A few years ago I constructed a simple Uninterruptible Power Supply using a stack of two super capacitors. It has been running continuously since then, but has some peculiarities and is not the most efficient solution. In addition, recent Raspberry Pi versions have gotten more power hungry and I decided to improve upon the previous design, increasing the output current capacity and making it more efficient.
I just completed the “new and improved” last gasp UPS that uses a single 2.7V 350-400F super capacitor. It provides power for 30 seconds when the mains power fails, which is enough time for the Raspberry Pi to shutdown safely, without damage to the SD card. It also charges and maintains the super capacitor at the proper voltage.
The details, and supporting python code example, are posted here: https://hackaday.io/project/25107-singl ... spberry-pi. I found that Hackaday provides a much friendlier space to describe project details than this forum, which restricts upload file sizes to a ridiculously small size and limits each posting to three uploads. All of the nitty-gritty is posted on the Hackaday project page.
I have no plans to make it into a commercial product, but I placed it into the public domain. If you want to build one of these yourself, I can post the link to purchase the PCBs from OSH Park, or make the gerber files available for download on the project site. But I’m warning you…it took me 3-4 hours to assemble this board (an experienced electronic design engineer) so I don’t recommend it for beginners. The LTC3759 is an MSOP-12EP (exposed pad) with a 0.5mm pad pitch. Soldering by hand is out of the question. You will need a hot air rework station or reflow oven to do it properly.
PaulV and I are also working on a UPS concept that uses a single Li-Ion battery. It is not yet complete but you can follow our progress here: https://hackaday.io/project/25116-singl ... spberry-pi. When we get it working it should provide at least 2.5A @ 5VDC for either a few minutes or a few hours, depending upon the battery capacity employed and the total current load. There are currently plans for a ~700mAh 14500 battery and also a ~2500mAh 18650 battery. The UPS communicates with the Raspberry Pi using a simple I2C interface and provides the Pi with regular updates of battery voltage. All the gory details are described on the Hackaday site. This is also freely available for you you copy and build yourself, but again we have no plans to make it a commercial product.