Battery for wearable pico / neopixel project

Greetings! I am building a wearable neopixel project with some 256 LEDs and a Pi Pico microcontroller. The project is meant to be used for a performance and as such my requirements may be slightly different to most - firstly I only need the rig to run 5-10 mins at a time, while size and weight are major concerns (must be as small and light as possible). Using the standard Adafruit math to work out how many amps I need gives 5.12 (256 * 20am per pixel) which is pretty ridiculous - seems I can get away with using a standard usb power bank (5v @ 2.1amps) however the size and weight become a problem if I go this route.

The thought of using a standard alkaline 9v battery with a
buck converter came to mind - if I’m reading the specs right a 9v 1.28 amp in (which seems to be standard 9v battery output?) will give 5v 2amp output.

Would such an approach work, or can someone suggest a better solution? NB I’d like to connect the pico to the same power supply if possible.

Many thanks in advance!

Hi James

No not quite right. Like nearly everyone else you have not considered conversion efficiency which would be about 85%. This converts to ((9V * 1.28A) / 5) * 0.85 = 1.83A.
That also assumes that the 9V battery will last longer than a couple of minutes at that rate. I don’t think they are designed for that sort of current. Alkaline batteries are about 550mAHr while your 1.2 AHr battery would be Lithium. Just because it is quoted at 1.2AHr does not mean it will deliver 1.2A for 1 hour. These rates are quoted at the 20hr rate, that is a discharge rate of 60mA. I have not got the curves for Lithium chemistry bur even though they would be better that lead acid I don’t think they are super better. But whatever the chemistry the capacity will reduce (sometimes drastically) as the discharge rate increases.
Cheers Bob

Hi James,

Welcome to the forum :slight_smile:

I’m with @Robert93820 here, only Lithium style batteries will give you the power and energy density you need for a costume that can be worn with that many LEDs.
I’ve made a light-up butterfly wing cape that uses around that amount of LEDs and ran it off a pair of these USB power banks. I think the real world battery life was around a couple of hours, but I’d chosen patterns that lit as few lights as possible, and kept the brightness low as it was only worn at night anyway.

Your battery life requirement is lower which is good, but your instantaneous current draw isn’t likely to change unless you’re ensuring only part of the LED array is lit at any given time. Reducing the brightness also doesn’t give much of a return as the quiescent draw is still there for the LEDs that are “off” according to the pattern, but still part of the array.

You probably won’t find a cheaper battery option than a USB power bank (which is really just an 18650 battery and a regulator in a mass produced package).

Pouch style batteries like those used in RC drones and racing cars will give higher current output but will require external regulators and won’t have the same small footprint. Something like the combo below might work, but it’s worth mentioning that care needs to be taken when using and recharging these kinds of batteries. If dropped or squashed by part of a performance they will make a fire that fire extinguishers for the most part can’t put out.

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Hi Trent

If that follows most trends that 2000mAh is at the battery (nominal 3.7V). Converted to 5V this becomes 1480mAh. If you allow 85% (a fairly safe number) conversion efficiency this is now 1258mAh. A deal different than the 2000.
Cheers Bob

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Thanks for the replies, all. I went and got this which does a decent job of powering 256 leds, however at full white the leds towards the end of the strip start to go yellow. I suppose I’m getting voltage drop off?

@Trent5487676 That video is horrendous. Hoping I’m not running these sorts of risks using a power bank? Is lion meant to be more stable that lipo?

Hi James
I might point out here that that 3600mAH is probably rated AT THE BATTERY (nominal 3.7V). By the time it gets to 5V and you allow for 85% conversion efficiency it becomes 2265mAH so dom’t be surprised if it doesn’t last as long as you might calculate.
Cheers Bob

Thanks for the info @Robert93820! Probably ok with the battery charge not lasting as long as it says on the box, but I’d love to get pure, bright white out of all 256 leds. I’m assuming I need more amps?

Hi James
You are probably right regarding voltage drop toward the end. It is not a matter of more amps but more like distribution of the amps you have got. Is it mechanically possible to apply the voltage (both positive and negative) to both ends at once. This will help current (amps) distribution.
If the extra voltage distribution has to be the whole length of the strip make sure the wire is large enough not to cause a significant voltage drop on its own. This is what you are trying to overcome.

If voltage drop is still a problem and LEDs are yellow in the middle you may have to feed the strip in 3 places, both ends and the middle. 256 is a fair few LEDs. Are you sure the supply is up to it???
Cheers Bob

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Hi Robert,

Yes it seems like the strip will support being powered at any place along the strip. This being said, I’m still rather inexperienced so not entirely sure if the battery is up to the task. If I connect the same battery to more points along the strip, this presumably doesn’t change how many volts / amps are being delivered, only that the existing power is being distributed more evenly, is that correct?

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Hi James

Yes that is correct. You will use a little more current as you are overcoming the resistance of the tracks in the strip which caused your yellowish lights in the first place. The percentage higher won’t be that much but as you surmised you are only evening out the current distribution.

If you can power anywhere along the strip you may get a more even distribution by connecting to about one third and two thirds (2 points) of the length. Or one quarter and threw quarters.
Cheers Bob

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Hi Bob,

Sorry for the delayed reply, I’ve been away all week. My recommended solution is still the USB battery bank, as imperfect as they are, they are a fully integrated product with lots of built in protection devices and they are the cheapest solution I know.

The primary advantage of the racing drone batteries is that you aren’t capped at 5V and 2.1Amps which every USB charger will be limited to. This means any USB power bank will only be able to give you 10.5 Watts at any given time. The mAh rating is for the total capacity which will make the battery last longer, but since our 256 LEDs will still require 25.6 Watts (using our 5.12 Amp estimate from earlier) you will need multiple USB banks regardless of how long the costume needs to be powered.

That racing drone battery can theoretically supply over 100 Watts instantaneously, the current discharge ratings are always overstated but even if we take half of what they’ve promised to counter the marketing fudge factor that’s still at least 50 Watts of power. All that instantaneous power naturally has a trade-off. The batteries require specialised converters, there is less protection for the batteries, and special balanced chargers are necessary to recharge the batteries.

Hi James,

I chose the most dramatic example I could find but it pays to be aware of what can go wrong if high power batteries are misused, as was the case when a wave of defective hoverboards all hit the market a couple of years ago.
Your USB battery bank has protection circuits built into it that will prevent that kind of fireworks display, the only way you’d get a result similar would be to drive a nail through the battery itself.

Sizing batteries for projects is tricky as there are a few factors that all need to be right at once for your battery to work. The main three you’ll need for your project are voltage output, instantaneous current required, and energy capacity.

Voltage output is the easiest, your LED strips want 5V and your USB bank provides that. Having the power distributed to the LED strip at multiple points throughout your LED strip as you’ve done will minimise the voltage drops so it’s worth doing. (If the voltage sags your current draw may also increase slightly to compensate which we want to avoid.

Instantaneous current is actually easier to work out using power, as power is conserved throughout the system even if the voltage levels change, this doesn’t account for conversion losses as Bob has indicated, but 85% is a good figure. Using your estimate of 5.12 Amps, for 5V strips this equals 25.6 Watts consumed by the whole strip. For an efficiency of 85% we then want our power supply to deliver 30.1 Watts to cover the losses. Each USB battery bank has a nominal 10.5 Watt maximum (5V * 2.1A) so you will need 3 of any USB battery bank to power all 256 LEDs.

Energy capacity is the Watt-hours rating that determines how long the battery will last. Battery banks will only have a mAh rating and assume you know to multiple by 3.7V to get the power.
For every hour the costume is powered on it will need 30.1Watt-hours of power supply. Since we only need to last 10 minutes we can get away with a sixth of that, so 5Wh. If we divide by 3.7 again to get back to mAh then we need around 1350mAH of capacity in our battery.

All this means that your single battery bank has more than enough energy to supply your LEDs total needs but it can’t deliver it fast enough to keep them all running at once. You will need multiple USB banks to supply your LEDs at once for the costume to have enough power.

Thanks again for all the responses, all. I really appreciate it. I’m currently experimenting with a usb power bank similar to that which I posted early however with two usb output sockets (one to power the LEDs and one to power the pico) and is a great deal heavier. While it’s rated at the save 5v 2.1A I assume the extra weight equates to longevity per charge in this case (is that what mAh is supposed to indicate?).

Both the slim-form USB and the larger one have the issue with light turning yellow towards the end of the strip (did not attempt to power the rail at the halfway point but will do that soon) which is not surprising considering the output is the same. That being said, the amount of light this thing throws out is proving to be more than enough (see attached pic), and when using single-channel colours only the power requirements go down considerably (by about a third, if I understand correctly - although I believe each colour consumes slightly differently). I might be able to get by with a single bank and just not use 100% white.

Re running several power banks at the same time. seems to me two of the slim usb power banks would be lighter than the single heavier one and will give me the boost I need if desired. Would I be wiring these in parallel? If I want to power my board with this set up will I now potentially fry the board, or myself?

You are unlikely to fry the board (the voltage stays the same when the power banks are in parallel) but it probably won’t work and could cause a problem with the power banks. The most likely outcome will be that one power bank will detect that its power is not being used and will shut down. It might or might not start up again when the other power bank voltage falls. Or the two might continually switch back and forth as they try to work out which one should be doing all the work. Or there might be other odd interactions between them, such as both shutting down. Generally, paralleling any power sources is not a good idea unless they are specifically designed to do that, or a special balancing circuit is used.

If two small power banks are lighter than one large one either they have different amp-hour ratings or they use different battery technologies. If it’s different battery technology then one power bank using that lighter technology would be the best choice.

Thanks @Jeff105671 !
Feels like single usb power bank with multiple power connections might be the way to go, and just manage the colours / intensity in order to compensate for the lack of amps. I still need to try attaching power halfway through the rail so will try that next.

Hi James,

You are spot on, you will find all 5V USB banks have the same instantaneous output and their mAh rating just indicates how many hours they can keep that up for.

I think I may have a suggestion that escapes some of the pitfalls that @Jeff105671 has outlined with parallel supplies.

  • The ground wire of all devices will need to be connected to the same single point somewhere in your system.
  • Your signal wire will need to travel through each LED in sequence so that they remain uniquely addressible.
  • However, you can inject the 5V power at any point along the LED strips, or in fact split the two strips in half with respect to power, but keep them as one continuous chain with respect to the controller signal.

If you can cut the 5V connection between your single length of LED strip, then inject power at both ends of the two strip segments you should be able to overcome the yellowing and get away with the two cheapest, lightest USB power banks you can find. The big caveat to this working is that ground MUST be tied back to one point for everything, and the signal wire must pass in the correct direction through each strip segment in order.

Hi Trent.
Sorry for the late reply.

Not disagreeing with that. I merely pointed out the quoted mAh is usually at the BATTERY and becomes somewhat less after conversion to 5V.

As said NOT at 5V (usually). The mAh rating has to be converted to what it will be at 5V.
Cheers Bob

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