I'm incorporating a HUZZAH32 (part of Adafruit's Feather line) into a small system, where it will be powered from a 5vdc power source. The obvious way to connect the 5vdc is to the USB pin (the 5v pin on the USB connector). But I need to occasionally plug in a micro-USB in order to download sketches; so I'd need to be able to disconnect the system connection so the system 5vdc and the USB master won't be in conflict. See the HUZZAH32 schematic here: https://learn.adafruit.com/assets/41630. The USB 5v signal is called VBUS (upper-left corner).
But I could instead connect the system 5vdc thru the BAT pin (VBAT in the schematic). Would this work ? Under normal conditions, with nothing plugged into the micro-USB, VBUS is pulled low by R12, turning on the PFET Q3, and VBAT (connected to my system's 5vdc supply) would be passed through to the 3.3v regulator, same as if a battery were connected. When I plug in a micro-USB, if its 5vdc signal VBUS is equal to or higher than VBAT, Q3 is turned off and VBUS passed thru to the 3.3v regulator. If VBAT is higher than VBUS, then Q3 is turned on and VBAT is passed through (as when the micro-USB is disconnected), but the diode D4 prevents VBAT from trying to source current into VBUS.
It's this last case (VBAT higher than VBUS) that is different from the normal situation when a battery (with significantly lower than 5vdc) is connected to VBAT. I've seen it stated that it will damage the Feather if 5vdc is connected to the BAT pin. I don't see how, according to my analysis above. I can imagine that the LIPO Charger chip might be unhappy if its VBAT output were driven higher than its VDD input, but I can't see anything in its data sheet forbidding this (https://cdn.sparkfun.com/assets/learn_tutorials/6/9/5/MCP738312.pdf). Anyhow, I could simply de-solder this chip if I like.