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Zflyer08/08/2020 15:42:21
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I intend to power a set of LED's using the balance lead of my lipo., The LED's are set to cell voltage. Will the below arrangement work.

3s lipo led.jpg

Denis Watkins08/08/2020 15:48:31
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Just the last two Zflyer, will not share equally, as one LED will be brighter

It is easily balanced by another resistor in series with the brighter LED

Phil Green08/08/2020 15:49:45
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Not sure what you mean by "The LED's are set to cell voltage" ?   LEDs have a fixed forward voltage and need some method of controlling current, either resistively or with PWM.   

Your arrangement will pull the Lipo out of balance, you have two leds running from one cell, another running from two cells, and another running from 3 cells. 

I would put all 4 in series across the whole pack, this would maintain balance, ensure they all get the same current, and waste less power in the current limiter.

To answer properly we need to know exactly what you're planning & what components you've used.

Cheers
Phil

 

Edited By Phil Green on 08/08/2020 17:52:28

Zflyer08/08/2020 19:04:02
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Thank you Dennis, that makes sense, though I would have thought as both are wired in parallel it would have been okay.

Phil, the leds are wired to separate cells via the balance led.

Denis Watkins08/08/2020 19:14:22
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Posted by Zflyer on 08/08/2020 19:04:02:

Thank you Dennis, that makes sense, though I would have thought as both are wired in parallel it would have been okay.

Phil, the leds are wired to separate cells via the balance led.

Logic with parallel bulbs comes close, but LEDs have no filament so try it

But be ready for one brighter than the other, or balance them beforehand

Geoff S08/08/2020 20:04:23
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Posted by Zflyer on 08/08/2020 19:04:02:

Thank you Dennis, that makes sense, though I would have thought as both are wired in parallel it would have been okay.

Phil, the leds are wired to separate cells via the balance led.

But even so, unless there's a series resistor built into the LED to set the current, you have no means of knowing how the circuit will behave and you could damage the LiPo.

An LED has a set voltage across it (around 0.7v) when it's forward biassed as it will be here. The usual way is to fit a series resistor depending on the current the LED needs to light up and the applied voltage (in this case about 4v). At a guess the each LED needs 10mA so you need to drop 4 - 0.7v = 3.3v. So the resistor needed is 3.3/0.01 = 330 ohms for each LED.

Note the spec for the LED is a guess on my part but it's reasonable for the LEDs I've used in the past.

Geoff

Engine Doctor08/08/2020 20:10:01
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Google Lipo battery balance lead wiring . It should then become clear idea

Zflyer08/08/2020 20:34:23
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Gents the LED's all have resistors and I have literally just tried using them in parallel and have found no difference in brightness.

Allan Bennett08/08/2020 20:43:39
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It might help us if we knew what is your purpose for connecting them that way? Depending on the battery size, the small current required by LEDs is unlikely to throw the pack out of balance by wiring it that way, but why not have all LEDs (with their resistors) in parallel and feed them from the red and black wires in your diagram in order to ensure pack balance?

Phil Green08/08/2020 20:45:16
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So these leds have inbuilt resistors, presumably they are the 5v ones.   So you can in theory put one (or more in parallel) leds across each single cell, NOT like your diagram but one (or a parallel bunch) between red & yellow, one (or another parallel bunch) between yellow and green, and one (or yet another parallel bunch) between green and black. However if they are different colours or different numbers in the bunches they will draw different amounts of current and your lipo would be pulled out of balance by the different loads on each cell.   So this would work but its a very bad idea.

With the components you have there you could run them all in parallel from the 5v BEC (or receiver battery) or you could run two similar bunches in series from the outer balance connections, ie from full battery voltage.  You would need to match the bunches pretty well to ensure both halves each saw about half the pack voltage.

Either way would preserve cell balance.

Taking your drawing as posted, the answer to your original question is no, you have some across one cell, some across two and some across 3 cells, so the ones on the left of your diagram are connected to 12.6v and 8.4v, and the ones on the right to 4.2v ....  so each led or parallel bunch sees a different voltage.
You say "Phil, the leds are wired to separate cells via the balance led" (lead?) but thats not what you've drawn smiley 
All your current-limited leds need to see roughly the same voltage though some variation wont hurt, it doesnt need to be exact, a 5v resistive led is happy with say 3.5 to 7v or so, the '5v' spec is conservative for long life.

The crucial point that you didnt mention is that you have leds with a built-in resistor.  Otherwise the forward voltage of a non-resistive LED is between 1.8 and 3.3 volts depending on the colour, unless they're high-power ones like Crees. Reds are about 1.7 to 2.0 volts, blues 3 to 3.3 volts, with yellows and greens inbetween. If you connect unprotected leds directly to a low impedance source of 4.2v (ie a lipo cell) without current limiting resistors, they will blow. The ones on the left of your diagram connected to 8.4v and 12.6v will blow spectacularly! 
This is irrelevant however since you do have resistive leds smiley

Cheers
Phil

 

 

 

Edited By Phil Green on 10/08/2020 09:56:55

Zflyer09/08/2020 13:54:49
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I have connected the LED's to the balance connectior on a 3s lipo as per my diagram. All working with no apparent problems. 20200809_124110 1.jpg

Phil Green09/08/2020 14:16:21
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Posted by Zflyer on 09/08/2020 13:54:49:

I have connected the LED's to the balance connector on a 3s lipo as per my diagram.

Some you just can't help wink

Posted by Zflyer on 09/08/2020 13:54:49:

All working with no apparent problems.

Whats not visibly apparent is that cell one is powering 4 leds, cell two is powering 2 leds and cell three is powering 1 led.  How do you propose to keep the cells balanced?   
You asked a question, several people took the time to competently answer, its your choice to ignore them all, but I wonder - why ask in the first place?  smiley

Your diagram as posted:

stupid_leds.jpg

 

Edited By Phil Green on 09/08/2020 14:52:46

Denis Watkins09/08/2020 14:40:09
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A photo appears, as if by magic or Photoshop

Of a scene not duplicated at all in the original diagram or dialogue.

What is the purpose of this jest as some unwitting soul may take it seriously.

Phil's post has just appeared before my similar question

Edited By Denis Watkins on 09/08/2020 14:51:42

gangster09/08/2020 14:50:37
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I have to ask the question ... Why do it like that? It would seem to be a whole load simpler just to connect 4 leds to either the BEC supply or even across the pack. I can see no merit at all in using the individual cells in the pack to supply just a few mA to light the led. I do not mean any criticism at all because it in many respect one of those seems a good idea at the time ideas. The other point is, and I am not disputing it at all because I am not certain but why would it unbalance the pack ? Nice low resistance high current cells And a few mA . As I said not disputing but what am I overlooking please?

Phil Green09/08/2020 14:55:49
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Posted by gangster on 09/08/2020 14:50:37:

what am I overlooking please?

I think its this bit:

Posted by Phil Green on 09/08/2020 14:16:21

Whats not visibly apparent is that cell one is powering 4 leds, cell two is powering 2 leds and cell three is powering 1 led.  How do you propose to keep the cells balanced?  

 

Edited By Phil Green on 09/08/2020 17:30:26

Cuban809/08/2020 17:11:58
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Had to have a giggle at this wink.......far too hot and humid outside and people are getting worked up over a piddling LED circuit.

Oh and by the way, a typical red LED will exhibit a Vforward of a nominal 2V rather than the typical 0.7V for a regular silicon rectifier diode as has been mentioned - probably not that critical in most simple applications but might catch you out in certain designs.

Back to the garden for a cold beer..............................

Edited By Cuban8 on 09/08/2020 17:16:00

Phil Green09/08/2020 17:26:20
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You're right C8, if someone asks a question its really not worth the trouble of answering it.... wink

No-one reads the replies anyway... for example I'd posted the typical Vf of various coloured leds...  laugh

 

Edited By Phil Green on 09/08/2020 19:11:41

Zflyer09/08/2020 17:33:11
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I chose to do it this way as I thought, perhaps wrongly that it would help with balance, however the ma is so small it probably makes no difference.

As for the diagram being different from the picture it is not.

In any event its working, but I may well consider just using the BEC.

Sheessh it wasn't intended to get people agitated.

Cold beer !!! In the UK LOL

cymaz09/08/2020 17:33:48
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Looks like you’re going for nav lights. If that’s all and they work, I say you’re plane do what you want.

Ben B09/08/2020 20:56:22
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It works but it's not the right way of doing it. As said you'll end up either with badly balanced packs or (if you balance charge) spending a long time on the charger.... And you could even damage a cell.

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