5v, high amperage mobile power for neopixels (WS2812 leds)

Other Hardware General Electronics
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Like a lot of people, I've been playing with the neopixels from Adafruit that are based on the WS2812 (and now WS2812B) leds, arranged either as strands of leds, rings, sticks, or individual pixels. These are spec'ed to run at 5 volts, and each led can draw up to 60 mA when it is at its brightest (20 mA for each of the red, green, and blue leds that are part of the WS2812B chip). Now, often times you don't have that many lights lit at the same time (or many lights lit at low power) that it will fit in the 100-500 mA that you can get from normal 5v USB charger batteries meant for charging cell phones. Sometimes you can even get away with 3.7 volt lipo batteries (though I have one ring that requires 5 volts, and I suspect over time more will).

I'm wondering how to set up powering these with batteries. I have one project coming up, where I will want to do a ring of 24 LEDs (or a set of 4 8-LED sticks) at full power for a small amount of time. It is a camera focus assist light, where you want a lot of light when the camera is focusing, and perhaps a lessor amount of light over a longer period to act as a video light. By my calculations, I need 5 volts and 1.5 amps of power.

One thought would be to use the normal cell phone chargers that are available that use USB cables. As I understand it, to get to the higher power levels, a device has to 'requrest' high power, otherwise it may only get 100-500 mA. Is there a commercial plug out there that has a standard USB plug/port on one end, and +/- wires on the other end, and the cable has the resistors set up so that if I plug it into a battery that claims to deliver 2 amps on a USB port, I will actually get 2 amps? Or is there a FAQ somewhere that shows how I need to wire up the D+/D- resistors to get the most amount of juice from a USB connection.

Another thought is to use voltage converters you can buy at places like pololu.com. Now, in my limited understanding of electronics (I'm primarily a software guy), I would think it would be better to start with more voltage, so that when the voltage is dropped, you get more of it as amps. Pololu.com sells one such converter, that you would hook up to a 2 cell 7.4v lipo battery, and it could deliver up to 5v, 3.5a. http://www.pololu.com/product/2110. However, there is a warning about in use, the voltage converter will get hot enough to burn you. I would worry about the excess heat, and venting it (particularly if I need to enclose the battery to protect it against rain).

A third option is the various converters using 18650 batteries. These typically have 1-2 USB ports, and sometimes a separate port for delivering a fixed amount of voltage. For example, this can deliver 5v/2a, 9v/1.5a, and 12v/1a: http://www.ebay.com/itm/301064928798?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649. Now, I would suspect the USB ports will need resistors like using standard USB chargers would, but I could use the 5/9/12v output directly for the LEDs. I don't have any experience with these types of units, can they deliver the power that they promise, and how hot do they get when used for a period of time? The units that have the selectable voltage output, tend to take 4-6 batteries, are there units that take fewer batteries, as sometimes space for the batteries is a big concern (now for the focus assist light, it isn't as much of an issue, but there are other things, I would be willing to change batteries every so often to get a smaller battery)?

Are there other options? Are there specific solutions people would recommend?

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I'm guessing, though I've never done this myself, is that some form of RC car battery and the pololu step down are your easiest and best bet. If you are worried about heat, then you'll need to worry about heat-sinking. One of the easiest things though is have one side of your enclosure be the heatsink, and so it will transfer heat to outside of the case. This makes it a fun mechanical problem, but is usually fairly easily solvable, though you'll have to figure out exactly what is getting hot first. Or you can add a low power fan and see if that is cooling it enough, but that won't be rain proof.

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I'm curious, why addressable leds?

Would it not be easier and cheaper to buy 2-3 1-5m LED strips ... they run from 12v so a small lead acid battery could run the strips for several hours and a lot easier to recharge. Switching them on/off is a doddle with an Arduino.

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Thanks for the comments.

In terms of using the LED strips and rectangles, yeah, I've thought of using those, and they might work just for the focus assist portion. But I was thinking that when I wasn't actually taking pictures with the camera, I would be doing some low level neopixel blinky light type things, and that I can't do with the mono-color LED strips that I've seen. Also, it was more of a general question for future projects using led strips. I would prefer to minimize the number of batteries I have (and that need power at the end of the day), due to weight issues, as well having to have extra replacement batteries, and having to charge n different types of battery at the end of the day.

In any case, I was planning on ordering some led strips to evaluate how they would fit in the project. The ones I've seen still have hefty power draws that I would have to deal with the batteries. For example, I was looking at some 5v versions (to minimize the number of different batteries/voltages I need to deal with), and one panel runs at 300 mA. One panel alone might put put me into jeopardy for single 5v USB supplies (where without the resistors to grab all of the power, you might only get 100-500 mA). Similarly with a lot of voltage regulators that boost current, many will only give 500 mA current (at present, I don't have the two cell lipos, so if I go that route, I need to get appropriate chargers, batteries, and I have to plan the purchases to fit in the weekly budget).

In terms of heat, I really would prefer something that does not generate a lot of heat. Some of these lights go in costumes, rather than it boxes (such as 2 rings on goggles that I have on a hat), and with a costume, I really would prefer not to have hot-spots that can touched by accident (or even cause a fire hazard if some other material touches it). Adding a fan will make the batteries drain even faster.

Basically in a science fiction/steampunk convention, you are going for most of the day with your costume/prop. Sometimes I am staying in the hotel where the convention is going and I can pop back to the room to change things, and sometimes not, where I need to carry everything for the full day. Some things are outdoors, where rain is always a possibility (as I said, the camera is weather sealed, but the Arduino/Teensy/etc. is not, but I would need to use an enclosure or some such). For the focus assist light part of the project, I would use it when I take pictures of other people at the convention, but if I was doing a light show, I would want to have it going most of the day.

Now, my icon picture shows a picture of me holding the big version of the camera box (working digital camera on the inside). It weighs some 10-20 pounds, while I am planning on downsize some parts of it, I do have to plan for carrying the thing all day long.