Advice needed - running an LED light off of 12V SLA?

[PirateBaker]

Hi, CPF!

I am looking into building/modifying a fishing light for my father. It would be used to illuminate the surface of the sea (at night) to attract european smelt (this apparently works incredibly well). My father wants it to be fairly small, something that can be carried with other fishing supplies, and attachable to an old fishing rod (to be used as a telescoping mount).

The simplest solution would be getting a 18650 LED flashlight and spare batteries. I have offered to give him some old 18650s (harvested from an old laptop battery, so free), but he has his own ideas. He wants to power the light from his 12V SLAs.

I searched around the forum, and came up with these ideas:

1) Get an LED flashlight, remove the battery tube, and run it off the SLA battery through a stepdown (buck) voltage regulator. Simple, but according to efficiency curves, small stepdown regulators regulating 12V to 3.7-4V are only 80% efficient (that seems low. Is that low?).

2) Use a 12V LED, like this one:
https://www.fasttech.com/products/1609/10010823/1924906-cree-mk-r-1200lm-3500k-led-emitter

And a 3-18V driver, like this one:
http://www.dx.com/p/18v-5w-cree-circuit-board-for-flashlights-16-8mm-5-5mm-26110

The driver in the example is 5W, the LED is 12W (so, 1A) (12V LEDs don't seem to come in low power). Would that work? Would the LED work being driven at 410 mA?

3) 12V driver and 3 LEDs in series?


I'm pretty sure any of these would work. My question is, which one would work most efficiently? The smelt are fished throughout the night, you obviously can't recharge batteries on the pier, and you can't just carry a pocket full of charged spares, due to SLAs being big and heavy.

So, any thoughts?

(There's also
4) Convince father that his idea is stupid and he should use a flashlight with my free 18650s

Not ruling that out)

 

[Modder]

This might help for the driver: http://www.amazon.com/dp/B00C4QLK52/?tag=cpf0b6-20
It can have a higher input (let's say 12v battery, which could be 12.6v really) and regulate (buck) the voltage to that of the led. I would suggest an LED like the Cree XM-L2 which works at around 3.2v (found here: http://www.mtnelectronics.com/index.php?route=product/product&path=60_61&product_id=504) or a Cree XT-G2 that works on 6v (found here: http://www.mtnelectronics.com/index.php?route=product/product&path=60_77&product_id=407) with >2000lumens @ 3000mA.
But you would need to manufacture the head for the LED, heatsink and reflector.

Or just get this: http://www.amazon.com/dp/B019EX18CW/?tag=cpf0b6-20 or http://www.amazon.com/dp/B0188I6YNM/?tag=cpf0b6-20 for 25 bucks shipped ea. and be done

 

[DIWdiver]

Indeed those look like nice heads for this project, but I wouldn't bet the performance would be anything like what they claim.

The first one has three 1000 lm emitters, yet they claim 6000 lm. Clearly they have no respect for the truth. The second one is almost reasonable, 5600 lm from five emitters, but to run it at that output, you'd need over 50W of input power. To run it two hours on high would take 100 W-H of battery, but they only claim 4.4 A-H, and it's probably 7.4V, so that's only 32.6 W-H. And I'd be stunned if the battery lived up to even that claim.

Also, neither of those heads is large enough to dissipate the heat that would be generated by running those emitters at full power, at least not in still air. On a bike cruising around, it would be better, but still insufficient, IMHO.

That said, and with a more realistic expectation of 1500-2000 lm, either of these might be a reasonable solution, or a reasonable starting point for a DIY project. You could either build a couple of spare LiIon packs or a single larger one to run for a longer time, or open the head, bypass the driver, and add your own (externally) to run from the SLA. The drivers in those lights are probably designed for 7.6V input, and might not work at 12V.

I don't know how big your SLAs are, but a 12V, 7.2 A-H battery isn't unreasonable to carry around. That would give you 84 W-H, and at 80% efficiency, deliver 70 W-H to the emitters. At 1500 lm (about 15W), they should run nearly 5 hours.

You asked about efficiency. Generally, the best efficiency is had from buck regulators, with the output being slightly below the input voltage. So your 12V SLA driving three LEDs in series is a pretty good setup. But driver efficiency varies a lot from one design to the next. A fairly simple but well made driver in this configuration could be 90% or better, with 95% being achievable with more complex designs. A crappy one might be 70% or even less. Cost and size considerations often force part selections that compromise the efficiency. So 80% is not great, but it's not awful either.

In certain cases where the input and output voltages are close, linear regulators can also achieve impressive efficiency. Ideally, the efficiency of a linear regulator is Vout/Vin. In practice, it's always slightly less than this, because of power used by the regulator, but in a good design the difference can be very small or even trivial.

The driver you linked is a buck driver, so it would not work well on a 12V LED with a 12V (or less) battery. If you had a higher battery voltage, it might work. However, it's designed to drive a single LED (notice it says 1*5W LED), and might not like the higher output voltage. The MK-R you linked to is four LEDs in a package, and they can be run all four in series at 12V or two in series, two in parallel (2S2P) at 6V. This would work better on the 12V battery, though with a different driver. Notice though, that the "star" board it is mounted on has them wired all in series. There are other ones available from the same vendor that have it set up as 2S2P. If you want to run a single package, this would be a good choice.

 

[TexLite]

If you want simple, get a 12v LED MR16 bulb and wire it directly to the battery with a toggle switch. Just make sure tou buy from a reputable manufacturer/dealer. There is a ton of unadulterated garbage out there on places such as eBay, not saying you can't get a good deal, just make sure it's from a reputable manufacturer. The only other thing I'd recommend is make sure it's outdoor/wet location rated, for obvious reasons.


For a DIY driver with high efficiency there's the Sandwich Shoppe or TaskLED. Both have buck drivers with efficiency levels over 90% that will drive 3 LED's in series;

http://theledguy.chainreactionweb.com/index.php?cPath=48_49_61

http://www.taskled.com

These folks have a variety of good quality drivers as well, made by LuxDrive;

http://www.ledsupply.com/luxdrive-constant-current-led-drivers

If you want a cheaper driver, you can go to some of the dealers mentioned by previous posters, just be sure to google the driver to make sure it's a solid performer, some are great, some are junk. I'll list a few stores that most all have had good experiences with;

http://www.mtnelectronics.com

http://intl-outdoor.com/components-drivers-c-57_88.html?page=2&sort=20a

http://www.illumn.com/leds-drivers-optics-mcpcbs.html?___store=default&cat=122

Also, I know you didn't ask for any advice concerning your father, and I don't want to offend you in any way, but my advice would be to suffer his "quirks" and enjoy every single minute you get to spend with him. I just lost my father last month and miss him greatly, in fact, it's why I'm up late binge posting on CPF when I should be in bed.

-Michael

 

[HarryN]

It might be helpful to check if the real plan is to plug it into the boat's 12 volt supply, which is charged from an engine, or really just plugged into a stand alone battery.

The voltage range of a stand alone battery will be 10 - 14 volts, while the "12 volt power" in a motor boat or car can have quite large spikes that will blow out many non - automotive rated electronics. TaskLed used to offer automotive rated drivers, but I am not sure if they still do or not. LED supply is another possible source.

A common method to help remove these spikes is to use what is called a zener diode. Essentially, they are wired in parallel (from supply to ground) along with your light. The zeners are rated for different voltages, and they ground out voltages higher than their "rated voltage. For example, maybe use a 20 V zener in a 12 volt nominal circuit to reduce the spikes. They are a very inexpensive protection method, on the order of 25 cents or less.

 

[cland72]

Why not just order a LED light bar?

http://www.ebay.com/itm/20inch-120W...ash=item3a9450a7fc:g:s8kAAOSwFNZWw-lU&vxp=mtr