On 14/01/2021 09:23, Folderol wrote:
> On Thu, 14 Jan 2021 03:46:08 +0000
> 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.
>
> To get round that use a latching relay 'start' circuit, and when the
> battery voltage gets too low unlatch the relay-  Zero power demand then.
>
>
Its getting as complicated as a renewable energy grid though isn't it?



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