I built/assembled them. I carried out some informal experiments and found that it requires roughly one million full screen refreshes on two coin cells to completely drain them. Furthermore, given that Badger operates on an RP2040 and a battery holder comes with a toggle, it is astonishingly durable device. It is like a smoke detector, which can operate for a decade straight on a single battery.
Note: CR2032 is awesome and all, but be careful about extrapolating like this out to decade+ timeframes.
CR2032 only has 235mA-hrs of life down to 2V. x2 and that's only 470mA-hrs and 2V probably browns-out the circuit (so... 470mA is already a stretch. You'll probably get less than that on practice).
Over 10 Years, that's 5uA of power usage on the average.
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IIRC, an aluminum capacitor has ~15 uA of leakage current, and Tantalum is ~1uA of leakage current. So Aluminum caps are already disqualified, and Tantalum capacitor leakage-current already uses 20% of your power budget. Given the "Burstiness" of this workload, I know that these capacitors need to exist somewhere.
You probably can get a year out of this in practice. To get better than that, you'll need to spend an incredible amount of energy on finding every 1uA "leak" and plugging the leak.
And its crazy how many things leak 1uA. Not only capacitors leak 1uA, but so do MOSFETs, diodes (reverse bias currents, especially in schottky diodes)... diode-protected MOSFETs (oh no, twice the leakage!).
After I completed the project, I made a somewhat lighthearted personal vow to try to design a custom PCB next time I fall down the rabbit hole of hardware tinkering. I suspect, these days it is a commodity skills, and curious if you happen to have any suggestions or articles that could serve as a starting point?
PCBs are a dark art, and I focus on lower-speed (below 30 MHz) to try and avoid any issues.
I know that the faster the PCB is, the more issues you get. Above a certain frequency, inductors look like capacitors, capacitors look like inductors, and PCB-traces look like transmission lines with reflections and other such nonsense. Staying at a slower speed helps negate these issues.
Most application notes, be it from STMicro (for STM32) or Microchip, or really any other microcontroller manufacturerer, will have recommended hardware designs + their thought process fully documented.
Study up on the "reference designs". For ATMega328p, that's Arduino Uno. For more recent AVR chips (such as AVR DD), that's "AVR DD Curiosity Nano". (See schematics here: https://www.microchip.com/en-us/development-tool/EV72Y42A)
The sibling comment is too specialized, I feel. It depends on what you want to do. If you just want to connect a few components together, you can learn the required skills in a day, watch some KiCAD videos.
I made a sensor board the other day (I'm just printing the case for it now), and it was very enjoyable, and even came assembled for $1.7 per board:
> The sibling comment is too specialized, I feel. It depends on what you want to do. If you just want to connect a few components together, you can learn the required skills in a day, watch some KiCAD videos.
That's fair.
Lets put it this way: if your circuit works on a breadboard, you don't need to know anything about PCB design. The PCB will pretty much always be better than the breadboard.
Things get troublesome as you enter mixed-signal (analog + digital), or high-frequency.
Agreed, but you still need to know a ton of things that seem hard when you haven't done them before. Even exporting the Gerbers, or the BOM for assembly, or any of those things seemed too hard to me before I did it for the first time, so I don't want to underestimate people asking "I want to connect a few components into a custom PCB, how do I do it?".
> That has to cut down on the thickness as well since the Badger 2040 is designed to take AAA.
Would it be ok with the lower voltage of NIMH rechargeables? I really dislike primary batteries.
Edit: found the answer.
> 2x AAA rechargeable (NiMH) batteries only puts out 2.4V which is, strictly speaking, not enough for Badger. However, in our tests it keeps on truckin' down to an input voltage of 2.05V (without the LED), so if you want to use rechargeable batteries that should be fine.
AAAA is popular and as cost-effective as CR2032 (aka: bad value, but less-bad value than most other batteries of this size). Note that CR2032 is toxic, so AAAA is somewhat preferred.
AAA and AA have much more energy-per-dollar. I mean, so does lead-acid but I guess that's too big lol.
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Specialty batteries, like CR123A, seem to fit the bill for this size much better. But those are so, so much more expensive. I feel like the only two cells worth really considering are AAAA and CR2032, despite their deficiencies.
Exactly! The device is approximately 12mm thick. I will upload a side shot shortly. I appreciate you bringing this up, it's an excellent point! To be honest, it is still a bit thicker than I had initially hoped for; however, when I weighed my options - solder 26 devices by hand vs use pre-made components -- the decision was much more clear.
