Light Bar Amp Draw

I am confused. I have bought a 700w lightbar, running on circa 13 Volts, which equates to 53 amps (W=VA), but the kit is supplied with 'wieny' small diameter cable and a 40amp fuse.

Is there some sort of conversion factor for LED light bars that I am not understanding? I am yet to check the volt drop - will do tomorrow, but i am think I need around a 16 - 25mm2 cable on pos and neg to get the best out of these lights?

Of course since they claim the lights to be able to run over voltages up to circa 30V, i am wondering if this is where the reduced amperage is assumed, even though they are sold for 12v systems - HELP PLS, totally baffled.
Phil

Have you got a link or spec sheet on the light bar. 700w seems extraordinary

Voltage amps and wattage are all relative to each other via ohms law.
So if it is 700w at 12vdc it will be 58.33amps
So based on that i would go around 8 guage wiring depending on how long your power run is. At least that gives you some headroom regarding voltage drop etc
Im guessing its a 52" light bar?


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This PDF may help you

another one on the redarc site might be better

https://www.redarc.com.au/images/upl..._worksheet.pdf

The problem with alot of automotive multi-volt (generally 9-36v) LED lighting manufacturers, is that the advertised specs are based on the highest input voltage. It helps with advertising, and is misleading but not a lie.

If this is the case, which I believe it is. It will not be a 700w unit @12v. But a bit more info on the the lightbar is needed to determine if I am on the right track or not.

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Originally Posted by MudRunnerTD

Have you got a link or spec sheet on the light bar. 700w seems extraordinary

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Check out ebay, about $110 from memory. 100 x 7w cree's

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Originally Posted by Mike02Ti

Voltage amps and wattage are all relative to each other via ohms law.
So if it is 700w at 12vdc it will be 58.33amps
So based on that i would go around 8 guage wiring depending on how long your power run is. At least that gives you some headroom regarding voltage drop etc
Im guessing its a 52" light bar?

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Yes 52" thanks. Just working on the mounts as the roofrack is curved on the ends.

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Originally Posted by Ben-e-boy

The problem with alot of automotive multi-volt (generally 9-36v) LED lighting manufacturers, is that the advertised specs are based on the highest input voltage. It helps with advertising, and is misleading but not a lie.

If this is the case, which I believe it is. It will not be a 700w unit @12v. But a bit more info on the the lightbar is needed to determine if I am on the right track or not.

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its on ebay, 52" 700w 100 x 7w cree, about $110 from memory.

700w @ 30V = 23.3 Amps - not counting the power used by the LED driver.
If the unit is current limited then 23.3 Amps @ 12V = 279w

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You can bet the lightbar will not actually be 700 watts, typically they will run anywhere between 50% and 70% of their claimed wattage. In lab conditions they may get the full 100%, but this is rarely true in the real world. My 400 watt bar (40x10watt crees) only drew 269watts when I tested it. Still ridiculously bright though.

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Originally Posted by Turtle_au

700w @ 30V = 23.3 Amps - not counting the power used by the LED driver.
If the unit is current limited then 23.3 Amps @ 12V = 279w

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How is the LED bar actually 'current limited? Is it by some controller board or by the supply fuse size/ I was going to have a dig around and see if I could understand this and the light output rating point. There is a similar issue with solar panels in that they are all lab rated at 1000w/m2 density yet there isn't a place on the earth with this light density and max in Australia from memory is 650w/m2 in a small couple of pockets. I think the solar panel ratings could be for satellite ratings in space?

Did you establish this output from current draw?

I found a pretty good article on the www.wranglertiforum.com site. Quote below. Light output governed by current, controlled by led controller, so output is independent of voltage as long as its within the operating range of the lightbar.

"Do not get the idea that if you apply more voltage that the LEDs should give off more light. The amount of light an LED gives off is determined by current, not voltage. The current at each LED is maintained within a narrow range by a device known as an LED Driver."

It sounds like you are taking a punt with this product. Even at 70% of the max wattage claimed, you would still need 40 + amps. If you have already ordered or got the unit, I suggest you hook it to which ever voltage you will run, and check the current draw. A friendly auto elec might test it for you.

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Originally Posted by PBBIZ2

I found a pretty good article on the www.wranglertiforum.com site. Quote below. Light output governed by current, controlled by led controller, so output is independent of voltage as long as its within the operating range of the lightbar.

"Do not get the idea that if you apply more voltage that the LEDs should give off more light. The amount of light an LED gives off is determined by current, not voltage. The current at each LED is maintained within a narrow range by a device known as an LED Driver."

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Typically the LED drivers will be in the main lightbar behind each LED. My single row lightbars on my old Pajero use a separate digital LED controller. I am guessing this is to save space in the single row bars ? Pretty sure the box just supplies constant voltage. I measured the current draw via a Victron BMV-702 battery monitor with a Negative shunt on the Aux battery. I connected these bars up to the Aux for testing purposes quite a while ago.

http://i1248.photobucket.com/albums/...ps5cfde55e.jpg

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Originally Posted by mudnut

It sounds like you are taking a punt with this product. Even at 70% of the max wattage claimed, you would still need 40 + amps. If you have already ordered or got the unit, I suggest you hook it to which ever voltage you will run, and check the current draw.

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Funny you mention 40amps as that's what the supplied fuse is. I have taken on board what stevec has sad also, so will see what the sparkies have on site and do a check at 13V.

thanks guys, appreciate the input.

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Originally Posted by Turtle_au

700w @ 30V = 23.3 Amps - not counting the power used by the LED driver.
If the unit is current limited then 23.3 Amps @ 12V = 279w

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I borrowed a high quality clamp meter with DC function and checked the current draw at 12V DC . 9.0 amps, which equates to 108W in my world. I have fired off a message to the ebay seller requesting either an exchange unit or clarification on what is going on - will advise. Luckily i have not mounted it yet - looks reasonable quality though.

Sort of reminds me of audio systems ratings - peak music power, max power or rms power.
100x7w leds (700w) if driven to their limit.
Output limited to avoid overheating when mounted so closely together in a sealed enclosure.
If they did run at full rating, the amount of heat generated would melt any plastic nearby...ie grille

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Still you can ignore all the readings, install the unit and actually see how effective the light is. You may be surprised at how well it works. And running below rating helps reliability.

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Originally Posted by PBBIZ2

I borrowed a high quality clamp meter with DC function and checked the current draw at 12V DC . 9.0 amps, which equates to 108W in my world. I have fired off a message to the ebay seller requesting either an exchange unit or clarification on what is going on - will advise. Luckily i have not mounted it yet - looks reasonable quality though.

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Yep, most of the ebay jobbies are woeful in their claimed wattage. Good Luck getting any satisfaction from the seller, hopefully they will give you a refund or a decent explanation. I'd check it out first and see if you are happy with the output. My 400 watt (claimed, 269 watt actual) bar, will singe the fleas off a roos gonads from a few hundred metres, so it's bright enough for me. Too bright actually if any road signs are around. This time around with the Patrol, I think I'll be going a set of Fyrlyt 9000 250watt halogens. The led bars are pretty harsh on the eyes over long distances.

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Originally Posted by PBBIZ2

I borrowed a high quality clamp meter with DC function and checked the current draw at 12V DC . 9.0 amps, which equates to 108W in my world. I have fired off a message to the ebay seller requesting either an exchange unit or clarification on what is going on - will advise. Luckily i have not mounted it yet - looks reasonable quality though.

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It has already been clarified on the first page. The specs are based on the highest input voltage. Given that my own calculation rated it at about 112W. I would say your readings are pretty much correct. Like I said, It's not a lie. But misleading

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Originally Posted by Ben-e-boy

It has already been clarified on the first page. The specs are based on the highest input voltage. Given that my own calculation rated it at about 112W. I would say your readings are pretty much correct. Like I said, It's not a lie. But misleading

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Ben=e=boy, the inference is noted except it is at odds with the statement below I believe. This is why i can't put the logic together.

"Do not get the idea that if you apply more voltage that the LEDs should give off more light. The amount of light an LED gives off is determined by current, not voltage. The current at each LED is maintained within a narrow range by a device known as an LED Driver."

What I am understanding is that Voltage only has a place within a performance band of the LED, but the current capacity of the controller is what delivers the 'shine'. So, in reality I am drawing a conclusion that the LED driver supplied for these 100 x 7w LEDS is rated at 9 amps total and in essence is too small to achieve the rated potential of the light bar combined output. The driver in effect should be closer to 58 amps. Does this make sense?

Simply saying the total capacity of the light bar is 100 x 7 and then driving it with a driver card too small is misleading and in effect fraudulent as there is no way the rated output would ever be achieved. May be a similar performance claim of some vehicles rated to run on 98 grade fuel, that exhibit acceleration and economy on that grade, but drop down to 91 grade and the performance is not delivered.

happy to hear your thoughts.

The driver itself is designed to operate the LED at its peak efficiency. All the comments you have posted all regard the driver output. Those LED's probably run on 3.5V

You are measuring the total current and working out total wattage for that circuit. The lightbar itself has a resistance, that basically remains unchanged. All loads have some resistance. It's a dead short if it doesn't.

In a circuit where the resistive load remains constant, the wattage is proportional to voltage squared ( this is a basic fundamental of electricity ) i.e. if you double the voltage, multiply the wattage by 4, triple the voltage multiply wattage by 9.

So.......you lightbar consumes 108W @ 12Vdc. Given that the maximum working voltage of that lightbar is (as claimed) approx 30Vdc. You'll find that 30Vdc is 2.5 times greater than 12Vdc that 2.5 has to be squared. That equals 6.25.
Therfore 108W x 6.25=675W. Thats calculated at exactly 30Vdc. Use more decimal places and assume the test was conducted at between 30.5 and 30.6Vdc and you'll pretty much get bang on 700W

Well bugger me, that's beautifully explained, thanks, I get it. So, I have got what I paid for in essence?

If the bar will only ever draw 9 amps, and it's got pissy thin wires, why do they supply a 40 amp fuse? Is this part of the 'deception'? So to speak.

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Originally Posted by PBBIZ2

Well bugger me, that's beautifully explained, thanks, I get it. So, I have got what I paid for in essence?

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Yep, I would also piss that 40A fuse off too

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Originally Posted by PBBIZ2

If the bar will only ever draw 9 amps, and it's got pissy thin wires, why do they supply a 40 amp fuse? Is this part of the 'deception'? So to speak.

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They're idiots. Anyone who protects a circuit with a fuse that's higher than the current carrying capacity of the cable shouldnt be playing with electricity