A Class-3 PoE PD front-end takes PoE power from an RPi 3B+ 4-pin header, a commercial Cosel DC-DC converter isolates and converts from 48V to 5V at 2A, a charger circuit keeps two series 25F super-caps topped up, a balancer circuit makes sure the super-caps stay in their voltage range, a boost converter circuit supplys the RPi with 4.9V, a microcontroller supervises operations and also provides a watch-dog function. All on a HAT sized RPi PCB, there's even room for a 20mm fan, super-capacitors are off-board but there's a full 40pin through-header to accept another HAT on top (eg a RAK2249 lora concentrator & GPS board).

The supercaps provide about 30s of backup time at 800mA load, that is enough to cover 10 seconds of power outage plus another 15 seconds during which the Pi is cleanly shut down (with the help of a service script) if necessary prior to power removal.

The impetus for this project was the construction of a LoRa gateway that must be highly reliable - trusted not to trash it's SD card and can be remotely located and managed.

I currently have a prototype up and running but have several bug fixes to implement on a revision-B PCB and am still working on the firmware for the microcontroller.  Watch this space and let me know if you're interested.