It took longer than expected to get results from using Raspberry Pi 2 and ZeroW. I first pugged in the RPi2, let the supercap charge and then pulled the wall wart. The red power light on the RPi2 began to flash hysterically and the dreaded lightning bolt appeared on the display. Something was wrong with the voltage delivered to the Pi. I measured 4.81V between the OUT+ and OUT- terminals on the UPS, but the PI was getting around 4.7V. It was that damn pink micro-USB cable that I got from the dollar store (for a dollar!). The power leads on that cable were pretty small -- 30 or 32 AWG. I did not want to cut into any of my other micro-USB cables, so I ordered a set of six micro-USB cables, 12 inches long, for $7 from Amazon. They arrived today.

I cut the USB type-A connector off the end of the cable and trimmed off the shielding and the unnecessary data wires. The power wires did look beefier. Substituting the new black power cable for the cheap(er) pink cable fixed the problem. I got a flash from the RPi2's red power LED when the AC adapter was disconnected, but the LED stayed on after that, and no lightning bolt appeared on the screen. I the ran the RPi2 with all 4 cores at 100% without issue. The 100F supercap voltage, after  going through 15 seconds of waiting and then 20 seconds of shutting down, was 2.3V -- very acceptable. 

I also tested the UPS with the Raspberry Pi ZeroW. The current drain is quite a bit lower -- the UPS can supply power to the ZeroW for 35 seconds (using a 100F supercap) before having to enter the shutdown sequence. I tried all of my various AC adapters to see if the dreaded relaxation oscillation would return, but the UPS reliably applied power and dutifully shutdown without incident, independent of the AC adapter.

I'm very close to the end of this project. The only task remaining is to swap the current UPS for this new UPS in my heating system controller. Since it is the middle of winter, and I'm a bit risk averse, that task will have to wait until warmer weather. 

OLD vs. NEW:

Just for grins and giggles I took a photo of the first version of the UPS next to the latest version.

Some items of note:

• The first version did provide 2.5A from a single supercap charged to 2.7V. The latest version can't do that -- it requires a stack of 2 supercaps to output 2.5A (per spec).
• There is no trim pot on the new version -- a savings of around $1 (and board space).
• The new version can charge the supercap much faster without heating the PCB too much since it uses a switched-mode charger. The old version used a linear charger with foldback current limit.
• Input power jack. Old -- barrel jack. New -- micro USB.
• The new version is less susceptible to noise between the Raspberry Pi and UPS. The PIC SHUTDN input noise rejection seems to be working.
• The new version uses a synchronous switcher for both charging and booster -- lower power dissipation in both cases.
• The old version required nearly 4 hours to populate the PCB. I can populate the new board in less than 1 hour.