On v1 I used a large UPS with since I had the space. For v2 I don't have space so I went with a UPS meant for a Pi Zero. One thing I like about this UPS is that the board has pads for the switch, 5V and GND.

That made it easier to integrate. The bad thing is it gets hot! Initial high current charging was a 170F. It's that little 4R7 (4.7uH) inductor. I contacted them to see. a quick search says these are typically rated to 185F so we're in the zone. The image looks weird because the camera didn't line up with the lepton sensor. FLIR MSX you fail.