Ahem A Rivet's Shot  wrote:
>         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.

Which is all very nice, but is not what the OP is doing.

They're monitoring *the input rail of the Pi*, because that's all the Pi
hardware does.  You put 5V in, it tells you if it sags below some threshold.
You don't get to put some other voltage in to test.  If you put in your raw
cell voltage that will under- or over-volt the Pi and it'll either not work
or fry[1].

If they want to monitor some other voltage, they'll have to start from
scratch because none of what they're doing (hardware or software) will work.

Theo

[1] the main SoC has some protection as it has its own voltage regulators,
but USB-side things do use that 5V directly and you may fry it that way

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