Battery Task Light - Optimizing Efficiency

High Efficiency · Aug 29, 2013

First off, a brief hello from a new member and long time observer.

I've been building several homemade battery operated LED task lights (dispersed light) over the last year and have finally narrowed down a design that works for the project but need help in optimizing the electronics.

The ultimate goal is obtaining the best battery life per lumen (aren't we all?)

Below is basic outline on my project which is a bit different than the high output projects I often peruse on the forums but thought someone might be able to help.

Maximum Battery Efficiency
8 X AA (Alkaline Cells) or 8 X C (Alkaline Cells)

LED Characteristics

Lumen Range: 80 - 120 Lm
Color Rendition Index Range: High
Beam Angle Range: 70 – 120
Emitting Color: Warm White
Color Range: 3200 K – 4500 K
Voltage DC: 12 Volts DC Preferred
Power Source: AA Alkaline Batteries, C Alkaline Batteries

I have been using LED Type 3014 strips for 9 - 14 volt, rated @155 lm/ft and power consumption/ft of 78 mA with current draw @12V DC of 127 mA.

The specs above are for the full 12 inch strip, of which I've been using half (hence the difference in product specs and those I've listed in the table). As you all know, simply doubling the number of these LEDs does not double the light output so that doesn't help. The solution likely lies in finding a better LED and combination of circuitry to increase brightness without a dismal fall off in runtime.

The task light is usually around 10" or so away from the subject. Light projection is usually about 24 inches wide.

I suppose the questions are:

1) What is the most energy efficient LED to run with 12 volt AA power supply for long battery life (150+ hours) under constant load?

2) What drivers and resistors are needed to operate the LEDs?

3) What is the best way to go for maximum battery efficiency while maintaining a good CRI?

Apologies in advance if this hasn't been posted in the proper forum.

Thanks

Toaster79 · Aug 29, 2013

Cree MK-R and Taskled b3flex.

DIWdiver · Aug 29, 2013

b3flex is a buck regulator that requires the input be somewhat higher than the output voltage. Since the MK-R may require 12V or more, this wouldn't be a good combination to run on 8 alkalines since they would rapidly drop well below 12V. It also can't be set low enough. At its minimum setting, the MK-R would be pumping out six times the desired light. Granted, the efficiency would be awesome.

You really want about 350 mA in a single die, not 500mA in 4 dice that the MK-R has.

The CC1W is 350 mA fixed output. It's also relatively inexpensive for the reliability you'd get from Taskled. It would only be around 75% efficient at 12V input. It would probably be better at 6V input if you just re-wired the battery pack, but unfortunately there's no data to say how much better. The BuckToot from LEDsupply and the BuckPuck from LuxDrive are the only other drivers I found in this range except the CCC (Cheap China Crap). The BuckPuck might have better efficiency, but you can't really tell from the available data. You might peruse CPF for more info.

I'd go to the LED forum on CPF to look for a good High-CRI LED. I know Philips and Nichia make them, but I'm no expert.

AA batteries will get you the best bang for your buck, because they are by FAR the highest volume sellers of any battery. C cells will be slightly more efficient and will give much better life, but will end up more expensive in the long run (not to mention bulkier).

As far as 150 hours, you're going to need more than 8 cells for that. To get around 100 lm, you need around 1W in the led (less for low-CRI). Because of driver efficiency, you need a bit more than a watt from the batteries. With premium alkalines, you'll get 2.5-3 W-H from AA's, and 6+ W-H from C's. Multiply by 8 and you don't get anywhere near 150 W-H.

High Efficiency · Sep 27, 2013

DIWdiver said:
b3flex is a buck regulator that requires the input be somewhat higher than the output voltage. Since the MK-R may require 12V or more, this wouldn't be a good combination to run on 8 alkalines since they would rapidly drop well below 12V. It also can't be set low enough. At its minimum setting, the MK-R would be pumping out six times the desired light. Granted, the efficiency would be awesome.

You really want about 350 mA in a single die, not 500mA in 4 dice that the MK-R has.

The CC1W is 350 mA fixed output. It's also relatively inexpensive for the reliability you'd get from Taskled. It would only be around 75% efficient at 12V input. It would probably be better at 6V input if you just re-wired the battery pack, but unfortunately there's no data to say how much better. The BuckToot from LEDsupply and the BuckPuck from LuxDrive are the only other drivers I found in this range except the CCC (Cheap China Crap). The BuckPuck might have better efficiency, but you can't really tell from the available data. You might peruse CPF for more info.

I'd go to the LED forum on CPF to look for a good High-CRI LED. I know Philips and Nichia make them, but I'm no expert.

AA batteries will get you the best bang for your buck, because they are by FAR the highest volume sellers of any battery. C cells will be slightly more efficient and will give much better life, but will end up more expensive in the long run (not to mention bulkier).

As far as 150 hours, you're going to need more than 8 cells for that. To get around 100 lm, you need around 1W in the led (less for low-CRI). Because of driver efficiency, you need a bit more than a watt from the batteries. With premium alkalines, you'll get 2.5-3 W-H from AA's, and 6+ W-H from C's. Multiply by 8 and you don't get anywhere near 150 W-H.

A big thanks for your input here. I've since tried the configurations you mentioned above. Unfortunately, I still can't get the light output I need given the form factor of my power constraints. So, this has brought me to the idea of using optics to capture and redirect the wasted light from the strip LEDs. The strip LEDs project light somewhere equal to or greater than 120 degrees. The usable light I'm after should be projected somewhere around 70 degrees. My thinking is most, if not all of the light being emitted to the side (in my case greater than 70 degrees) is being wasted.

Does anyone have any experience or recommendations or some type of low profile optic that would work with these strip LEDs? I've toyed with some half round acrylic rods and they seem to help but aren't quite there. Some more poking around online (can we call this research?) has brought up the idea of using a low profile (actually tape) fresnel lens. This is an option, or perhaps even combining this with a half round type of lens (dome). Now I'm just rambling as ideas turn.

Any help?

DIWdiver · Sep 27, 2013

If you're looking for a 70 degree beam, I'd look at some shallow reflectors to capture the otherwise wasted light. Most lenses are narrower than 70 deg.

High Efficiency · Sep 29, 2013

Anything in particular to recommend for a shallow reflector?

Thanks again (and congrats on your 1000th post....)

Battery Task Light - Optimizing Efficiency

High Efficiency

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Toaster79

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DIWdiver

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High Efficiency

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DIWdiver

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High Efficiency

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