On 14/01/2021 11:46, Ahem A Rivet's Shot wrote:
> On Thu, 14 Jan 2021 11:04:55 +0000
> The Natural Philosopher  wrote:
>
>> On 14/01/2021 10:18, Ahem A Rivet's Shot wrote:
>>> On 13 Jan 2021 23:37:18 +0000 (GMT)
>>> 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 and a DC-DC converter to give you a regulated supply,
>>> monitor the cell so you can shut down (well) before there's too little
>>> to run the converter.
>>>
>> It's slightly simpler and more efficient to use a step down from two
>> cells.
>
>  It's only simpler if you look at what's on the converter board :)
>
No, that's not true. To step up you need to use a buck converter with a
choke big enough to store energy to push the voltage up. To step down
you only need a choke to smooth HF ripple, and transistor efficiency
goes up with voltage and down with current, so that's a win win too.

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