What’s a good way to power a smallish portable project? My wife has a clear “bubble” umbrella, and I’ve been wanting to add LEDs to it. There are 8 struts, each about 2ft long. So with 30/m strips, I should get 20 LEDs per strut, for a total of 160 lights.
Obviously, I’m going to avoid trying to light them all simultaneously at full brightness (9.6A). I have a bunch of 18650 LiPos (3.7V, 3000 mAh). I figure I’d probably need to put some in series @ 7.4V, then use a regulator back down to 5V. Any suggestion for one that will get a decent current output?
I’ll probably use a Teensy (3.2?) to drive things. Should I use an OctoWS2812, too, or could I get away with skipping that? Also, could a Teensy LC drive 8 outputs, or would it be too limited for this?
@Ken_White Hmmm, I hadn’t considered daisy-chaining the strips. I’m still not sure if I want to deal with the extra wiring, but I’ll keep it in mind, just in case. Thanks for the links!
@dougal you certainly don’t need a OCTOWS2811 to make this work, the Teensy 3.2 will work fine. You might want to use a resistor for each data line. You can see how they do this in the OCTOWS2811library documentation.
As for a decent high amp buck converter, this one is kind of big but can go as high as 12A:
All of the ratings are max, but you can do a lot with these buck converters.
Also, question for the group, does brightness scale linearly within FastLED? For example: brightness(255) is not 2x as bright as brightness(127). But does brightness(255) require 2x the power of brightness(127)?
@dougal you got 2 choices: a 5V powerbank - select carefully, usually their output range is 1… 3A
If you want Battery packs check out the new LiFePo4. They are 3.3V, cannot explode and have up to 15Ah.
Some also fit into standard AA battery holders, some have screw terminals.
I’m unsure as to why we all make this stuff so complicated. I just powered 4x 16x16 led matrices (yes 1024 leds) from one single large usb battery pack that has a 5.5a rated output and 13.5ah capacity.
Trick is to a) limit the current draw in software using the power management functions, and b) draw from multiple outputs on your battery pack. My pack has 3 outputs each rated for 2.5amp. I set my max power draw to 4amps, powered the teensy from one output, and then put two panels each on the other outputs. That’s 512 leds per usb port drawing 2amps each. Works perfectly, nice and simple no additional regulators required and easy to recharge from any usb charger.
Have I overlooked something? Will this blow up in my face one day?
@Ben_Delarre please have respect from Li-Po and Li-Io. They can be dangerous if mistreated at charging and discharging.Once a mistake occurs it starts a mostly slow but irreversible chemical process: formation of dendrites.https://en.wikipedia.org/wiki/Dendrite_(crystal)
Those crystals will poke through the insulation membranes and create local shortcuts, what triggers in the end a “thermal event”. This can happen days or weeks later. They burn at a typical temperature of 1700C and issue nasty chemicals like boric acid https://en.wikipedia.org/wiki/Boric_acid
Same happens when those batteries are mechanically mistreated.
If you know what you are doing nothing will blow up. Best practices (Li-Io): Never overcharge, stay strictly below 4.2 V (at 4.215V the process starts); never discharge more than specified, Min Voltage of 2.7V is safe, at 2.4V you have to throw the battery away. Use a special Li-Io Charger with balancer. Then you can keep each cell on the same voltage. If you have different voltages in your pack you will unintendedly over- charge and discharge one or more cells in your pack.
Good news: the new LiFePo4 cells do not have any of the above mentioned issues.
@Juergen_Bruegl Thanks for the info on the LiFePO4 cells. I vaguely knew that they had some “better” properties, but I hadn’t really looked into the differences before. There are some really compact, high-capacity versions out there with reasonable prices!
Of course, wouldn’t you know that I just recently bought a new charger, but it doesn’t do LiFePO4 (it handles Li-Ion/IMR and Ni-Cd/Ni-MH). But I’m definitely going to consider investing in some LiFePo4 (AKA “IFR”) cells and charger.
@Ben_Delarre Yes, for now, I’m probably going to go the route of just using a USB power bank or a couple of 18650 LiPo cells, with a buck down to 3V3 for the MCU. And using software to limit the current, as you mentioned.
@Juergen_Bruegl , I’ve been looking into LiFePo4 batteries for most of the evening now and had some questions from your experience. I typically use 5V powerbanks, but, like anyone wanting the best of all possible worlds for wearables, I’d like to reduce weight, increase capacity, all while not catch on fire, etc.
Your post above talks both about the 3.3V nominal voltage, high Ah, but then the board you reference is a step-down. What is the downside to having say four LiFePo4 18650s with a 5A discharge rating in parallel using a boost converter like this to power the leds?
@Chris_Creel it is all about efficiency and depends mainly on how many LEDs you want to drive. On the 3.3V side a Teensy needs about 30mA, an ESP8266 up to 200mA. If you step-up this voltage to 5V you get around 90% at 1000mA and less than 80% at 2000mA
Let’s assume you want to drive 50LEDs at average 50% you need 1500mA.
15% losses in the step-up account for 225mA; so you got a total powerdraw (with an ESP) of around 2000mA
Btw, the Step-Down I’m referencing is the CHARGER. It has 3 pots, one to set the max current allowed for the batteries. 700mAh batteries should be charged with max. 700mA
