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Having just successfully completed a basic single Luxeon 3W LED as a trial to see if it was worth taking this further, I'm after something more powerful. A lot more powerful. I'm planning on a triple or Quad Cree XR-E unit at either 700ma or 1000ma. The problem is that all of my lighting stuff to date is based around 6v-7.4v battery packs, of which I have a lot, including some nice light smallish but high capacity Li-Ion packs (8ah and 12ah). I'd like to use these for this new project so that all of my light kit is compatible - useful for 24hr solos etc and general ease of use (charging, using different battery/lamp combinations etc). So I'm assuming that I need a boost driver, rather than the stepdown type I used on the Luxeon 3 (the CC5W from TaskLed).

So - is this a reasonable thing to do? I've read somewhere, here or on Candlepower, that it makes a lot more sense to use a correct voltage batt and step down driver than a boost one. Why would this be? Is it small %'s of difference in efficiency, or a far more major problem? Has anyone run a similar setup? Another reason I was looking at this is that, if the Crees are assumed to be about 4v forwards voltage each at 700ma, and you usually need 1v 'overhead' for the driver to work, it's going to be close running 3 even with a 14.4v Li-ion battery, and it certainly won't be able to drive 4. The TaskLed Fatman boost driver claims 90% efficiency or better, runs up to 4 Luxeon 3's (so I'm assuming 4 Crees, could be wrong) and looking at the spec sheets and carrying out the calculations, might be run at a higher input current than reccomended (2.4A instead of the reccommmened 2A) with 4 Crees, but is 1.8A so fine with 3 LEDs. I'll email george too about this, but wanted peoples thoughts who may or may not have used the drivers in the same application. Or are there any other drivers out there that are suitable?

Ian
 

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SSBonty said:
Having just successfully completed a basic single Luxeon 3W LED as a trial to see if it was worth taking this further, I'm after something more powerful. A lot more powerful. I'm planning on a triple or Quad Cree XR-E unit at either 700ma or 1000ma. The problem is that all of my lighting stuff to date is based around 6v-7.4v battery packs, of which I have a lot, including some nice light smallish but high capacity Li-Ion packs (8ah and 12ah). I'd like to use these for this new project so that all of my light kit is compatible - useful for 24hr solos etc and general ease of use (charging, using different battery/lamp combinations etc). So I'm assuming that I need a boost driver, rather than the stepdown type I used on the Luxeon 3 (the CC5W from TaskLed).

So - is this a reasonable thing to do? I've read somewhere, here or on Candlepower, that it makes a lot more sense to use a correct voltage batt and step down driver than a boost one. Why would this be? Is it small %'s of difference in efficiency, or a far more major problem? Has anyone run a similar setup? Another reason I was looking at this is that, if the Crees are assumed to be about 4v forwards voltage each at 700ma, and you usually need 1v 'overhead' for the driver to work, it's going to be close running 3 even with a 14.4v Li-ion battery, and it certainly won't be able to drive 4. The TaskLed Fatman boost driver claims 90% efficiency or better, runs up to 4 Luxeon 3's (so I'm assuming 4 Crees, could be wrong) and looking at the spec sheets and carrying out the calculations, might be run at a higher input current than reccomended (2.4A instead of the reccommmened 2A) with 4 Crees, but is 1.8A so fine with 3 LEDs. I'll email george too about this, but wanted peoples thoughts who may or may not have used the drivers in the same application. Or are there any other drivers out there that are suitable?

Ian
Too many questions :p ...

It used to be very difficult to find a boost converter for the kind of current you need, and the efficiency was pretty low. But things change quickly with electronics, and with increased demand - who knows :eek: .

When I last looked at this, it was hard for any converter circuits to maintain high efficiency over the wide range of input voltages you get in battery applications. It was easier to get the step-down (buck) converters in high efficiency than the step-up (boost) ones. The disadvantage of step-up is that you will need proportionally higher amp-hour batteries for the lights and you may be better off just adding more cells of a lower rating - a trade off point which changes as battery and electronic converters advance.

So... I use step down for my light systems. They work fine and I don't need to change that for now. Either 13.2V Nimh or 14.4V Li-ion. I have had success using matched 7.2V 2000mahr L-ion packs in series to get 14.4V. You may want to try that... A 14.4V Li-ion 2200mahr pack with triple K2 leds and a bFlex converter at 1000ma gives me 3 hours burn time and a really bright light for 24 hour races....

Good luck!
:cool:
 

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buck/boost question

Do they just regulate current in or do they also reuglate voltage--ie would there be a different light output running a 13.2, 14.4 or 16.8v battery with a buck puck (Constant current)

Thanks
 

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twelve_volt_man said:
Do they just regulate current in or do they also reuglate voltage--ie would there be a different light output running a 13.2, 14.4 or 16.8v battery with a buck puck (Constant current)

Thanks
All leddrivers are a current source. So the output is always say 700mA, no matter or there is one led connected or there are 3 leds in series connected. A normal regulator is a non-switching driver, when you have a led with a forward voltage of 3,6V for 700mA and your battery has a voltage of 7V the driver just dissipates (7V-3,6V)*0,7A= 2,38W. A step-down driver doesn't dissipate but there need to be more components on the PCB which makes them a bit more expensive.

@TS. When you really want to build a quad led light with the batteries you have I would do the follow. Connect to Li-ion packs in serie, that gives you a voltage of 14.4V. That is not enough when you want to run a Cree (or SSC P4 or Edison K-series) at 1A. So I would use to step-down regulators for 2 leds. You could use these drivers for example, it will be very easy with them to make several output levels with them.
 

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Despite what others are saying, I'd suggest that using a Fatman boost converter with your batteries to drive 3 or 4 Crees is a perfectly reasonable thing to do. I'm using a Fatman with my current triple Cree light http://forums.mtbr.com/showthread.php?t=258729 for similar reasons to you, and it works just fine - tested efficiency is up around the 90% level with a 2 cell LiIon (nominal 7.4V) battery, which is no worse than a buck converter will be. I did also test it with a single cell LiIon at which point the input current is getting towards the recommended limit, where it does become noticeably less efficient

My Crees are only about 3.3V Vf at 700mA, so you could if you wanted run 3 in regulation using a 14.4V pack and a buck converter, but not 4. This also means your current calcs for input current are wrong - I reckon you only need a maximum of about 1.7A input current for 4 (when the LiIon battery is discharged at 6V), or only 1.25A for 3, which is about what I measured.
 

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Yep you're spot on Chris. Generally you see better efficiency numbers when the overhead between the input / output of the regulator is as close to the minimum recommended as possible.

Depending on cell / regulator / emitter choices you could quite possibly make a boost regulated system more efficient then a buck, or vice versa. It's just a matter of sitting down and doing the numbers to see what will work or not. Keeping in mind your current packs and undecided emitter quantity and forward voltage, it sounds like a boost regulator would be a suitable choice. :thumbsup:

Dave.
 

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Crap, I thought I was all set with my 4 Cree's, b-flex and a 14.4v li-on battery. I guess I'm not.:madman: Will the fatman work with a 14.4v pack? The specs say 2.4v to 12v. Any recommendations? I've almost finishing my housing so I'd like to stay with 4 Cree's if possible.
 

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Do you know the exact forward voltage of your emitters at your expected drive current? You’re going to be right on the borderline…

Depending on how hard you were going to drive them, you may be better off with three emitters driven a little harder. You’ll loose a little efficiency and possibly light output, but it may be negligible depending on your original planned setup. ;)
 

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Yeah. What Dave said. It might work alright, you are on the borderline... and actually your 14.4v (probably 14.8v?) Li-Ion battery will typically run well over 16v when charged. You might also consider running the bFlex at a lower drive current (500ma) with all four, but driving 3 LEDs harder might work better (as Dave said). Just experiment. That's what it's all about :)
 

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Remember that to start with it is likely the forward voltage of your emitters will shift a little after a bit of use. It might be worth running them for a while on a suitable, well cooled heat sink.

Also the Cree XR-E generally has a pretty low forward voltage when driven at lower drive currents, obviously dependent on what bin you end up with. It's really something you need to measure yourself.

Dave Not Thinking said:
I'm not really thinking straight today so please correct me if I'm wrong here, but two series connected emitters in parallel with another two series connected emitters should halve the forward voltage. It would also halve the current too assuming a constant current driver, so a 1 amp driver would allow each emitter to draw 500mA? It might be a way of saving you from cutting up that nice housing, and may be a better idea thermally too.

And 4 emitters at 500mA would still be bucket loads of light! ;)
Dave.

EDIT: Please ignore the text in quotes, I just wasn't thinking... :nono:
 

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Dave,

Running any high power LEDs in parallel from a single current source is not a good idea at all. All the diodes are not the same, and they have different internal resistances. So if you run them in parallel with 1A (with the idea of getting 2 times 500ma), then the one with lower forward resistance will get most of the current. Basically this is a gambling not worth playing, as the resistance changes a little with time, I think.

Candlepowerforums had a quite extensive discussion on the subject from much more knowledgeable guys than me.

Stefan

Low_Rider said:
Remember that to start with it is likely the forward voltage of your emitters will shift a little after a bit of use. It might be worth running them for a while on a suitable, well cooled heat sink.

Also the Cree XR-E generally has a pretty low forward voltage when driven at lower drive currents, obviously dependent on what bin you end up with. It's really something you need to measure yourself.

I'm not really thinking straight today so please correct me if I'm wrong here, but two series connected emitters in parallel with another two series connected emitters should halve the forward voltage. It would also halve the current too assuming a constant current driver, so a 1 amp driver would allow each emitter to draw 500mA? It might be a way of saving you from cutting up that nice housing, and may be a better idea thermally too.

And 4 emitters at 500mA would still be bucket loads of light! ;)

Dave.
 

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Hi Stefan,

Sorry about that, yes you are correct on both accounts as it isn't an ideal solution. I told you I wasn't thinking straight today! :p Come to think of it, I think I've actually mentioned this here on MTBR in the past… :madman:

The problem is that as one emitter pulls more current it will warm up and the forward voltage of that emitter will drop, allowing it to pull more current. The emitter next to it will cool down and the forward voltage will rise, allowing the first emitter to draw even more current. In the long term you will begin to see a significant difference.

The forward voltages of the Cree XR-E emitters that I have seen have all been very close, and you could probably *just* get away with it if the emitters were thermally managed really well, but it's not worth the risk. Sorry wittyname, a three emitter setup may be your best option at this stage.

Sorry to drag your thread off topic Ian, maybe we could remember to start individual threads for new topics in the future please everyone?

It's worth emailing George but I would be inclined to do the same, use three emitters and drive them nice and hard on a well made heat sink. As you may have noticed browsing CPF, there are quite a few boost regulators available besides the Taskled Fatman, but I love George's drivers and highly recommend using them. :thumbsup:

Cheers, Dave.
 

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@Wittyname
Your housing looks really good, it would a pity to get one led of. I would use 2 drivers, so 2 leds with one driver. A Dutch guy that lives in Shangai sells a step-down Pcb that have an input spec of 12 to 24V. The drivers are 30x23x10 mm so I don't know or you can fit 2 of them in your housing.

I am getting these drivers and some step-up driver in a few weeks, I will let you guys know how they to their job.
 

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Just in case any of you here haven't got it (or somebody new is coming to this topic) the issue with running 4 Crees with a nominal 14.4V LiIon pack is that that voltage is only nominal - it will go from just under 16.8V fully charged down to 12V when totally empty. Since the total Vf of your Crees is likely to be about 13.4V and you need 1.5V headroom for a bFlex, then it will only stay in regulation when the battery is above 14.9V, or less than half the discharge.

You can't use a boost driver either - the issue being that when the input voltage of your battery goes above the regulated Vf of the LEDs, a boost driver goes direct drive - ie the LEDs get full battery voltage. With 16.8V they will be toast very quickly!

In fact the only solution for 4 Crees and a nominal 14.4V pack is a buck/boost converter.You can get these - I think the Sandwich Shoppe sells one - but they are less efficient.

Alternatively you need to either go for a lower voltage (2 or 3 cell LiIon) pack with a boost, or a higher voltage (5 cell or more LiIon) with a buck.
 

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Discussion Starter · #17 ·
Low_Rider - don't mind at all, it's all about similar stuff and this is the kind of thread I've learnt the most from in the past where different people contribute different stuff, some at a tangent, some straight at the original question. All good!

I emailed george @ TaskLed about the original question - does a boost driver (specifically his Fatman) work with my planned 7.4v Li-Ion/4x Cree Q2 @ 1A setup. Here's the reply:

"Hi Ian,
the problem with boost converters is they draw more and more current as the battery discharges. So, as the battery discharges and becomes weaker and less capable of providing current, the boost converter demands more and more current. Not really a good situation for a bike light where you want good runtime and no sudden battery cutoff.

Without circuitry to taper off the current demands of a boost converter, it wouldn't be the type of driver that I'd recommend of a bike light.

4 Crees at 1A would definitely be too much of Fatman. Especially with only a 7.4V nominal pack. 3 Crees would be ok - but of course you still have the issues of a boost converter.

You could also use 2 of your li-ion packs in series and run a buck converter. Check out the latest bFlex firmware that's now available on my website. I've put in a bike specific interface that might appeal to you.

cheers,
george."

Starting at the end - my two 7.4v li-ion packs are different voltages and shapes and I don't really want to combine them, so that's out. As is another suggestion from above, using two drivers - no room in the housing! George's thoughts on running 3 Crees at 1amp with a Fatman is that it should be fine, 4 less good, but the exact figures will depend on the Vfs of the emitters. So my first question:

- Does the Vf rise with input ampage i.e. if I drive at 1A as opposed to ChrisM's 0.7A will I get a higher Vf? And will it simply be a case of measuring when I get the LEDS before working out if the figures are safe? That was why my figures for input amps for the Fatman were a bit higher than yours ChrisM and above the reccommended ratings, I used what I guessed (from the basic specs) would be the Vf for a cree at 1A, and input 4v.

Anyway, it looks as though I can either run a 3 @ 1A or 4 @ 0.7A setup with a Fatman and my preferred batteries, either of which seems reasonable. In real world conditions is the 3 @ 1A or the 4 @ 0.7A preferable, and which is likely to give the better light output? I'm leaning towards running 4 @ 0.7A, and after getting it working maybe using the trimpot on the Fatman to up the drive current to somewhere between 0.7A and 1A.

Final question, and relating to what George mentioned about suitability of the fatman in a bike light:

- How do I go about calculating run time with this setup (lets assume 4xQ2 Cree, Fatman at 0.7A, and nominal 7.4v 12ah battery? I'm OK with Halogens! This leads into Georges concern about sudden battery cutoff, and presumably also runtime as with a big enough battery I shouldn't hit the lowered voltage - higher current drain - further lowered voltage positive feedback loop... I'm looking at 24hr solos by the way if this kind of battery capacity seems excessive!

Ian
 

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When you want to send a higher current through the the forward voltage is a bit higher. This is the graph from the datasheet from Cree, I added somelines to make it clear:


So the difference in forward voltage between 750 and 1000 mA is not more then 0,2V. With the 4 leds you have you win only 0,8V in the best case.

Calculating batterylife is quite easy:
1: Calculate how much power there is in your battery: in your case 7,2V * 12Ah = 86,4 Wh
2: Calculate how much the battery's are going to dissipate: power = number of leds * forward voltage * current = 4 * 0,750 * 3,7 = 11,1 W
3: You need to know the efficiency of the leddriver, when you don't know I would always use something like 80% or 85%.
4: the final calculation: I assume the efficiency is 85% so, (0,85*86,4Wh) / 11,1W=6,6 hours

But it isn't a problem to connect batteries that have different voltages in serie with each other. When you have a battery that is 7V and the other one is 8V, you simply will have a voltage of 15V. There is only one think that can be a pity, when your 7V battery has a capacity of let's say 5 Ah and your 8V battery 2Ah then your "new" battery can only deliver 2Ah and it is over.
 

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SSBonty said:
Anyway, it looks as though I can either run a 3 @ 1A or 4 @ 0.7A setup with a Fatman and my preferred batteries, either of which seems reasonable. In real world conditions is the 3 @ 1A or the 4 @ 0.7A preferable, and which is likely to give the better light output? I'm leaning towards running 4 @ 0.7A, and after getting it working maybe using the trimpot on the Fatman to up the drive current to somewhere between 0.7A and 1A.
Sorry - I missed the fact you were planning on running at 1A - in which case I'd agree with George that 4 is too much, and you've identified your options correctly.

I've thought about running my triple at 1A - in fact I've actually tried it out to see the difference. Theoretically you get 25% more light for 50% more power, which I'm not convinced is worth it. Practically there really wasn't a huge difference, so I think I'm sticking with a top end of 700mA.

In theory, the quad at 700mA will give you ~560lm (with the latest bin Crees) and the triple at 1A 525lm. Meanwhile the quad at 700mA takes ~10W from the output of the driver and the triple at 1A takes about 11W. So you'll get better runtime and light from the quad, but obviously you have to trade that off against the added size and complexity compared to a triple. One other thing in the quad's favour with a Fatman and that battery is that if you are turning it off via the shutdown pin on the driver it will turn off, whereas the triple will still shine slightly as the battery voltage when fully charged is high enough to drive a few mA through the LEDs on direct drive (even more an issue if using a MaxFlex when those become available).

One point I don't agree with George about - and I'm really surprised he doesn't seem to have thought this through properly - is the issue with the increasing current with a boost driver. In reality if you are using LiIon cells with a low voltage cutoff it makes no difference at all whether you run a buck or a boost - in both cases the current will increase as you discharge. You could take 4 LiIon cells and wire them either all in series and use a buck or in series parallel (to make a 7.4V pack) and use a boost, and use either to drive 3 Cree LEDs. At the end of discharge cutoff voltage the current each cell sees is the same for both examples - assuming 8W input to the driver about 0.67A for both. Also the rate the current ramps up is the same in both cases. About the only justification for the argument is if running your cells into the ground. In this case the buck goes out of regulation, after which point the battery current drops, whilst the current the batteries see keeps rising with a boost - though since we're not doing this it's not a useful argument.
 

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Sorry, I miss this one earlier.

The main issue with boost converters is that they are less efficient, not only in conversion, but general battery performance and wiring. A good boost converter is usually a few % less efficient than a good buck converter. However, drawing power using a high current and a low voltage is less efficient than using a high voltage and a low current because of resistive losses and battery performance/capacity. So it does add up, especially if you are trying to maximize efficiency.

However! I have triple Luxeon-III units that run off a 6V battery (boost converter) and similar unit (plus a dual Lux-5 unit) that runs off buck converters, and in general they work fine.

In both cases you can limit the battery draw by design - with the 6V system, my boost converters are running close to their limit, which means if the battery voltage depresses a significant amount, the boost converter starts reducing it's output current since it has reached it's limit of "boosting". Basically my system will passively reduce the output current as the battery voltage drops. I do not use a "smart" low battery cutoff, most of our night riding is in winter (<0C) and most smart circuits fail miserably at low temperature, especially with cheap or older batteries. And I can run any type of battery on the same unit.
 
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