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  1. #1
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    most efficient way of getting 1k lumen?

    I'm planning on building a light but i want it to be super efficient so I don't have to recharge it so often.

    And now I wonder what the most efficient way of getting 1000lm is. I've been looking at cree leds and those seems to deliver more lumen per watt if you drive them with less current.

    I was thinking maybe 8-10 leds of some sort as maximum. But will the drivers eat up the efficiency if I have so many of them?

    Any ideas?
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  2. #2
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    Quote Originally Posted by car bone View Post

    I was thinking maybe 8-10 leds of some sort as maximum. But will the drivers eat up the efficiency if I have so many of them?

    Any ideas?
    Having that many leds in one light will take a lot of battery power to light them all up. You'll need at least 12 batteries (18650s) in a 3S4P config. You don't need that many leds to reach 1000 lumens.

    You can most likely reach this goal with one or two leds. especially with the new XM-L2 out now. So with a dual XM-L2 set up in theory you should get about 2200lumens. With four good 18650s in a 2S2P config you should get at least 2.5hrs run time on high.

    Are you planing a D.I.Y. build or purchasing a set? I saw your other post asking for which set of light would be better to purchase.
    "By Your Command"

  3. #3
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    I'm buying a set for now and when I have more time i would like to build one myself, for fun.

    Can LEDs be connecter in both parallell and series? So i would need less amounts of batteries I'm thinking.

    I was looking at the xpe/xpg and those looks much more efficient when ran at low amps rather than max.

    I would like to build something that gets me about double the battery life compared to a maxed out xml2. If that is possible.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.

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  4. #4
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    Can LEDs be connecter in both parallell and series? Yes

    So i would need less amounts of batteries I'm thinking. No, the amount of power LEDs use isn't changed by wiring them parallel vs. series.

    I was looking at the xpe/xpg and those looks much more efficient when ran at low amps rather than max. This isn't unique to xpg. Or to xpe emitters.

    I would like to build something that gets me about double the battery life compared to a maxed out xml2. If that is possible. No, this isn't feasible. XML2 emitters are among the highest efficiency emitters available.

    My $0.02: You are looking for a magic configuration to get extraordinary battery life plus high output. There isn't any such animal. Any efficiency you would get by running lots of emitters at low output will get eaten up by the loss in the multiple drivers you will need to run all those emitters. Plus it will cost a lot more money to build, and will have lots of solder joints that can fail. Plus it will be big, and look bad.

    A much better route to longer battery life is to use a driver that has a way to reduce the power output (low, medium, high settings). Turning the power down increases battery life a lot and is simple and cheap to implement compared to what you are contemplating. Half of 1000 lumens is plenty for slower riding like hill climbs.

    Walt

  5. #5
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    So there is no silver bullet here? The xml2's are as good and efficient as it gets in practice?
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.

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  6. #6
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    Quote Originally Posted by Walt Dizzy View Post
    the amount of power LEDs use isn't changed by wiring them parallel vs. series.
    IMO the best way to keep track of all this is to convert all measurements to Watts. LED drive current x Vf (at that current) = LED Watts. Battery voltage x Amp hour = battery Watt hours. Battery Watt hours / LED Watts - 10% (efficiency factor) = approximate runtime.

  7. #7
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    what i meant with wiring in parallell and series was this: pucked up said i would need at least 12 batteries and I enterpreted that as each led needed its own battery or a certain configuration of volts or similar and I just wondered if it works similar to speakers and impedances where you can for example hook up 2 8ohms in parallel to get 4ohms and connect them in series with another pair to end up at 8 ohms again and be able to drive them with the same amp (at probably max 1/4 the amplitude before you run out of juice).

    i just wondered if it works like that here too so I could use less than 12 batteries and/or drivers and so on but still use many leds.

    Anyone following here?
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.

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  8. #8
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    what i meant with wiring in parallell and series was this: pucked up said i would need at least 12 batteries and I enterpreted that as each led needed its own battery or a certain configuration of volts or similar and I just wondered if it works similar to speakers and impedances where you can for example hook up 2 8ohms in parallel to get 4ohms and connect them in series with another pair to end up at 8 ohms again and be able to drive them with the same amp (at probably max 1/4 the amplitude before you run out of juice).

    i just wondered if it works like that here too so I could use less than 12 batteries and/or drivers and so on but still use many leds.

    Anyone following here?

    Short answer: No, it doesn't work that way.

    Sure, knock your socks off. You can wire a whole bunch of emitters in a series-parallel circuit to get the power spread out as you wish. On paper at least.

    Want to know why nobody does it that way? Because LEDs aren't like "normal" loads. Their resistance *drops* as they heat up. Worse, the operating voltage range for an emitter is ~ 0.1-0.2 volt. The reason that everyone avoids parallel wiring for LEDs is even if you were able to match the impedances at room temperature, they won't track exactly as they heat up. The probable result is most of your power will get siphoned off into one branch of the circuit, killing any efficiency gains you got by running them at low power.

    That's why driver circuits for LEDs are current controllers, not voltage controllers. That's why everyone who can do so runs series circuits for multiple emitters.

    Ever seen an LED traffic light start to fail? It's not random emitters going out, entire strings of 10 or more emitters go at the same time. That's because a driver circuit has gone, taking its series string of emitters with it. The lights were converted to save electricity (and for reliability). And they are wired in series strings. What you are proposing would have been done in this application if it worked.

    It doesn't.

    Walt

  9. #9
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    Quote Originally Posted by Walt Dizzy View Post
    Want to know why nobody does it that way? Because LEDs aren't like "normal" loads. Their resistance *drops* as they heat up. Worse, the operating voltage range for an emitter is ~ 0.1-0.2 volt. The reason that everyone avoids parallel wiring for LEDs is even if you were able to match the impedances at room temperature, they won't track exactly as they heat up. The probable result is most of your power will get siphoned off into one branch of the circuit, killing any efficiency gains you got by running them at low power.
    While in a purely electrical sense, parallel LEDs are a poor idea, my practical experience is that it works fine. I have built lights with up to 3 parallel strings of 2 series LEDs (6 LEDs total). Also have 4S2P builds. Never had a failure yet. I do take pains to match the room temp Vf of the pairs and provide a very good heat path for all the LEDs.

    Actually, the only emitter failure I've had was on a 2 LED series light where one of the emitters failed in a closed/shorted condition. This occurred in the first hour or so of use so I attribute it to a faulty LED.

  10. #10
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    Quote Originally Posted by Vancbiker View Post
    While in a purely electrical sense, parallel LEDs are a poor idea, my practical experience is that it works fine. I have built lights with up to 3 parallel strings of 2 series LEDs (6 LEDs total). Also have 4S2P builds. Never had a failure yet. I do take pains to match the room temp Vf of the pairs and provide a very good heat path for all the LEDs.

    Actually, the only emitter failure I've had was on a 2 LED series light where one of the emitters failed in a closed/shorted condition. This occurred in the first hour or so of use so I attribute it to a faulty LED.
    That's great. Glad it worked for you. However, the question the original poster asked was not whether a series parallel circuit was reliable, but whether this would allow him to run 12 emitters at low power to gain electrical efficiency. I stand my my assertion that it's going to divide the power between series strings unevenly and at least partially defeat the goal of gaining efficiency.

    A much more straightforward and less expensive way to achieve low power efficiency with 1000 lumen output is to make a series circuit of 2-4 XML emitters and run them at a current that yields the target output. The total Vf in this case is something that could be handled with a 4-cell battery.

    If you have evidence that your setup is more power efficient than a series only circuit, I'd appreciate hearing about it.

    Walt

  11. #11
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    No claims to greater efficiency, just working around the driver ratings, battery voltage, and LEDs to drive.

  12. #12
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    car bone, you sort of have the right idea - more emitters running at lower current will put out more light than fewer emitters being driven harder for the exact same power (wattage). However, these are bike lights we're talking about, so while theoretically you're right, in practice you'll have to find a housing to fit all those LEDs in and a suitably sized battery to power them.

    Your best bet would be to go for a 2 to 3 LED system and use a driver to drop the drive current when you don't need the light output. As your eyes don't react linearly to light, running a light at half the power (as long as that's enough to see by) isn't half the light according to your eyes. I won't say precisely what it is, as that varies from person to person and depends on a bunch of other things (plus it seems someone argumentative has wandered into this thread). That way you can considerably extend your runtime.

    As for not wanting to recharge your light often, I won't ask why that particular preference, but I will point out that li-ion batteries generally prefer not to be repeatedly discharged to empty. It's much better to recharge after each use than to use try and use all the capacity. Besides, if the worst happened and you ended up stranded or some other kind of calamity occurred, wouldn't you be completely pi$$ed if your light ran out because you didn't recharge it?!

  13. #13
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    IMO one big disadvantage to trying to get efficient by using a large number of LEDs at low current is the size of the light. 20mm is the smallest optic or reflector usable with XMLs and those are pretty floody. If you want a tighter beam to throw more without making the area near the bike too bright you need to go to a larger optic or reflector. Best beam I have seen so far with an XML is the 45mm reflector from Cutter and a 40mm from KD. Two of those make an awesome bar light but any more than that and the housing gets pretty darn huge. A single one of those would be tough to build a low mounted helmet light with.

    If long runtime or not having to recharge after every ride is the goal, add some battery capacity. I made an 18650 holder for my bar light that can hold up to 8 cells. I usually only recharge after every third ride. It will drive the bar light on high for more than 5 hours. Since I rarely run it on high, I easily get 3 rides in the ~2 hour range each with capacity to spare.

  14. #14
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    Thanks for all your answers guys, I really appreciate you taking time to answer.

    I figured more lumens per watt gives me either longer run time and/or more light for any given battery capacity.

    I design and build lots of stuff myself, for my specific situations and needs and all those things are super special tailored just as I want them, so i have no real problem building a light with 10 leds just for fun even if it costs money, I actually like thinking about the problems involved and how to solve them.

    I of course do understand that getting a higher capacity battery gives more more runtime and/or more lumens, but I kinda regard that similar to to building a higher roof when your toilet explodes and floods your whole house, its a solution yes, but I mean, there are more elegant solutions if you know what I mean. So I wanted to find out if it was practically possible at all to get more lumens/W than off the shelf solutions.

    That being said I have never built a light before so I'm a total newbie here. the equipment I have: I have a really good soldering iron and 20-30 tips (at leat 10 smd), a multimeter (fluke 83), tools and spare time. Will this equipment be sufficient to build a light with a finished bought driver?

    So what do you guys recommend I build? I can get stuff cnc'd, but i would prefer to do stuff myself (currently only have access to manual lathe and drill press and material) since I enjoy it more then.

    Can you give examples of builds to look into? I have electronics books from school, about 6-700 pages in total (not mine, got them from my brother), I have to look into current regulators and related stuff.

    I'm not really that knowledgeable about electronics but I have quite a good grasp of the physics involved with frequencies and wavelenghts, and waveforms and wavefronts and similar stuff. I will grind, polish (and test) my own telescope mirrors soon so I'm not totally lost with optics. I also have quite good knowledge of audio synthesis and waveforms and how those spread out spectrally and filters and such and this I feel is somewhat related since i can imagine the drivers are pushing some kind of waveforms somewhere.

    Yeah well I'm buying an oscilloscope soon, a nos crt one to "learn by doing" (measuring) stuff. I know how to use lab measurement equipment.

    I would like to build a light with as many leds as a single driver can drive and then I would like to build a really skinny "flashlight" with my own designed and built driver. How many leds can a single driver drive really?

    Yeah well I'm "all over the map" here but I'm exited, I love building ****.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.

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  15. #15
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    Quote Originally Posted by car bone View Post
    How many leds can a single driver drive really?
    Well the Taskled hyperboost says it can output up to 80V so that would be like 24-25 XMLs. Search that site, they are arguably the best drivers available to the DIY builder.

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    9 XMLs wired 3S3P, TaskLed b3Flex driver and 4S battery. Adjust the current to get the desired lumen output. Done.

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    If you like to tinker, you can always build something like this and strap it on to your bike!

    "By Your Command"

  18. #18
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    The best possible solution would be to use a Taskled driver (high efficiency and tunability) with a remote switch on the handlebar.
    I have a 15W light which is almost always used at 7.5W and I can quickly turn on full power without getting my hand off the handlebar.

    You can also try this simulator: Cree Product Characterization Tool
    the result for 1000 lumen with optical efficiency 90% and electrical efficiency 90%:
    7 XM-L2 U2 @ 350mA - 7.7W total
    3 XM-L2 U2 @ 800mA - 7.9W total

    So a triple XM-L 35mm board with a Taskled driver can be assembled in this housing: M36 LED housing kit - $25.00 : Easy2LED.com, The store for LED DIY
    + a remote switch
    CNC LED light housing for DIY projects

  19. #19
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    lol thats almost nothing to gain there by going with 7 instead of 3 now I understand.

    I wonder what the actual efficiency from battery to photons in the woods are with things you can actually buy today.

    The lens if not coated lose 4% per glass/air surface so thats 96% first crossing and 96% of the remaining 96% out the next one, thats 8% gone in the glass. Unless its broad band multicoated, then its almost nothing. And the lens must have some internal losses too I suppose since its not made from vacuum.

    Also if there is a reflector the best alu coatings I have seen (front side coated so no glass/air crossing) have like 92-94% reflectance and the enhanced coatings just shuffle the spectrum around a bit.

    Anti-reflective coating - Wikipedia, the free encyclopedia
    Optical coating - Wikipedia, the free encyclopedia

    Anyway I bought a light! A gemini Xera flashlight, its for my commuter so I need to be able to remove quickly. It uses good regular 18650s unlike the other lights I was looking at, like the lezynes and niterider 700 or is it 750, well the self contained one. No big difference in price so I feel it was a good deal. Most people probably regard it as overpriced but I think it should be more suitable than the lezynes and niteriders and the shipping was free.

    I will probably have to take it apart to see how and why it works.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.

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  20. #20
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    Quote Originally Posted by mattthemuppet View Post
    (plus it seems someone argumentative has wandered into this thread). !
    Apologies if I came on too strong. Good luck with your build, car bone.

    Walt

  21. #21
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    It is an interesting mental exercise....I went through a similar thought process when I broke my leg a few years a ago...I was bored .

    More leds driven less hard is more efficient and not just becasue of the curved lumens vs current characteristic. If you drive 4 leds with same number of watts as 2, assuming the heatsink has the same overall thermal resistance,

    - each LED dissipates half the power, so gets half as hot. LEDs are brighter at lower junction temps


    Is it worth it? probably not
    What exactly is a rigid hard tail?

  22. #22
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    Quote Originally Posted by Stuart B View Post
    It is an interesting mental exercise....I went through a similar thought process when I broke my leg a few years a ago...I was bored .

    More leds driven less hard is more efficient and not just becasue of the curved lumens vs current characteristic. If you drive 4 leds with same number of watts as 2, assuming the heatsink has the same overall thermal resistance,

    - each LED dissipates half the power, so gets half as hot. LEDs are brighter at lower junction temps


    Is it worth it? probably not
    I don't think that there is much in it as what you gain electronically and thermally will probably be lost again due to the fact that you'll end up using more optics and there will be something like a 10 to 15 % loss with each optic used.

  23. #23
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    Quote Originally Posted by yetibetty View Post
    I don't think that there is much in it as what you gain electronically and thermally will probably be lost again due to the fact that you'll end up using more optics and there will be something like a 10 to 15 % loss with each optic used.
    The losses don't accumulate... 15% from two 500lm leds is 150lm, the same as 15% from one 1000lm led... of course if you want the most lumens you wouldn't use an optic at all... but if you wanted the best beam you might use one led with less lumens and a better optic.
    Last edited by znomit; 10-24-2013 at 10:28 PM.
    DIY LED Bike Lights:
    A few Dynamo builds and some Small battery lights

  24. #24
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    most efficient way of getting 1k lumen?

    Quote Originally Posted by yetibetty View Post
    I don't think that there is much in it as what you gain electronically and thermally will probably be lost again due to the fact that you'll end up using more optics and there will be something like a 10 to 15 % loss with each optic used.
    As already said more optics won't make it worse. Percentage loss is same no matter how many are used.
    What exactly is a rigid hard tail?

  25. #25
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    How does spectrum play in here? As in visibility i mean. If the spectrum is continous from 400 to 800 and flat would that make the light seem brighter somehow?

    I once had some type of light that had a spectrum like that (from some store/shop to display products) and it had this magical shine to it. Like the sun.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.

    Quote Originally Posted by iheartbicycles View Post
    Specialized sucks ass.

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