On 12 Jan 2021 21:43:03 +0000 (GMT), in
, 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?

Yes, except...

At the low current draw of the Pi, unless the battery is rather small
a cutoff of 5 V for a 6 V battery is 1.67 Volts per cell (Vpc) and
that's way too low if you want to prevent premature failure due to
overdischarge.

You might get away with this long term if the battery capacity is low
to begin with because then 1.67 Vpc might be say 80 - 90% discharged.
But for a larger battery, say an automotive starting battery, a
voltage of 1.67 Vpc at the current rate drawn by a Pi is way
overdischarged and the battery will experience short life.

The upside to all of this is that there probably won't be many
outages. If regular outages are expected, then a much higher cutoff
voltage needs to be set to prolong battery life... the bigger the
battery the higher the cutoff.

--
Jim H

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