On 12 Jan 2021 21:43:03 +0000 (GMT)
Theo  wrote:

> 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.
>
> I'd expect it to be so steep you don't get enough time to do any
> meaningful shutdown, but I could be wrong.  What battery setup do you
> have and how does it work out in practice?
>

Lithium cells have a linear-ish discharge curve, from 4.2V down to
around 3V. Any sensibly designed lithium battery will cut off its
output at the chosen lower bound, because if it completely discharges,
it's dead forever. Not a 'steep' decline, a 'fall off the wall' decline.

It's a good idea to have some independent means of anticipating this
point.

--
Joe

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