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I have a fun project I want to do. It's really simple: I want to power a long LED strip using a bunch of 18650 batteries. The batteries I use are 4.2v fully charged and about 3.5v when discharged (I don't want to push them further down than this).

The LEDs are WS2812b and they work from 3.3v to 5v easily. I successfully power 10 of them with a single 3.3v signal from my ESP8266 and they're bright enough.

Now, what would be best?

  1. Wiring 2x 18650 in series, making a maximum of 8.4v and a minimum of 7v, then use a buck converter to make it 5v all the time.
  2. A single 18650 battery at 4.2v maximum and 3.5v minimum.

I will make a bunch of these in parallel, so I have more current to run the strip for a longer time. Option #2 would definitely be a better option for me, as it doesn't take that much work.

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  • \$\begingroup\$ Option 2 is the simplest and possibly the most flexible. But what is best is a subjective question that only you can answer, depending on your wants/needs and situation. \$\endgroup\$ – Dampmaskin Nov 22 '17 at 23:02
  • \$\begingroup\$ @Dampmaskin I appreciate your reply. Yeah the most flexible will probably be #2. I'm more thinking about the pros/cons of the two options. I'm not sure if there are any cons to #2 except they might now be less bright. \$\endgroup\$ – MortenMoulder Nov 22 '17 at 23:43
  • \$\begingroup\$ Buy a 4 or 8 cell usb power bank. \$\endgroup\$ – Passerby Nov 23 '17 at 4:11
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I've done similar using a cheap ($3) "USB power bank module". These are designed to provide a constant 5 volts from an 18650 battery. I bought some with "134N3P program" which protect the battery by dropping out when the battery voltage goes down to 3.7v. You can even buy these things in an empty case to put your 18650 batteries in.

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  • \$\begingroup\$ Oh yeah definitely a good idea. I took apart a small 1x 18650 powerbank the other day, which revealed an awesome small circuit for charging the battery and outputting 5v. I could definitely use that and just take a whole pack of batteries and wire them in parallel for maximum current. I want to power about 150 of these WS2812b LEDs. They're about 60 mA each, according to many websites, which results in 9A total. However, I can easily power these using 4A. Like easily. I don't even see the LEDs go above 3.6A, actually. The 18650 batteries I got came from a laptop. Do you think they can handle? \$\endgroup\$ – MortenMoulder Nov 23 '17 at 12:40
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Option 2 is the best, and you don't need any other hardware to manage/regulate/buck the voltage.
The WS2812b is a current controlled 3 LED system, providing the input voltage is high enough for the current regulators to work. The brightness does not vary if you alter the voltage within spec.
It is not sensitive to input voltage unless you drop below its lower limits 3.5 V, when it stops working or go above the absolute maxima of 5.3 V when you can cause damage.

If you read the datasheet you will notice the following:
"Also include a precision internal oscillator and a 12V voltage programmable constant current control part, effectively ensuring the pixel point light color height consistent."

Chinese for it has it's own onboard Boost convertor that regulates the voltage for the current sources.

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  • \$\begingroup\$ Awesome, man. I will probably do option 2 then and if I simply take a bunch of these in parallel, I should be able to keep it going for a while. Question: I took apart a powerbank and it revealed an awesome small circuit for charging and outputting. If I simply hook up all the batteries (5-6) in parallel to the "B+" and "B-" (for battery charging), I should be able to charge all at once, right? It might take a while, if we assume I charge at just 1A, but I can leave it charging overnight easily. Here's the circuit: imgur.com/a/7almr \$\endgroup\$ – MortenMoulder Nov 23 '17 at 12:48

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