A fully charged healthy battery cannot accept much current, even at voltage approaching 15.
So is your maintainer actually holding the battery at 100%?
You will require an Ammeter and a charging source capable of forcing the battery upto the high 14 volt range to know.
If the single mid size battery takes 6 amps at 14.8v and these amps do not taper as you watch the ammeter, then the battery is not fully charged and the maintainer is not doing what it claims to do.
If it requires only 2 amps to bring the battery to 14.8v and amps taper from there, then the battery is indeed near full charge.
The exact amperage will depend on the battery itself, and it will also change as the battery ages.
A voltmeter by itself tells very little. Amps flowing into the battery at a certain voltage reveal much much more. Without a hydrometer, or an Ammeter, you are basically blind as to what and where your battery is.
An adjustable voltage power source with a voltmeter and ammeter is very revealing when higher voltages are forced and the gauges monitored.
