# Understanding Multi-Emitters and Drivers.

I’ve recently branched out into into multiemitter flashlights. I gained some insight into running them in parallel vs series direct drive; however, I’m a bit confused as far as running them with a driver.

The lights in question are a Sky Ray King (3x XML) and a BTU Shocker.

SRK runs 4 batteries in parallel.
BTU runs 3 batteries in series.

Let’s upgrade the BTU to a 5 amp regulated driver. Alright, so we’ve now got 12v across 3 batteries in series running at 5 amps… Is Each emitter pulling 1.7 amps then?.. That can’t be… Each pulling 5?.. :~

SRK runs reasonable stock at about 6amps iirc. So we’ve got 4v supplied with 1.5 amps supplied by each battery in parallel. Each of the 3 emitters pulling 2 amps? That would make sense to me, reasonable 700 lumens for each emitter.

I think someone should make a good video on how to use a DMM ? I no there is one but that’s with pics but I’d like to see a video

Like what does the symbols that u use mean and how to read on how many amps your driver is putting out ect……just saying because I’m not the best at using a DMM

5A to a series string of LEDs = 5A to each LED. What goes into the first one has to come out, and the only place for it to go is into the next LED, and so on... but it's at a higher voltage. Convert to watts (amps x volts = watts) to get numbers that make sense when comparing series vs. parallel LEDs or series cells/parallel LEDs or the opposite. Input power is calculated separately. The difference between the input watts and output watts is the driver efficiency.

If you only have one set of numbers, either input or output, you can make a wild guess at the driver efficiency and get a fair estimate of the other set of numbers. If output is 5 amps at 11.4 volts, the power is 57 watts. Optimistic guess at the driver efficiency of 90% means the input will have to be 10% higher than the output. 57 + 10% = 62.7W. Assume an input voltage of 4v/cell under load gives Vin of 12v, 62.7W / 12v = 5.23A input current. But it won't stay there for long...

3 cells in series + 3 LEDs in series doesn't really have enough voltage overhead for good regulation. It's the same as 1 cell+1 LED x 3. You'll run into the same cell voltage vs. forward voltage issues as you get in single cell lights, which will limit the max current you can get out of it. You really need 4 cells in series to properly run & regulate 3 series LEDs.

XML2's Vf at 5 amps is damn near 3.8v, because of driver losses that means the current & light output will start to fall off when the cells are down to only 3.9-4.0v each. Which is likely to be only a few dozen seconds after you first switch it on with fresh cells.

If you have 4 cells in series, the minimum voltage when the cells are totally flat is 12v (3v x 4). The 3 LED string's total Vf is 11.4v (3.8v x 3) at 5 amps. It's likely to drop off a little towards the end, but not until cell voltage is down to 3.1-3.2v. Many times longer than the same driver with the same LEDs, just with one more cell to keep the minimum voltage above what the LEDs need.

If you're limited to 3 cells, driver selection is tricky. Are there any boost drivers that can do 5A? No, and besides a boost driver wouldn't be happy at the start with fresh cells, since input voltage would be higher than output voltage. A buck or linear driver will be essentially direct drive, where the 'regulation' comes from the falling cell voltage limiting the current through the LEDs. Switch the LEDs to parallel though, and you'd need a buck driver capable of 15A (15A / 3 = 5A each), and there aren't any of those available, either.

Great chunk of info! Thank you.

So a 5amp driver (from lck led) in a btu turns it into an amazingly bright, but brief light launcher.

How would calculate a rough runtime with this setup?

Comfychair that little lesson helped clear things up for me a great deal. All the different threads on SRK’s and ideas on how to pump them up now start to make sense- I think. So tell me if I have this right; I would get more efficiency for longer on an SRK with 3XML’s using 4 cells- I could extend that even longer with 18650’s that can be charged to 4.35 volts because of the higher voltage across the 4 cells. As opposed to 4 cells charged to 4.2v and driving 4 XML’s. ( or even driving 3 XML’s )

Runtime before it drops below 5A or runtime until the low voltage warning/shutdown?

You'd need to find a discharge curve for your cells, and a current/forward voltage graph for the LEDs, and then do a bunch of guessing and estimating. If the output current stayed constant it would be easy, but once it's dropped regulation and voltage falling causing current to fall too, it's easier to put it together and measure it directly. Too much guessing. It gets into a situation kinda like a feedback loop where voltage falls which reduces the current, and less current means less load/voltage sag and less sag means MORE current, and so on. It finds its own balance and just does what it's gonna do. There's even more to it that's not taken into account, like as the LED gets hotter the Vf drops, and lower Vf = more current. Which makes even more heat, and... you get the point.

It all gets much much easier when you have enough voltage overhead (4 cells instead of 3) to keep it truly regulated from start to finish.

I have thought about this before. I think this explains why Fenix TK75 uses 4 cells in series rather than 3 cells in series (like the BTU Shocker).

No, that was all to do with the BTU example, with series cells & LEDs. The parallel/parallel SRK-like example is different.

Adding more cells in parallel helps a little bit, but not nearly the same thing as adding cells in series. In the SRK, you could have fifty cells in parallel but they will suffer the same poor-regulation thing, the total input voltage will still be below the LED's forward voltage when the cells are flat, and current/light output will fall off. The optimum situation is to make sure the lowest input voltage will always be higher than the LED's Vf at your desired output current, even when the cells are fully discharged. Or, use a boost driver, but the choices there are rather limited (boost drivers that will run from single cell voltage won't do the big amps needed). You still end up needing to change the cell configuration, and if you do that, there's usually no reason to restrict yourself to a boost driver anymore.

Alright, I think I’m getting it.

So when it drops off regulation, it fluctuates a bit and eventually finds an equilibrium of sorts… Seems like it would be a bit of a crapshoot, but any idea as to a rough, ballpark guesstimate as to what that equilibrium would be? I imagine quite a bit situational, but are we looking at less than 4 amps, less than 3?

http://lygte-info.dk/review/batteries2012/Common18650comparator.php

You need around 3.8v for each LED to be able to hit 5A, which due to driver losses will be around 4v per cell on the input side. At 3.7v per cell, you'll be down to an output voltage of 3.5v per LED, and the max current the LEDs will run at with that voltage is around 3.4A. And it just keeps going down. When the 3 cells are dead flat at 3v each, the max current the LEDs will allow is approximately nothing.

So you see why you can't take a straightforward battery discharge curve, overlay it on the LED Vf chart, and get a solid number out the other end. The battery discharge is done at one set current for the entire run, while in a direct drive(ish) situation, the current is constantly falling as the voltage falls.

So this brings me back to a conversation we had on another thread. If I try to run 3 XML’s on 4 18650’s- I forgot how to say it, but 2 pairs of 2 in series?… So 8.4 volts to the driver and more amps gives too much power and melts the springs. I think I understand. Please correct me if I have it wrong. This is great, I haven’t learned so much since my senior year at the U’.

So 3 XML in parallel, 8.4v input? If you use a driver, and pick the right one, it will be fine. If it's direct drive, I don't think it would work with only 3 XMLs. Five XMLs in parallel, and weak cells, maybe.

For a driver, any buck driver that's safe with the 8.4v input will work, but, with LEDs in parallel the output current will be equally divided between them. A 3 amp driver would only run each LED at 1A. A 9A driver would run them at 3A each (but, all the drivers I know of that'll do that kind of current run too hot and trip their own overheat protection, even when they are kept separate from the heat produced by the LEDs).

There are some decent choices for boost drivers if you go with 8.4v input & 3 LEDs in series. As always though the catch is finding one with the right specs, and the right dimensions to go where you want it. In some builds there just isn't a driver that that will do both.

For the battery config terminology, it's usually stated as '_S_P', the number of cells in series comes first as that gives the total voltage, adding more series strings in parallel just increases the capacity and the potential peak amperage. Four cells, two pairs of two series cells, is '2S2P'.

Cells in series keep the same capacity, in mAh, as that's a number that totally ignores the working voltage. Two 3000mAh cells in series are still 3000mAh total. But the voltage is doubled - you need the numbers converted to power (watt-hours) before they're directly comparable.

Cells in parallel keep the same voltage, but double the mAh. The watt-hours for 4S, 4P, 2S2P, assuming the same cells just connected differently, keep the same power or watt-hours, just at different voltages and capacities. It all works out to the same power no matter what.

2S2P for me. Thanks for that explanation. The more I learn the more I understand I have a lot more to learn. The idea of incorporating a driver to safely regulate the energy sounds like a nice layer of safety especially in relation to being a novice at this. I can’t wait for Christmas so can start modding.

Hi comfychair, I have other questions though.

Assuming with the same LED, same current supplied at 1A, and everything else the same. There are two identical 3400mAh 18650, and wiring them in series will give the input numbers of 8.4V and 3400mAh; wiring them in parallel will gives 4.2V and 6800mAh, is my understanding correct?

If yes, so under which condition (series/parallel) will give longer runtime? Or will they both give the same runtime?

8.4 x 3400 = 28560

4.2 x 6800 = 28560

That's what the cells are capable of, what the drivers used for each setup do with that energy is a different story.

8.4 x 3400 = 28560

4.2 x 6800 = 28560

That's what the cells are capable of, what the drivers for each setup do with that energy is a different story.

28560mW = 28.56 watt-hours. 28.56 watts, for one hour. Or 1 watt for 28.56 hours.

So that means both of them will give the same runtime if everything else being equal?

It only means they have the same amount of energy. What the driver does with it, and if the voltages are compatible with using all the potential energy in a practical way is the catch.

In theory, a 90% efficient driver with input spec of 3-8.4v should require ~3.3A input to get 3A output from 2 cells in parallel, and half that, or 1.65A, to get the same 3A output with 2 cells in series. But that's not taking into account the input voltage vs. LED forward voltage issue when run from 2P/4.2v input though. The 8.4v/2S setup, even though the potential energy is the same as the 2P, will be fully regulated and be able to output the full 3A all the way down to the low voltage shutoff at 5.5-6v. The 4.2v/2P setup will suffer a drop in output, and with less current draw & output, it will take longer for the cells to go flat. The overall runtime may be longer, but only because it gets continuously dimmer.

Boost drivers are typically much less efficient than buck drivers. Apparently it's really easy to make a bad boost driver, and fairly easy to make a good buck driver.