Michael J. Mahon  wrote:
> A small, non-backed-up 5v wall wart will provide a logic level that goes
> low whenever mains power fails. Two resistors will convert it to a 3.3v
> logic level.

Good idea.  I'd probably make those resistors fairly low value, otherwise
the output capacitor may hold charge as long as it can sustain the leakage
on the Pi's GPIO (very long) before the voltage drops enough to signal the
failure.

However it won't tell you if the power bank is getting low - eg if the power
came back after an outage (so the power bank is near empty) and then goes
out again.

Theo

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