I'll now vouch for the accuracy of the little digital voltmeter.
With engine off it read 12.2 volts. A bit low. Engine running it read 14.6. A bit high. I wrote that off to being a cheap Chinese made voltmeter, which, in fact, it is.
But then the battery in my truck finally died. Replaced a rather expensive Group 65 and low and behold, engine off it reads 12.8 and engine running about 13.8. About where it should be.
Oh, and it does a dandy job of charging my phone while I drive.