# Running LED's direct off a 12v battery?



## mnm99 (Jan 8, 2015)

I posted this in the Dive forum. I think it would be better off here. 

He's running them without a driver. What happens with the charging and battery fluctuations? http://loomisled.com/shop/2-24-led-f...rging-battery/ 

I understand they are in strings of 3 each needing 3.2v-3.8v each and in parallel. It looks like he uses these led's http://www.ebay.com/itm/271619050749...%3AMEBIDX%3AIT My question is he's regulating the light with a 22' piece of 22awg wire connected to a battery. 
Is it because of the type of cable and length? With the online calculator I came up with these numbers. I'm looking to do the same thing with the same amount of lights. Do I need a driver If I do this?

12v 5A draw = 8.766v at 22feet
14.5v 5A draw = 10.95v at 22 feet



It says No expensive current or voltage drivers are needed! The current is controlled by the size and length of the wire. Made to hook up to a charging or non charging (12-14.5vdc) battery. ​


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## Steve K (Jan 9, 2015)

It could work, depending on the LED characteristics. The info available from the ebay page isn't sufficient to do any sort of calculations. 

The only real calculations that can be done is the calculation of the wire resistance. 
Using the wiki page for American Wire Gauges, we can calculate that 44 feet of 22 ga wire has a resistance of 0.704 ohms (based on 16milliohms/foot). At 5A, that's a voltage drop of 3.52v, which is considerable when compared to the 9 to 10V across the LED string. Still, a variation of 2.5v at the battery is quite a bit... it all depends on how much variation in LED current you can allow.

An alternative to using 22 feet of wire is to use shorter wire and add a power resistor to the circuit board. I just bought some 1 watt metal oxide resistors from Mouser for $0.10 each (I think). My guess is that this could be cheaper than 10 or 20 feet of wire.


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## mnm99 (Jan 9, 2015)

Steve K said:


> It could work, depending on the LED characteristics. The info available from the ebay page isn't sufficient to do any sort of calculations.
> 
> The only real calculations that can be done is the calculation of the wire resistance.
> Using the wiki page for American Wire Gauges, we can calculate that 44 feet of 22 ga wire has a resistance of 0.704 ohms (based on 16milliohms/foot). At 5A, that's a voltage drop of 3.52v, which is considerable when compared to the 9 to 10V across the LED string. Still, a variation of 2.5v at the battery is quite a bit... it all depends on how much variation in LED current you can allow.
> ...




I'm not to familiar with power resistors. The other setup I have I'm using a driver to power the LEDs. 700ma that can put out up to 48v. I believe loomis is using the ebay led and it states in his description 22' of twisted wire that cannot be cut. It falls in the voltage range of the led with his setup. The 18 led setup says he uses 35' of wire. Makes sense with the resistance , but not voltage. I don't know...I'm trying to learn..lol


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## KITROBASKIN (Jan 9, 2015)

My unschooled solution at the dawn of white LED's way back when, was to take cheap amber LED's wired with the white emitter, then start removing one amber at a time until the white looked bright without getting hot. This was with a solar system. I also used thin wire from the 12V outlet plug to the LED's and twisted the LED leads together. They continue to work to this day.


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## Steve K (Jan 9, 2015)

here's a link to the metal oxide resistors that Mouser sells:
http://www.mouser.com/Passive-Components/Resistors/Metal-Oxide-Resistors/_/N-7h8z0?Ns=Pricing|0

I just bought the KOA Speer parts that are rated for 1 watt and cost $0.10 each. Seems like it would have to less expensive than a lot of extra wire (unless you need 20 feet of wire).

Places like Mouser or Digi-key are also good sources of LEDs. They will sell you LEDs that are from a known manufacturer that has good quality and actual data on the parts. You'd hate to put so much work into building your light and then have cheap LEDs die suddenly.


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## mnm99 (Jan 9, 2015)

Steve K said:


> here's a link to the metal oxide resistors that Mouser sells:
> http://www.mouser.com/Passive-Components/Resistors/Metal-Oxide-Resistors/_/N-7h8z0?Ns=Pricing|0
> 
> I just bought the KOA Speer parts that are rated for 1 watt and cost $0.10 each. Seems like it would have to less expensive than a lot of extra wire (unless you need 20 feet of wire).
> ...



Would it be set up something like this? Using the calculator. Does a resistor have to go on every leg or is there a way just to use one? And how? Thanks

input =12.5v
FV= 3.2 low end
FC =700ma
24 array
=4.7 ohm 

input = 14.5
FV = 3.8 high end
FC= 700ma
24 array
=4.7 ohm

http://led.linear1.org/led.wiz


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## Steve K (Jan 9, 2015)

There is a school of thought that it is possible to run parallel strings of LEDs and they will share current equally (or close enough). The more conservative school of thought is that there should be a small resistor in series with each string to ensure more equal sharing. I'd say try 1 ohm in series with each string and add a resistor in series with the power wire to control the current.

I'd recommend running the calculations and see where you end up. Then expect the actual results to be a bit different and buy resistors to accommodate the various possibilities. For me, that means assuming that you'll need a couple standard values, and can try parallel and series combinations to get the desired values.


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## mnm99 (Jan 10, 2015)

Steve K said:


> There is a school of thought that it is possible to run parallel strings of LEDs and they will share current equally (or close enough). The more conservative school of thought is that there should be a small resistor in series with each string to ensure more equal sharing. I'd say try 1 ohm in series with each string and add a resistor in series with the power wire to control the current.
> 
> I'd recommend running the calculations and see where you end up. Then expect the actual results to be a bit different and buy resistors to accommodate the various possibilities. For me, that means assuming that you'll need a couple standard values, and can try parallel and series combinations to get the desired values.



Thanks, but how would I figure that out with varying voltage? Say the battery is at 12.5 volts then when charging it goes up to 14.5 Volts? All LED's have a min/max voltage say 3.2-3.7 input. Say I pick 3.5 and put in the calculations to this calculator http://led.linear1.org/led.wiz It comes out with 3.3ohms putting the resistor on the - side with a 8 string array. If I input 14.5 volts it comes out with 1ohm on a 6 string array. That's where I'm getting lost. With the varying voltage what should I shoot for? Example..How would you set this up?..

24 Leds AND 18 LED's setup
12-14.5 volts input
input voltage to each led = 3.5
Current = 700ma


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## Steve K (Jan 10, 2015)

I did a sample calculation where there is a resistor in each string, and a resistor in series with the battery wire. The resistor in each string is really there just to ensure current sharing between strings.

The important part of the calculation is to consider the different worst-case conditions. These are generally the things that cause the LED current to vary. In my calculations, I limited it to the battery voltage and the LED forward voltage, Vf. 

My calculations are posted to Flickr. 
page 1: https://farm8.staticflickr.com/7516/16247150461_2db31f800a_o_d.jpg
page 2: https://farm8.staticflickr.com/7510/16248994895_d168e52c2e_o_d.jpg

my values for max and min battery voltage and Vf are a bit different than yours, but the calculation results do show quite a bit of difference between the max and min LED current. My calculations were used to design for a max current of 350mA, and then the minimum current ended up being 141mA. 

My calculations are general enough that you should be able to adapt them for your values for the max and min voltages, etc.


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## mnm99 (Jan 10, 2015)

So what resistors would I use on this setup? Parallel / series

Input = 12-14.5v
FV = 3.2-3.8
MA = 700
24 LED
+ -
0-0-0
0-0-0
0-0-0
0-0-0
0-0-0
0-0-0
0-0-0
0-0-0


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