Using a supercap with ATXRaspi to power shutdown after primary power loss

[ant_bite]

After reading the blog post about using supercaps in place of batteries (and using solar panels to recharge them) got me to thinking about how I could use a supercap to provide enough power for the ATXRaspi to properly run the shutdown procedure in case of power loss.  It would be like a miniature "smart" battery backup.

The blog post noted that the cap was inserted inline with the panels so that they provided power to both the cap and pi, with the cap providing power during low/no light times.

It seems simple enough to add a cap (and shotky diode) inline with whatever power is supplied, but where I am stuck is how to close the "power button" circuit when the primary power is lost.

My assumption is that a lead would have to come from the positive lead of the main power supply going into a GPIO, with an addition to shutdown.sh that would run if voltage on that pin dropped to 0.

In your opinion, is that the best course of action or am I under/over thinking this solution?

Thank you.

[Felix]

Not sure what blog post you refer to but ...
You'd need a very super cap to do it. Then you'd have to control the cap voltage, not to drop it below 5V to ensure everything is stable.
When you add a shottky diode, you will drop 200mV+ from your cap rail.
The ATXRaspi continues to draw current from your cap, which will drain it.

FWIW back before there was MightyBoost and MightyHat and I think in the very early stages of ATXRaspi I put a lot of thought into this very idea. Eventually it never made it to production due to high costs and many complications and potential issues of using super caps in such a setup, with only very little ability to sustain a 1A load and for a very short time ... so it was NOT WORTH IT.
There have actually been some commercial products that backed up the Pi with supercaps, while cool and challenging in concept, too much to bother. A much simpler (and compact) scheme is a 5v boost with backup from lipo pack (IMO).

[ant_bite]

Thank you for the enlightenment.  A lipo battery sounds like the right way to go.

Oh, the blog post was from 9/15/17 where you were demonstrating how a super cap could be used along with a solar panel to provide power to a moteino with a weather shield.

Thank you again.

[Felix]

Oh yes, that is a different setup with the Moteino drawing only a few uA of current 
So in that case the tiny solar panel and supercap are sufficient to run the Moteino for several days between charges!
A Pi is a much different "beast"