My AGM battery, when stuffed full to the brim, well at 14.7v it accepts 0.0x amps, the X is because I have no Ammeter which can read below 0.05 of an Amp.
So with this absolutely fully chrged battery, I start my engine. When amps again taper to 0.0x amps, at 14.7v, the battery can again be considered fully charged.
So how long does it take for amps to taper back to 0.0x amps at 14.7v after starting my engine?
About 45 seconds.
I don;t buy the short trip driving as a killer of batteries. I think it it parasitic loads and the fact that it takes a long time for a 95% charged battery to get to 100% charged.
One can discharge a battery to 95% in a few minutes with the stereo on with engine off, but recharging 95% to 100%, you are basically looking at 2 hours when held at 14.5ish volts.
Plugging in a charger overnight is wise, as long as the charger does not exceed the high 14 volt range, and this could be excessive.
How to know if a batery is fully charged? Well voltage alone is not a good indicator, unless on has earlier verified what the fully charged resting voltage was when the battery was new at a certain temperature.
Get a hydrometer. OTC 4619 or similar, that has a thermometer on it for temp compensation.
For AGM batteries, when they can accept 0.5 amps or less per 100AH of capacity, when held at absorption voltage( generally 14.4 to 14.7v), then they can be considered fully charged.
Most AGM batteries will actually decline to very low levels of amperage to be held at just about any voltage under 14.8v, given enough time.
Some maintenance free batteries will also exhibit this behavior of accepting very little current when fully charged at high voltages, where as regular starting batteries will usually require 0.7 or more amps to be held at 14.5ish volts.
They will all vary on these numbers, they are not written in stone, but the Ammeter and hydrometer are great tools for establishing full charge. The green light full charge indicator on automatic charging sources, should be viewed with high suspicion when it first alights. They are just following an algorithm, they do not know the size of the battery or its 'numbers'.
They guess, and hopefully when they drop to float voltage, well they are eigher close to full charge, or there is enough time at float voltage to finish the task.
Abused batteries might never revert to maximum specific gravity at 'normal' charging voltages.
An Equalization charge is upto 16.2 volts at 77f, but there are some safety considerations to performing this procedure, and it might or might not restore lost capacity/performance of an abused battery.