@Gonzini, the reason I bought the cheap plug in DVM was maybe two fold, first, it lets me see the voltage at a glance as it is at a convenient angle for easy viewing from the driver's seat - and due to its construction, always in that fixed position, and second because having it plugged in at all times avoids the need to have an expensive DMM in the car when out and about and finding a way to make its screen viewable and "awake".
Starting with a fully charged battery, I was initially curious to observe that driving up a long hill with moderate engine revs, the displayed battery voltage dropped slowly - then got charged on the overrun on the other side of the hill - it behaviour is pretty understandable when you think about it, and would not necessarily be the same behaviour if starting with an "not topped up with a charger" battery.
Another behaviour you might find is, if road traffic is flowing reasonably freely, even when stopping after a 10 miles run in daylight and the heater fan off, auto Stop/Start might not "happen", reason being that the battery is in a "just surviving" charge condition - mind you, that will only work if there has not been any or much accelerating or braking/overrunning. Obviously this will only happen "now and again" - but as I've seen this happening, I thought that I'd mention it to you.