If you only test your batteries while a battery charger is runnning, then you
know very little about batteries.

You must have a very clean hog, with all that wash on hand.

Real life scenario for you:

When I go aboard my boat and prepare to get underway, I check the battery
voltmeter on the distribution panel. It always reads just over 13 volts. Before
I start the engine, I shut off the shorepower to the vessel. When the
shorepower is shut off, the battery charger is not "running".

Does the voltmeter take a nosedive to 12.6? No, it does not. It remains at over
13.

Yes, if I wait long enough the voltage will begin to drop.

But my fully charged batteries show 13.2 volts *without* the charger running.

Are you one
of those bozos that runs a genset all night long?

I don't run a genset at all. Propane galley boat with a diesel furnace. Two
Rolls 8D batteries. Not even equipped with a genset.

Are you one of those bozos who prefers to believe that the state of charge
represented by the .6 volts between 13.2 and 12.6 never existed, simply because
the first .6 volt bleeds off?