On a sunny day (12 Jan 2021 21:43:03 +0000 (GMT)) it happened Theo
 wrote in
:

>Simple Simon  wrote:
>> Brilliant - many thanks - works a treat - I was almost there!!
>
>I'm curious... does this actually work for low battery detection?
>
>Unless you're wiring the Pi directly across a battery that's giving out
>somewhere around 5V (for example a 6v lead acid), most of the time you'll
>have a voltage regulator between you and the battery.  That regulator will
>aim to keep providing 5V as the battery is going flat.  When the voltage
>sags below 5V it means the battery is so empty it can't maintain that, which
>means it's in a very steep part of the voltage decline.

That is a valid argument for say detecting a low mains voltage.
It does not however cover a fault in the regulator itself.

I like the idea as extra test for fault conditions
like too much voltage drop over USB cable by high loads etc..
You can make it log the error, or switch off high load / routines first
before shutdown, sound an alarm, etc.
A very common error in wallwart supplies is bad filter caps on the output,
causing a high ripple on the 5V, this could perhaps detect that.
Did no know about the low voltage check option.

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