I thought he was talking about a proper battery tester, they are not the same thing at all. The monitor is just a voltmeter. The testers are larger do bunch of other functions.
One at a time so on a long trip I used my wife's phone. This is what I found, the battery's never get fully charged 90 percent max. Going down hill for miles the battery voltage went up 0.2V. I think it is intentional not to charge the battery's all the way, to protect the battery's from over...
I have 2 one on the AUX, one on the main battery. They have rolling displays, as I drive the ESS event is captured. I can see the restart as a dip in the main battery voltage. I can see the AUX battery during the ESS event, goes down a bit. The rolling display goes as long as the phone is near...
The meter does what you were doing with your Fluke meter. It can be programed from AGM. I also Have A 1000A shunt so I know how much current draw there is.
I bought a battery monitor $25 from Amazon. It gives battery voltage and temperature to your phone. Measures percent of charge, staring test, charging test, all while you drive. It gives a rolling graph for all the tests.