Can I use a fuel gauge and a solar charger at the same time?

This question is perhaps a subset of my previous question here.

I would like to set up a remote CCTV station using solar power. This will need a solar panel, a battery, and a board to ensure the solar panel keeps the battery charged and supplies clean power to a Pi.

However, to my surprise, the solar/charging board does not have any feature to examine battery charge state, other than whether it is charging. While that feels like an opportunity missed, fuel gauge boards are available - my question is whether it would work.

One could put the fuel gauge between the charge board and the Pi (i.e. in series) but that would give a reading of the health of the charge from the solar board, and not necessarily the battery. For example, the charge board might be sending some solar power to the Pi, and thus it would look healthy, even if the battery was depleted. Or, it might tinker with the voltage, and give an inaccurate reading.

That would look like this daisy-chained system:

So I wonder if that is not ideal. The alternative is to put the charge board and the solar board in parallel upon the battery, but I am not sure what the impact would be to feed power from the charge board into both the battery and the fuel gauge.

The second option would look a bit like this:

But would it work?

The reason why I want to monitor battery health is to go to a low-power mode if a poor battery health is detected. I expect the battery to last for a full night, when solar energy is not being received.

Alternative options

I did look at PiJuice, but it is not clear if their Zero board would work with a Zero 2. Their issues list suggests that folks are experiencing a lot of problems with this option, and most distributors are out of stock anyway. I don’t want to spend time on tech where the supplier might soon close up shop!

Finally, are there any solar/charge boards that would resolve the issue I’m identifying here? A built-in battery ADC feels like an obvious feature that most users would need.

Hmm, I wonder if I could experiment with the serial arrangement. From the tutorial:

[The regulated load output] pin will provide a regulated output when the input voltage is below the over voltage protection threshold and above the regulation voltage. It will never be higher than 4.4V (but it may dip down to 3V or whatever the LiPo battery voltage is at, if USB/DC [isn’t] plugged in)

That said, elsewhere it says:

DC/USB/Solar is prioritized over battery power when available, to keep the battery from cycling

That would mean that during the day, current measurements would be meaningless, as they might represent the output of the panel!

That does give me an idea though - perhaps I could use a light sensor to determine whether the panel is doing anything. When it has gone dark for more than a few minutes, it can be assumed that the fuel gauge is measuring the battery 🤔