On 13/01/2021 22:04, Joe wrote:

>
> Lithium cells have a linear-ish discharge curve, from 4.2V down to
> around 3V.

They do.

> Any sensibly designed lithium battery will cut off its
> output at the chosen lower bound,

No, it wont. That isn't in the *battery* - that's in whatever is drawing
current off it. I have many lthium packs with no such ptotection



> because if it completely discharges,
> it's dead forever. Not a 'steep' decline, a 'fall off the wall' decline.

No, again it goes on down to zero, 3V is just approximately the point at
which irreversible damage starts to happen.

>
> It's a good idea to have some independent means of anticipating this
> point.
>
The way to run a lithium in this application is to use a 2 cell lithium,
a constant voltage constant current charger limited to 8.4V and probably
no more than the battery mAh capacity divided by one hour... and a
switched mode 5V regulator to feed the Pi and then somehow monitor raw
battery voltage and switch it all off when it gets to say 6.6V and hope
that the SMPS and monitoring circuit don't then still draw enough to
damage the battery, or even if it is take it on the chin and replace the
battery when that happens.

I would imagine that the whole setup would cost more than the Pi itself



--
“It is dangerous to be right in matters on which the established
authorities are wrong.”

― Voltaire, The Age of Louis XIV

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