On 13/01/2021 23:37, Theo wrote:
> Joe  wrote:
>> 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.
>
> But lithium ion cells don't work for this application.  We need 5V.
> One cell is too little, two cells is too much.
> One cell would start at 4.2V which is too low to reliably run a Pi.
> Two cells would start at 8.4V which is dangerously high for the Pi, and the
> proposed 5V cutoff would only come in below 2.5V per cell which is bad for
> the cells.
>
> As mentioned 6V lead acid doesn't work for a starting battery - it might be
> OK for a 'leisure' battery that is fine with deep discharge, but again we're
> on a part of the curve where the battery is absolutely flat and declining
> fast.
>
> Possibly LiFePO4 might be better, but a pair of cells at 3.6V fully charged
> is still too dangerous for the Pi.
>
> 4x alkaline cells might just about manage it - 1.6V per cell when new, ie
> 6.4V total, which is a bit dangerous but maybe OK.  A gradual decline - I
> don't know where the Pi low voltage detection is but flat would be about
> 1.1V per cell or 4.4V total which is probably detectable.  So if you don't
> mind throwing away cells at 20% capacity it might just work.
>
> Theo
>
switch mode regulator will work with any of them.

Given that you want around a one amp capability

https://hobbyking.com/en_us/turnigy-1800mah-2s-12c-lipo-receiver-pack.html

is a suitable pack

and

https://hobbyking.com/en_us/turnigy-hv-sbec-5a-switch-regulator-8-42v-input.htm
l

is a suitable regulator.

HOWEVER that is still only half the story, as, presumably, you want to
run it off mains as well - or do you?

My point here is that a charger that will both charge the pack correctly
AND have a spare amp to drive the Pi is not so easy.

If you are happy to go and change batteries every so often, well then
presumably you will want a much bigger battery.

Anyway hobbyking is the go-to place for chargers regulators and
batteries - ship world wide, and product is good quality.


--
Future generations will wonder in bemused amazement that the early
twenty-first century’s developed world went into hysterical panic over a
globally average temperature increase of a few tenths of a degree, and,
on the basis of gross exaggerations of highly uncertain computer
projections combined into implausible chains of inference, proceeded to
contemplate a rollback of the industrial age.

Richard Lindzen

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