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I am trying to build a portable project with some form of RGB LED (probably APA102) and a controller such as a Raspberry Pi. I am trying to figure out what is required to safely power both the LEDs and the controller, but based on my calculations (which are probably incorrect), I am having trouble coming up with a way to do so:

  • ~200 APA102 LEDs (which, at most, should be drawing around 60 mA per RGB LED)
  • Raspberry Pi (draws at the very most, 500-700 mA)

In total, this comes to ~12,000 mA maximum draw. Now, I am never going to have the Raspberry Pi or LEDs on full blast, so this figure is an absolute maximum. I can pretty easily add in software limits to prevent the LEDs from being completely lit at any one time.

But, even if this drops down to half (6A draw), this is way beyond what a standard AA/AAA battery pack can provide. So, the obvious choice is to use Lithium Ion.

Adafruit has a 6600 mAh battery pack (3 x 2200 mA: http://www.adafruit.com/products/353) with a stated max draw of ~13 A-- perfect for my situation, since I will rarely if ever be at that point. However, the voltage is only 3.7v when I need 5v.

So, then I would need a step up converter, correct? Is there a step up converter that even supports the 13 A peak draw (and probably around 3-4 A constant at very most)? Generally, I've only been able to find 500 mA to 1 A converters in my research.

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  • \$\begingroup\$ Why such an oversized controller? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 3 '15 at 1:15
  • \$\begingroup\$ I prefer the Pi's full ARM processor, even though I can use a powered powered controller like an Arduino. The LEDs are the bulk of the power consumption anyways in this project. \$\endgroup\$ – Andrew M Jun 3 '15 at 1:49
  • \$\begingroup\$ shrug There are smaller ARMs, in particular ones that don't need a boost regulator to run off a Li-ion battery. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 3 '15 at 2:11
  • \$\begingroup\$ The LEDs are 5v, however, and I don't know of any 3.7v digitally addressable RGB LEDs \$\endgroup\$ – Andrew M Jun 3 '15 at 4:42
  • \$\begingroup\$ I've run 5V LEDs off a Li-ion cell before. You'd be surprised what you can get away with. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 3 '15 at 4:49
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Cheap 3.7V to 5V voltage boosters that can do 2.5A are readily available. You might be able run several of these boosters, each one supplying power to a part of the circuit.

However, while the Adafruit 6600mAh 3.7V li-ion battery can deliver a maximum of 13A (though not with the supplied cable and connector, which can only handle a mere 2 Amps!) you need more. As the booster steps up voltage so its output current must drop proportionally. You want up to 12A @ 5V which equates to 60W. The booster cannot be 100% efficient, so it must draw more than 60W from the battery. For example a booster with 85% efficiency would draw 70.6W = 19A at 3.7V, way over the battery's rating. Also you will need thick wires and beefy connectors to carry this high current!

A better solution might be to use a higher voltage battery (eg. 7.4V) and step it down to 5V. That way the input current will be more manageable, and you can choose from a wide variety of high current buck mode regulators. At 7.4V you still want a battery that deliver at least 10A of real current. I would use an RC car lipo such as this 5000mAh 7.4V pack, which has 50% more energy storage capacity than the Adafruit 6600mAh 3.7V battery, and will have no trouble handling 10A.

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  • \$\begingroup\$ @AndrewM Or, you can just combine 2 of these batteries in series to increase voltage to 7.4V. I find that doing so is easier than looking for a battery pack that supports a 12A draw. \$\endgroup\$ – Alex Jun 22 '15 at 1:31

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