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# 40W LED array power source

## 40W LED array power source

#1
02-01-15, 10:01 AM
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Join Date: Jan 2015
Posts: 2
40W LED array power source

I'm building an array of 40, 1W Leds that needs to be battery powered and last 12 hours of continuos draw at at least 85-100% the LEDs' full brightness. I was thinking of stringing a few dozen rechargeable AAs in a series, but don't want to go blow a lot of cash on batteries without an idea of how many I'll need exactly. Is there an equation with which I can find out how to pull a continuous 40W current for 12 hours off of a series or parallel-series? If other batteries (D,C,9V...) would work better, I'm all ears - but a car battery is not applicable here. Heat issues will be dealt with if needed. My electrical knowledge is rather slim, but I know the basics of making a simple system, including soddering and using an Ohmeter.

#2
02-01-15, 11:16 AM
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Join Date: Nov 2012
Posts: 5,086
What voltage are you planning on using? Why is a car battery out? Could you use a smaller motor cycle battery or similar? If you did go with a 12VDC battery the load would be about 4 amp hour, times 12 hours the battery would need to be good for a max of 50 amp hours,that's basically a deep cycle car battery,have you considered some other type of power supply?
You may want to contact theses folks.
https://www.superbrightleds.com/
Geo

#3
02-01-15, 01:18 PM
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Location: NC, USA
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If you do the math you'll see that 40 watts for 12 hours from AA batteries is not practical. Secondly you'll have to pick your LED's. Then you can determine what you will need to power them.

Look into how LEDs are powered (drivers). They can just be fed current as you mentioned but it is very inefficient and heat becomes a limiting factor. LEDs are not like a regular incandescent light bulbs where you just feed them power. What is often done because the human eye can only see so fast the LED is rapidly turned on and off. Because the LED is off part of the time it can be driven to a higher peak power and brightness without burning out the emitter and the human eye perceives the peak brightness and doesn't notice that the LED is off part of the time. This scenario also greatly reduces the current needed and extends the batteries run time.

Still, obtaining 40w of brightness for 12 hours is not practical with AA batteries with current technology. Even with a LED driver your going to need a lot more battery. A traditional lead acid deep cycle battery would be the cheapest but also the biggest and heaviest. Next would probably be lithium and maybe a series of 18650 cells or possibly a large LiPo pack intended for electric RC vehicles but you would be talking more money than a lead deep cycle battery and lithiums demand more care in how you charge them.

#4
02-01-15, 04:20 PM
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Join Date: Jan 2015
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Response

I don't mind impracticality, just if it can be done with rechargeable AAs (or C,D,9v...), because they are compact in size, even when in a series (and cheap). I can recharge them, saving money in the long run for more batteries every 12 hour cycle. The only problem I have is a heat load, but I have a fan for that set up, which will need its own series/parallel power source. Doing the math I figure 12-20 D cells could do the job, but needed assurance and recommendation by someone who knows what they are doing haha

#5
02-01-15, 04:45 PM
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Location: NC, USA
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Well, that's the first time I've heard 12-20 D cell batteries described as compact and I'm still fuzzy how you can say it's impractical in one sentence and then "saving money" in the next. There is a reason you don't see battery powered devices with 12-20 D cells. When you need that much capacity there are far better options. It's sorta like saying you want to build an airplane and you want to power it with chainsaw motors simply because you can buy them at the local home center. Yea, it can be done (and it has) but there is a reason it's generally not done.

#6
02-01-15, 04:47 PM
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Location: New England
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As suggested above, we need to know what LED's you are going to use. The voltage for batteries in series adds where the current (amp hours) for batteries in parallel adds. 8 batteries in series will give you 12 volts but will only have the amp hours of a single battery, just at 12 volts.

It's been many years, but a basic LED is just a diode and you would then add other components to set the design required current. Mote complicated, but uses less power.

An LED that is sold to operate at a specific voltage already has internal circuitry to set the proper current, but may also dissipate (waste) more power.

A more basic question is, what is the light output (lumens) you are looking for? There are very bright 12 volt LED bulbs designed for automobiles. They will already be a combination of many LEDs mounted onto a single assembly. A single bulb up to 1,000 lumens if memory serves me.

Bud

#7
02-01-15, 04:50 PM
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Join Date: Mar 2006
Location: USA
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When you say D cells and 12 hours it is like you want to use a pint jar to provide five gallons of water. Possible with enough jars but why not just use one five gallon bucket?