I built up two more of my Raspberry Pi LiPoly battery back up boards today. After doing so, it occurred to me that, in addition to allowing for portable use of a Raspberry Pi Zero, as I had originally intended, the board, with a few minor modifications, will work quite well as a UPS (uninterruptible power supply).
I wired the output from the comparitor to GPIO 21 on the Pi expansion header. I’ll be able to monitor it to detect when the external power is off. I can then gracefully shut everything down.
Without anything else running, a small 150 mAh battery will give me about 40 minutes run time with a Pi A+, and about 20 minutes with a Pi 3. Add in peripherals, and it will go down a bit from there. Regardless, it will have plenty of time for a clean shut down, or to wait out a short power outage.
The cover image here shows my Pi A+, and my Pi 3 with their UPS integrated. I still need to drill a hole in the case lids so I can flip the power switch. Otherwise, I’ll have to pop the lid off if I want to completely shut it down. Not a big deal, but still something I should do.