Ykhjuy

On the most recent pi I've been playing with I've noticed the under-voltage lightning bolt warning, discussed on this question.

My question is whether there's a way I can access the voltage reading used to trigger this warning, so that I can see how under-voltage it is, and whether removing peripherals / using a bigger supply fixes is.

There are related questions such as this one one what might be causing voltage drift of the 5V rail, and this one one what the power requirements of the pi are, and finally this one for how to measure the voltage and current from a battery, but I couldn't find an explanation of how to access the pi's onboard supply voltage measurement (assuming there is one).

I do not think there is any Pi circuitry to return the current supply voltage.

As far as I am aware the under voltage circuitry is a piece of hardware which triggers at 4.65V. So you could discriminate between two values, more than 4.65V or less than 4.65V.

The only justification for this answer is I remember dozens of posts where people have asked the same question and I do not remember any other answer.

As noted in Raspberry Pi Power Limitations (which you referenced) The newer Pi(3/2/B+) have a voltage monitor chip (APX803) which triggers at 4.63±0.07V. The Pi3B+ Pi3A+ use a MxL7704 chip to manage power, which has the same nominal trigger point.

The Pi has NO voltage measurement circuitry, this is an on/off trigger and there is no analog measurement circuitry. If you want to measure the voltage, you need a meter or one of the in-line USB monitors.

The GUI had a lightning bolt which comes up in the top right if the voltage is inadequate.

You do not need "a bigger supply" (whatever that means) you need a quality PSU whose voltage is adequate at the rated current - which most inexpensive supplies are not. Even with a decent Power Supply if you use poor quality cables you will have problems.