Having a digital Ammeter on my dashboard showing amps into and out of the battery, and a temp sensor on the battery, and being able to spin a dial and change that voltage, is incredibly useful not ony for observation and state of charge/health of abtteries, but a great learning tool as well. The amperage accepted at absorption voltage where known 'full charge' should occur on AGMs is also convenient
My AGM says it is is full when it accepts 0.5 amps or less at 14.7v, but say at this point I lower voltage to 13.6v, and leave it there for a few more hours with no significant DC loads on the system, amperage tapers to zero, well 0.0x amps, below my ability to measure. If at this point I reboost it to 14.7v amperage tapers to 0.0x quickly again, well below that 0.5amp threshold of 'full'. so it seems while 0.5 amps at 14.7v is considered full, there is a secondary 'full' at 0.0x amps, at least while the battery is still healthy.
When driving, I have noticed if I leave it at 13.6v once fully charged, and at night with headlights on and perhaps 20 to 25 amps of total DC load,( my engine requires 12.2 amps at 2K rpm to run ignition and fuel pump, not including any field current to alternator) then the battery actually discharges slightly at 13.6v, registering 0.2 to 0.4 amps flowing from the battery, and I must get closer to 13.9v to prevent it from discharging he colder the temp the higher the voltage must be to prevent it discharging. But turn off the headlamps and blowermotor and 13.6v back to a 12 to 15 amp total dc load on alternator, is adequate to hold it full even with battery temps colder than 77f.
The reblasting to 14.7v after a few hours of my ammeter registering - 0.2 to 0.4 amps at the recommednded float voltage, shows than indeed the ammeter was right, amps were flowing out of my battery at the correct float voltage, and some period of time at 14.7v is required to return amperage acceptance below the 0.5 amp threshold considered full.
I do compensate for battery temperature.
So it seems the battery must be held above its normal float voltage in order to remain full, when there is a significant DC load on the system, to keep the amps flowing from only the alternator, and not some from the battery to help support that load.
It's not like the battery is providing much during this 'event', and I don;t know how a flooded battery compares, or other brands of AGM, only that I've witnessed it occur more than once, more than a dozen times, with two different Northstar AGMS in two different cross country road trips, California to Florida and back, twice subsequent Xmas seasons. It's easily repeatable, and with a 38 hour drive, not much else to do and I try to complete as much driving at night as I can for traffic and less wind reasons.
As such I only really lower my alternator's voltage below 13.8v, during the day when I know the battery is full and I want the solar to be powering as much of the electrical load as I can get it to, and this requires I set alternator voltage to below the float voltage setpoint of my solar controller, 13.6v.
Anyway, its not really consequential either way, but I thought it interesting that the recommended float voltage held, would still allow some slight discharging when there are significant dc loads on the system, of ~ 25 amps but not when they are under ~15. None of this would be possible without manually adjustable voltage, or the tools to display amps in out at what voltage I choose, and there is certainly ignorance in bliss and bliss in ignorance.
I know these data and observations fly in the face of accepted 'wisdom' regarding battery charging, at least that which one reads on forums like this, and no doubt despite my Speedometer saying I am doing 60mph, verified with GPS, and mile markers and a stop watch, someone will claim I am wrong and only doing 55. I tend to doubt anything written by them forever after, and question much conventional widespread 'knowledge' which is often just oft repeated/parrotted incorrect, or inaccurate opinions, shouted with an authority they actually lack.
