right. More than likely a parallel set-up. The question that remains is what the true current draw actually is. I would hope that the draw be somewhere between 3-4amps. At 3A you might get an actual 1500 lumen output on high. I doubt though that it will go higher than 3A but if it did go to 4A you might actually get a 2000 lumen output....ehhh, we all know that's not going to happen though

    Now back to battery run time: If the battery is 6.6Ah and the current about 2.8A the on-line battery calculator says it will run 1.65hrs. Now if you take into consideration that the battery might not actual supply a "true" 6.6Ah rating than what you will likely get is something around 1.5 hrs. ( Didn't someone say that )...

  14. #14
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    Quote Originally Posted by Cat-man-do View Post
    Now back to battery run time: If the battery is 6.6Ah and the current about 2.8A the on-line battery calculator says it will run 1.65hrs.
    The formula for runtime in the online calculator does not work when you use the output of the driver as the load value. You need to know the input current to the driver for it to work.

    The formula in post 11 is pretty good as it converts the Volt and Amp values to Watts and applies an estimated efficiency factor. The 70% used in post 11 is just a guess on the posters part. Taskled, makers of arguably the highest quality LED drivers available to the DIY crowd, list efficiencies in the 90%+ range depending on model and application. Cheapo driver at 70% efficiency is quite possible.

  15. #15
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    Vancbiker, thanks for the analysis. From what you are saying, these led are very underdriven otherwise this thing would melt down. So the extra led doesn't really give you anything, it's more of a gimmick. I'm still curious if there is an optimal number of emitters of give the most lumen output for the smallest heat generated.

    Cat, I don't think that the calculator works properly. The equation is correct, but if you plug in the numbers from example you don't get the correct answer.

  16. #16
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    Quote Originally Posted by varider View Post
    I'm still curious if there is an optimal number of emitters of give the most lumen output for the smallest heat generated.
    The XML gives its best Lumen to Watt ratio (most efficient) at the lowest drive current (100mA) per the datasheet. Unfortunately that is not a lot of light. Many DIY builders have decided that 2000-2500mA seems to strike a good balance between perceivable brightness and heat and battery consumption.

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    It's good that they're not including the battery. I don't want to pay extra for a poor quality battery TBH.

    It would be good if they could line up the LEDs in a more efficient way to allow better cooling, with more surface area going on, but it seems it's cheaper and easier to just upscale the 3x T6.

    I didn't buy the Tri-clone or Quad because I saw this and wanted to wait and see how far things would go before it started getting stale.

    Ultimately, i'm thinking it may be better for to get two triclones instead of something like this. One more LED and way better cooling per LED.

    Then again, I am tempted but I'm really happy with what I have right now.

  18. #18
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    Quote Originally Posted by Vancbiker View Post
    The formula for runtime in the online calculator does not work when you use the output of the driver as the load value. You need to know the input current to the driver for it to work.

    The formula in post 11 is pretty good as it converts the Volt and Amp values to Watts and applies an estimated efficiency factor. The 70% used in post 11 is just a guess on the posters part. Taskled, makers of arguably the highest quality LED drivers available to the DIY crowd, list efficiencies in the 90%+ range depending on model and application. Cheapo driver at 70% efficiency is quite possible.
    I assumed the battery calculator is giving you a ball park estimate based on a constant current draw. When you start factoring in all the variables in the complete circuit ( including the battery ) you will get a different ( lowered ) figure. For the moment all I was concerned about was the run-time for the battery. The current that leaves the battery is really the only figure you need to determine this. If you want to factor in efficiencies and variables in the complete circuit that will change the numbers no doubt. Once again I usually "ball park" stuff like that too for simplicities sake. ( If you factor in a ball park 20% loss you get something like 1.32 hrs. )...( FWIW )....10% loss would be 1.48 hrs. All of this means nothing though without a fairly accurate reading on the current draw coming out of the battery. Usually if your ball park estimate is between ( + or - ) 10 minutes of the actual run time you've done a decent job with your estimate.

    Now if you have the output current curve of the battery while under load ( till depletion ) you have a more accurate number to use for doing the math. Then again if you know the current curve you probably already know the time it takes before the battery cuts out. This is why "Ball Park" estimates only get you so far. To be sure you really do need to do a run-time test with your set-up....it's the only way to be sure.

  19. #19
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    Quote Originally Posted by Cat-man-do View Post
    I assumed the battery calculator is giving you a ball park estimate based on a constant current draw.
    The calculator you linked works fine when the current draw from the battery is known. The driver draws power, in this case 7.4V, from the battery and regulates it to 2.8A and ~3.4V to drive the LEDs. Since the voltage is higher on the battery than the driver output the current draw from the battery is lower than the 2.8A that the driver is outputting to the LEDs. Presuming this 5 XML light is running in a parallel configuration, that 6600mAh battery will run it around 4 hours. If the light has the LEDs in series that runtime will be less than an hour. The reason it is good to convert the the battery capacity to Watt hours and the light output to Watts for runtime estimates is you have a common unit of measurement. Here is a good calculator for LED applications.

    LED Runtime Calculator

  20. #20
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    A friend went to Euro Bike last year. He sent me a pic from a Chinese manufacturer that had a light with 10 or 12 LEDs in it...2 LEDs high and then 5 or 6 across with a water bottle battery.
    The people at the booth had very limited English...so he couldn't get details.
    I just pinged him to see if he still had the photo....

  21. #21
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    Quote Originally Posted by the mayor View Post
    A friend went to Euro Bike last year. He sent me a pic from a Chinese manufacturer that had a light with 10 or 12 LEDs in it...2 LEDs high and then 5 or 6 across with a water bottle battery.
    The people at the booth had very limited English...so he couldn't get details.
    I just pinged him to see if he still had the photo....
    Do you have contact details of the Chinese manufacturer? Maybe i can contact with the manufacture and get a sample to write a review here.


    Andy

  22. #22
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    Quote Originally Posted by Vancbiker View Post
    The calculator you linked works fine when the current draw from the battery is known. The driver draws power, in this case 7.4V, from the battery and regulates it to 2.8A and ~3.4V to drive the LEDs. Since the voltage is higher on the battery than the driver output the current draw from the battery is lower than the 2.8A that the driver is outputting to the LEDs. Presuming this 5 XML light is running in a parallel configuration, that 6600mAh battery will run it around 4 hours. If the light has the LEDs in series that runtime will be less than an hour. The reason it is good to convert the the battery capacity to Watt hours and the light output to Watts for runtime estimates is you have a common unit of measurement. Here is a good calculator for LED applications.

    LED Runtime Calculator
    Okay, over the last couple days I gave this all some thought based on what you said above. After doing some homework I discovered I had forgotten some things about how "some" drivers are set-up to operate. Basically I had forgotten how Op amps work and how they can be used to boost current. Without getting too technical this means the current that leaves the battery is not always the figure you use to calculate run time. The figure you use is the total current used to actually drive the load, in this case the 5 LED's in parallel.

    Using the calculator that you linked to I plugged some numbers in to try to get a theoretical output that made sense. To do this I had to make some assumptions because I don't know the actual numbers so bear with me. When I make an assumption I will put it in bold so you know it is an assumption.

    First assumption: the 2.8A listed on D/X website is the current leaving the battery. The next: The desired LED current ( total ) is about 5.1A With this total each LED would see about 1A. This would get you an actual output on high somewhere close to 2000 lumen. This makes sense to me because why build a lamp with 5 LED's if you can't get a little more output. It's not going to get an actual "3000" lumen as listed on the D/X website but that shouldn't surprise anyone.

    Converter efficiency I listed as 85%
    LED forward voltage as 3.4 ( with all LED's in parallel )
    Battery voltage as 7.4 nominal. Battery capacity I list as 6400mAh which is likely the actual capacity.
    With these numbers plugged in to the calculator the run time comes out as about 1.34hrs with the battery current listed @ 2757ma. This is pretty close to the 2800ma listed by D/X "IF" indeed it is the battery current. At least with this calculation all the numbers seem to fit as long as my assumptions are right.

    Now if the current leaving the battery turns out to be less than 2.8A the actual output will be somewhat less. Does this make sense?

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    True Cat, there will be differences because we do not know all the info. My calcs are based on the assumption that the 2.8A is the LED drive current. I make that assumption based the DX spec of a 5-12V operating voltage. As LEDs need a constant drive current, any variation in supply voltage to the driver must be countered by a change in supply current to the driver.

  24. #24
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    Quote Originally Posted by Vancbiker View Post
    True Cat, there will be differences because we do not know all the info. My calcs are based on the assumption that the 2.8A is the LED drive current. I make that assumption based the DX spec of a 5-12V operating voltage. As LEDs need a constant drive current, any variation in supply voltage to the driver must be countered by a change in supply current to the driver.
    Here's more food for thought; D/X also sells the quad as lamp only. The quad has listed
    as "circuitry"- 3800ma. Assuming the 3800ma. current is for the LED load, to determine run time I ran the numbers for both a total parallel LED set-up and a 2-parallel, 2 in series configuration. The parallel set-up runs about 3hrs. The 2/2 set-up about 1.5 hrs.

    My tri-clone lamp runs about 3.25 hrs. It would be a shame if the quad was a parallel setup because the output would likely be no brighter than than the tri-clone. The 2/2 setup would have a much shorter run time but would be much brighter ( maybe over 2000 lumen(?) ) If I was buying one of these I would want the brighter set-up. No way to know for sure unless you buy one. That goes for the 5-up too. Roll the dice and take your chances.

    Actually I was considering buying the 5-up ( just for fun ) but there are too many unanswered questions. Now I'm thinking the quad might be the all-round better buy. Likely the quad and 5-up have close to the same output so since the quad has the better mode setup the quad IMO is the better bet. Once again, roll the dice.

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    Quote Originally Posted by Cat-man-do View Post
    Roll the dice and take your chances.
    Absolutely true! As I said in a similar thread some time ago about the chinese lights, the only spec you can trust is that the housing is aluminum and it has LEDs in it.

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    Quote Originally Posted by Vancbiker View Post
    Absolutely true! As I said in a similar thread some time ago about the chinese lights, the only spec you can trust is that the housing is aluminum and it has LEDs in it.
    And that is part of the problem.
    If a person is having major fretting over a $35 cheap light.....they are probably NOT a good candidate for that light.
    These things are a pig in a poke ( google that)
    You don't know what you're getting.
    I bought a couple that I am pretty happy with....but I had little idea ( with the exception of a few reviews here) what I was getting. I have no idea on the longevity. And am pretty sure I will get ZERO support if one dies in a few months...I'm betting my email to the Ebay seller will go unanswered,
    But I rolled the dice. So far....I have no real regret.
    But in the end...I didn't really save a whole lot of money either.

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    Quote Originally Posted by Vancbiker View Post
    Absolutely true! As I said in a similar thread some time ago about the chinese lights, the only spec you can trust is that the housing is aluminum and it has LEDs in it.
    ( LOL ) Yep, unless someone else reputable buys one and reports on it no one really knows what they are buying. Someone has to be the guinea pig. I thought about taking the plunge but in all honesty I have too many spare Chinese lamps floating around as it is. If I buy one I want to have some idea of what the output is like before I buy. I figure sooner or later someone with a more pressing need for a bike light will buy and then post up. Nevertheless, if no one posts-up by July I might get one just to mess with it. Right now I have no pressing need for more lumen output so I'm opting to wait.

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    I blame you, Cat man, for my 3X clone.
    It is a good light...with a so-so battery.
    We'll see how long it lasts......or if something new comes up first.

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    Quote Originally Posted by the mayor View Post
    Holy crap!
    I bet that is going to get China Syndrome HOT!!!!
    And I bet that will make the Xeccon battery put up a white flag.
    I might buy one just to see how strong Xeccon's battery is.....
    kkkkkkkkkkkkkk Ohh yes Sir!!! Thats is Goddamn Right!

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    Quote Originally Posted by Cat-man-do View Post
    D/X also sells the quad as lamp only. T
    Could you please give a link to it? I spent quite a time trying to find but found nothing. Thanks.

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    LInks

    Quote Originally Posted by freebird74 View Post
    Could you please give a link to it? I spent quite a time trying to find but found nothing. Thanks.
    There you have it:

    4 leds Head only

    dx.com/p/4-x-cree-xm-l-t6-2800lm-4-mode-white-bicycle-headlamp-grey-187987

    5 Leds Head Only

    dx.com/p/5-x-cree-xm-l-t6-3000lm-3-mode-white-bicycle-headlamp-dark-grey-4-x-18650-192065

    Justa add the www before each link if necessary.

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    LED systems basics

    Guys, the calculations are simple, you only need to know the basics of Ohm's law.
    So here we go:
    1) no matter the setup of the LEDs, series or parallel, the power consumed by the LEDs for a given light output is the same.
    2) most drivers have an efficiency around 75-85%. Higher efficiency is achieved for series connection of the LEDs, lower efficiency for parallel. Can you guess why? Because power loss is proportional to resistance and square of current. Ploss=R*I^2. For a given resistance of the wires, for example 0.1 ohm, the loss is: a) at 3A current for a series connection of two XM-L leds Ploss=0.1*9=0.9W, b) at 6A for the two leds connected in parallel we have Ploss=0.1*36=3.6W.

    Let alone current, the driver will take care of it. Use power to calculate the discharge time of a battery, because the driver efficiency refers to power, not voltage or current.

    Calculate the LEDs power: Pled_total=V*I*n, where n= number of leds
    Calculate the input power: Pin=Pled_total/Eff, where Eff is the efficiency as subunitary number (e.g. 0.85).
    Calculate the battery pack energy: En=V*Icap [Watt*hour], where Icap=capacity in Ah
    Calculate discharge time: T=En/Pin [h]
    Example:
    The 5 leds can be powered at maximum with 3A each, for a Pled=3A*3.3V=10W (from the datasheet of Cree XM-L). That means 50W total power on the leds. The driver will take
    Pin=50/0.75=66.6W.

    With this power you can calculate for how long a battery of 7.4V and 6.6Ah will last.

    En=7.4*6.6Ah=48Wh. Then
    T=48/66.6=0.72h=43 minutes.

    A reasonable case for this light can be 3000 lumen total, so a 600 lumen per LED. From the datasheet we can estimate about 6W per LED, a total of 30W,

    Pin=30/0.75=40W, hence we can get
    T=48/40=1.2h=72 minutes

    About the LEDs configuration, there is a lot to know. It is determined by their number, the battery voltage and the type of converter/driver. In principle:

    1) buck converter cannot output a voltage equal to the input one because of the switch drop and the necessity to work with a duty-cycle around 50-60%, so from a 7.4V input a buck will output maximum 6.4V, but it will be at the bottom of efficiency curve. With some drop also on the wires, this is not enough to power two LEDs in series connection (~6.4-6.5V), mind that duty-cycle of the switch will get 100%. So, the leds will be paralleled if a buck is used with 7.4V battery. It will work with leds in series with a 3*3.7V=11.1V pack or 4*3.7V=14.8V pack.

    2) boost or buck-boost driver can take a lower voltage and raise it to something about 2-3 times higher. If the number of leds is 3, then it will be better to use a series configuration, that leads to a Vled=3*3.4V=10.2V max. A boost will work fine from a 7.4V to increase the voltage to 10.2V or 4*3.4V=13.6V.

    Magicshine MJ-872 uses a boost to power the 4 leds from 7.4V pack, while MJ-870 uses a buck to power the 3 leds from the same 7.4V. The efficiencies are 89.6% for boost and 84.9% for buck.

    I hope this helps to understand the basics of the LED lights.

  33. #33
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    Quote Originally Posted by uiflorin View Post
    Guys, the calculations are simple, you only need to know the basics of Ohm's law.
    So here we go:
    1) no matter the setup of the LEDs, series or parallel, the power consumed by the LEDs for a given light output is the same.
    2) most drivers have an efficiency around 75-85%. Higher efficiency is achieved for series connection of the LEDs, lower efficiency for parallel. Can you guess why? Because power loss is proportional to resistance and square of current. Ploss=R*I^2. For a given resistance of the wires, for example 0.1 ohm, the loss is: a) at 3A current for a series connection of two XM-L leds Ploss=0.1*9=0.9W, b) at 6A for the two leds connected in parallel we have Ploss=0.1*36=3.6W.

    Let alone current, the driver will take care of it. Use power to calculate the discharge time of a battery, because the driver efficiency refers to power, not voltage or current.

    Calculate the LEDs power: Pled_total=V*I*n, where n= number of leds
    Calculate the input power: Pin=Pled_total/Eff, where Eff is the efficiency as subunitary number (e.g. 0.85).
    Calculate the battery pack energy: En=V*Icap [Watt*hour], where Icap=capacity in Ah
    Calculate discharge time: T=En/Pin [h]
    Example:
    The 5 leds can be powered at maximum with 3A each, for a Pled=3A*3.3V=10W (from the datasheet of Cree XM-L). That means 50W total power on the leds. The driver will take
    Pin=50/0.75=66.6W.

    With this power you can calculate for how long a battery of 7.4V and 6.6Ah will last.

    En=7.4*6.6Ah=48Wh. Then
    T=48/66.6=0.72h=43 minutes.

    A reasonable case for this light can be 3000 lumen total, so a 600 lumen per LED. From the datasheet we can estimate about 6W per LED, a total of 30W,

    Pin=30/0.75=40W, hence we can get
    T=48/40=1.2h=72 minutes

    About the LEDs configuration, there is a lot to know. It is determined by their number, the battery voltage and the type of converter/driver. In principle:

    1) buck converter cannot output a voltage equal to the input one because of the switch drop and the necessity to work with a duty-cycle around 50-60%, so from a 7.4V input a buck will output maximum 6.4V, but it will be at the bottom of efficiency curve. With some drop also on the wires, this is not enough to power two LEDs in series connection (~6.4-6.5V), mind that duty-cycle of the switch will get 100%. So, the leds will be paralleled if a buck is used with 7.4V battery. It will work with leds in series with a 3*3.7V=11.1V pack or 4*3.7V=14.8V pack.

    2) boost or buck-boost driver can take a lower voltage and raise it to something about 2-3 times higher. If the number of leds is 3, then it will be better to use a series configuration, that leads to a Vled=3*3.4V=10.2V max. A boost will work fine from a 7.4V to increase the voltage to 10.2V or 4*3.4V=13.6V.

    Magicshine MJ-872 uses a boost to power the 4 leds from 7.4V pack, while MJ-870 uses a buck to power the 3 leds from the same 7.4V. The efficiencies are 89.6% for boost and 84.9% for buck.

    I hope this helps to understand the basics of the LED lights.
    Thanks for clarifying this matter for us. Its good to know capable users are arround to help out with the hard staff!

    Did any one buy or has some info on the characteristics of this 5 xml light?

  34. #34
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    Quote Originally Posted by uiflorin View Post
    ...A reasonable case for this light can be 3000 lumen total, so a 600 lumen per LED. From the datasheet we can estimate about 6W per LED, a total of 30W,

    Pin=30/0.75=40W, hence we can get
    T=48/40=1.2h=72 minutes

    About the LEDs configuration, there is a lot to know... .
    As you say, there is a lot to know that we don't know. If we knew the "actually" power draw of the 5-up lamp it would answer most questions but we don't.

    I think it very doubtful that this lamp is drawing 30 watts. If the lamp was using that much power it would have a real hard time getting rid of the heat. That said the actual light output would drop significantly once the lamp heated up. Not to mention that as you said the run time would be very low. Personally I'd like to see a 30watt ouput in a configuration like this as it would serve my purpose to used as a *"High output secondary lamp". ( * for momentary use only ).

    All things considered, I think it more likely that it is using much less power. That is because when you market a lamp for bike use you have to consider that most people are going to want a lamp that can be used for at least a couple hours while in high mode. The available batteries are only going to be able to run a lamp like this for so long ( on high ). That is why I was doing estimates based on 1.5 hrs run time. At least at that level you might see something around 2000 lumen. It is very possible though that the output could be more like 1600 lumen on high ( with a ~ 2hr run time ). Once again these are "ball park" guesses based on what I think is "most likely" to expect out of a low quality Chinese bike light.

  35. #35
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    Efficiency graph

    What I meant by "reasonable" power is actually "maximum reasonable". Yes, the heat is a major factor and 80% of the energy is wasted as heat. If the mechanical design is decent, meaning the thermal resistance diode-ambient is reasonable for the geometry of the case, then this unit can be much more efficient than a 2 led unit FOR THE SAME POWER LEVEL.

    The XM-L diode has a lm/W characteristic (like all leds) not a straight line with a given slope, but a curve with the slope decreasing with power injected into the diode. So, for a 20W total power, 5 diodes will run at 4W each and be much more efficient than 2 diodes running at full 10W each.

    For instance, the heat split in 5 sources placed over a larger area rather than only 2 in the middle is an advantage, cooling is better.
    Then, each diode from the 5 will dissipate less and the temperature difference between the silicon die and heatsink will be less, so each diode will be cooler by 4-5 degrees C (assuming max 1C/W contact thermal resistance).

    Theoretically 5 XM-L diodes powered with 20W total power will output 2650 lumens, compared to 1950 lumens for 2 diodes. With some tweaking of the driver (maybe the diodes replaced with better ones - low drop Schottky, some copper added on the pcb board, thicker wires, overall efficiency increased to 90%), I think it is entirely possible to get 3000 lumens with ~27W power from the battery. That is definitely what I will do with such light. Ofcourse, things can be very differently in real life and the design can be so bad (at this price...) that nothing can be done to improve it in such a way to deserve a try. The aluminum case can be so thin, that thermal resistance is the limiting factor of the max power.
    Attached Thumbnails Attached Thumbnails 5x XM-L bike light at DX ...-led_eff.png  


  36. #36
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    Quote Originally Posted by Lichitisky View Post
    kkkkkkkkkkkkkk Ohh yes Sir!!! Thats is Goddamn Right!
    Hey Mayor, this battery for me is the best bfb arrounf town right now. This can hold those multi xml lights for quite sometime. The capacity is guaranteed by the USA seller. Check it out:

    USA 8 4V 7 4V Battery Pack for CREE XML T6 SSC P7 12400mAh Capacity Guaranteed | eBay

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