Does the Pi (Pi3, in particular) have any ability to report interal power supply voltages? Spot checks with a voltmeter on the GPIO header are some help, but a running test that can be logged over time would be much more informative.
I found a web page suggesting "vcgencmd measure_volts core" but on my Pi3 it reports VCHI "initialization failed". Apropos didn't hit on voltage or volt
As it happens I want to do this under FreeBSD, but if it can't be done under Raspbian there's no hope. If it can, then I'll look further.
Thanks for reading,