I assembled two Hat versions and two 18650 versions. All PCBs are the latest revisions. All of the boards are functional, but I haven't bothered to take extensive parametrics. One of the 18650 boards is in service to my seismometer. The other 18650 board will be a spare. One of the Hat versions will find use in support of the various Raspberry Pi boards performing various functions around the house. The other has been shipped to a friend in The Netherlands that has been an invaluable consultant and sounding board for these kinds of things.

Discovered A Design Flaw:

I confronted one of my design assumptions today when swapping out an older UPS version for this version to service my seismometer. When I upgraded my seismometer I did not connect the UPS back into the system properly. Consequently, the backup battery drained to unsafe levels. When I transferred the battery, which had now drained to a voltage around 1V, into the U1LiUPSRPi-18650 version the UPS failed to supply power to the Raspberry Pi. 

The problem was that I assumed that the battery would almost always have a potential greater than the POR threshold of the PIC16F18323. In this particular case it did not, so the PIC kept the LTC4040 shutdown. Without the LTC4040 activated the output voltage of the UPS did not increase to the point where the PIC could operate. I removed the battery and performed a recovery operation on it (10% charge current rating until the battery voltage exceeded 2.5V), then reinserting the battery when the voltage exceeded 3.2V. At this point the UPS worked properly again and proceeded to charge the battery to full.

If this was a commercial product I would be iterating the design at least once more. The UPS should function whenever a valid supply voltage is available at its inputs. But since it is primarily for my own use...it's good enough, as long as I'm aware of its shortcomings. BTW: I think the flaw could be corrected with another low current Schottky diode from the input to the PIC VDD pin. 

LiFePO4 Disappointment:

I purchased four PKCELL 14500 LiFePO4 batteries from Amazon to see how well they perform compared to the 14500 Li-Ion cells. Just comparing W-hr the LiFePO4 are inferior:

W-hr = 600mAh x 3.2V = 1.9 W-hr for LiFePO4

W-hr = 650mAh x 3.7V = 2.4 W-hr for Li-Ion.

It also appears that the LiFe cells have a relatively high internal resistance and high leakage current. I measured the current that the UPS drains from the battery terminal as 45µA, so it should be weeks between recharge events. But the log is reporting that the battery need recharging every day:

2019-03-06 08:07:50,872 INFO     *** UPS Monitor is active ***
Battery Type is LiFePO4
Charge Termination Voltage = 3.6

2019-03-06 14:25:14,430 INFO     Battery is charging. VBAT = 3.46
2019-03-06 14:25:34,453 INFO     Battery charging terminated. VBAT = 3.63
2019-03-07 14:31:23,550 INFO     Battery is charging. VBAT = 3.63
2019-03-07 14:31:33,563 INFO     Battery charging terminated. VBAT = 3.63
2019-03-08 14:34:55,227 INFO     Battery is charging. VBAT = 3.60
2019-03-08 14:35:05,277 INFO     Battery charging terminated. VBAT = 3.63
2019-03-09 14:35:14,757 INFO     Battery is charging. VBAT = 3.47
2019-03-09 14:35:34,780 INFO     Battery charging terminated. VBAT = 3.63
2019-03-10 15:33:21,720 INFO     Battery is charging. VBAT = 3.47
2019-03-10 15:33:31,732 INFO     Battery charging terminated. VBAT = 3.63

The recharge event is usually less than 1 minute. I had to change the update time for the UPS monitor program to 10 seconds in order to catch the event. I don't know what effect that will have on battery longevity. I do know that the LiIon 14500 cell did not require a recharge for at least three weeks.

If I find anything else that's noteworthy I'll post it here.