ay ON with either supply, but you can turn OFF under software control
No, not quite. It only latches ON when the computer asserts GPIO 19. Note there is no direct path to the base of Q3 from +12V, +9.6V, or +5V, so not hing from any power supply can energize Q3, which is the only means of turn ing on O1. GPIO19 is floating until the software asserts it to +3.3V. Oth erwise, R8 makes certain no current flows though D5 and the LED of O1. Onc e the computer shuts down, GPIO 19 once again floats, and the battery is is olated from the rest of the electronics, with the exception of U1, which I must choose carefully so that it won't be killed by having ~10V on its posi tive input and its output with no power to its positive supply rail. A sma ll leakage through U1 - less than 0.1ma, won't be significant.