I did some testing to get an idea how long different Raspberry Pi versions run on a fully charged LiFePO4wered/Pi, and how much difference there is between the 3.2V and 5V (4.75V) versions.
All of these tests were run using the same LiFePO4 cell, and using the same SD card with Raspbian Jessie Lite and a USB WiFi dongle over which I maintained an SSH connection throughout the test. In each case, I ran `sudo apt-get update` one time, and `lifepo4wered-cli get VIN` regularly to track the battery voltage.
This is by no means a worst-case scenario, but the addition of the USB WiFi dongle and active SSH connection makes it not entirely theoretical either. So here are the run times, in minutes:
|3.2V LiFePO4wered/Pi||5V LiFePO4wered/Pi|
|Raspberry Pi Model A+||145||115|
|Raspberry Pi Model B+||80||65|
|Raspberry Pi 2||72||62|
I'm very happy to see that even the Raspberry Pi 2 runs for over an hour, even with the less efficient 5V LiFePO4wered/Pi version! The much-lower-power Model A+ gets over 2.5 hours!
The more efficient 3.2V version squeezes 15-25% more run time out of the battery, which I consider significant enough to think I favour it over the 5V version.
Another nice thing to note is that the RPi 2 is not much worse than the Model B+ when mostly idle.
I have a Raspberry Pi 3 on order, and I'm curious to see if I'll get an hour of backup time with that one as well. What I'm even more excited about is that thanks to Hackaday.io, Adafruit and this project being one of the Adafruit Pi Zero Contest Round 1 finalists, there's now an otherwise unobtainable Pi Zero on it's way to me! :-)
Since the Pi Zero is supposed to be the lowest power Raspberry Pi yet, I can't wait to see how much run time it can squeeze out of the LiFePO4wered/Pi.