Programmatically check for loss of VIN power

[ScruffR]

Somewhere on this forum is a solution proposed by @rickkas7 to do what you are looking for. I just haven't got time to search for it now.
Since Vin is not that different from USB you may search for either of the two.

But since most GPIOs on the Electron are 5V tolerant you could just use an input pin to read the voltage on Vin as this won't be showing any voltage when the device is running off LiPo only.

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I found Rick's post

When is Electron Powered By Battery

This is a deceptively difficult thing to detect. I’m not sure the method here is the best way to do it, but this is the first method I’ve found that actually produces the correct result, so there is that. I believe the problem is that when there’s no battery, the PMIC doesn’t go into some easy-to-read no battery state. It rapidly switches between fast charging and charge complete states, continuously. it’s possible to detect this, and this sample code breaks out three things that are sort of u…