On Thu, 14 Jan 2021 03:46:08 +0000, in ,
The Natural Philosopher  wrote:

>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.

Yes... with special care to observe that safe cutoff voltage so that
when power returns the constant current, constant voltage charger
doesn't charge the battery any faster than it can accept safely. But
wait... the constant current needs to be all that is needed to run the
Pi PLUS the additional safe amount to recharge the battery, but... how
do you assure the Pi and the battery share the current appropriately
so that the Pi runs and the battery recharges safely? Maybe a bit more
control is needed to assure a battery voltage above a certain value
before the Pi is rebooted.

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

Likely so, but I've always wondered at comparing the COST of the Pi to
the cost of supporting equipment vs comparing the CAPABILITIES of the
Pi (vs desktops/laptops/etc) to the cost of support. One can't really
expect support costs to scale down proportionally to the lower cost of
the Pi.
--
Jim H

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