While well depleted batteries, with a high amp charging source seeking to attain mid 14 volts range, can quickly get the batteries to about 80% charged, that last 20% takes no less than 3.5 hours, and that is when they are held at absorption voltage.
You cannot count on your vehicle to seek and hold absorption voltages.
Battery longevity requires achieving regular full recharges\
Regular full rechrges are best done by achieving absorption voltage and holding it as long as required.
perhaps if plugged in and time is not a factor, healthy batteries can be brough to full held at lesser voltages, but hard worked AGMS will likely appreciate higher amps to achieve a higher voltage quicker and thus getting to full quicker.
The alternators will only produce whatever amperage is to get the system voltage upto what the voltage regulator demands.
When one can control this target voltage and set it to the mid 14 range, and not freak out the engine computer, then anytime one drives one can recharge the batteries much much faster to a higher state of charge whenever the engine is running. Thus making the battery happier than if it were recharged to a lesser % and then asked to support another load for X amount of time and depleted further than if charged faster.
I was able to achieve this, but with an '89 Dodge Twist a dial and i can get well over 15 volts, with 120 amps at 3300+ rpm to achieve it.
If my engine computer sees more than 30 seconds over 14.7v, when it was itself demanding 13.7v, then it illuminates the check engine light and defaults to certain sensor readings. MPG and engine smoothness takes a dump until I reset engine computer.
Solution,... never ask external VR for more than 14.7v.
I generally choose 13.6 or 14.7v, and have an Ammeter on my dash counting amps into or out of battery to tell me when to dial it down to 13.6v.
The Ammeter also gives a good indication as to how charged the battery is. The closer to 0.4 amps it is at 14.7v, the closer it is to fully charged. Still taking 20 amps, and my 90AH Northstar AGM is just about 20AH from full assuming it is coming up from well below 20Ah from fully charged. The rate at which amps taper at absorption voltage are also a good indicator.
Recommend an ammeter.
this one makes things simple:
https://www.amazon.com/bayite-Digital-Current-Voltage-Transformer/dp/B01DDQM5PK/ref=sr_1_1?s=sporting-goods&ie=UTF8&qid=1509933813&sr=8-1&keywords=bayite+ammeterI extended the 6 foot 3 wire ribbon cable by about 8 feet to reach the display on my dashboard.
Knowing how many amps flow into the battery at a certain voltage is extremely enlightening as to state of charge and how well any charging source is doing, and more so when one can control that voltage by twisting a dial.
Voltage alone and one is blind and mostly ignorant but a good digital voltmeter on the truck battery, or house battery while underway, will say whether the battery is charging slow medium or fast.
My engine computer voltage regulator chose 13.7 or 14.9 with no regards for logical or informed battery charging. Usually it would start at 14.9 though, but during longer drives would decide that 14.9 was again needed. My ammeter said 14.9v was definitely NOT necessary.
If you want to guestimate how well your truck can charge youor proposed battery bank, knowing the voltage behavior the VR chooses, would allow one to make an edumuncated guess.
I hear later model GMs only fireup the VRs to 14.8+v when braking to increase load on the engine, and generally try to keep the batteries at 80% charged, so this 14.8 blast is effective for increasing load on the engine when braking. Underway it seeks lesser voltages to help try and eek out a 0.06% increase in MPG, to help meet CAFE regulations.
No idea how true this is, but alternator recharging is dependent on the voltages they are told to seek, and bean counters and lawyers and maximum profit are involved. Max battery longevity is Not a consideration when designing the algorithms modern vehicle voltage regulators now run.
So your twin 156 amp alternators might never even come close to providing this, unless there is 150+ amps of load and voltage is struggling to maintain 12.9v.
Get one of those ciggy plug digital voltmeters just to see the voltages the VR allows in general driving. While the ciggy receptacle does not truly represent voltage at battery terminals it is good enough to see whether mid 14s are achieved, or whether GMs newer system does allow system voltage to drop to the high 12s or low 13's most of the time and only fires it up to 14.5+ when one lifts off the gas.
If this is the case, perhaps dedicating one of those 156 alternators to the house battery, with external regulation, would greatly increase charging amps.
no idea how this would be accomplished without freaking out engine computer and GM techs would likely look at you like you have 3.5 eyes and 6 breasts if you ask them how as, few auto techs know anything about deep cycle battery charging or lead acid battery requirements to attain a useful lifespan.
So yes, charging houose batteries via alternator works, how well it works depends on the voltage regulator controlling the alternator output, and the resistance of the circuit path, and the resistance of the batteries themselves.
It could work well, OK, or poor. Too many unknown variables,m especially on a more modern GM whose designer hade C.A.F.E fuel economy standards in mind.
Observe the voltages you now see when driving, do not be one of those people that check battery terminal voltage after starting the engine, and assumes that is the voltage always held. It swings around wildly, at least as far back as vehcles made in 1989.
Also say 4 100AH AGMS batteries are depleted to 50% and read 12.2 rested, Twin alternators trying to get system voltage, which includes thos 4 depleted AGMS, will require everything those alternators can make, and voltage will not instantly rise to mid 14.s, but take perhaps 25 minutes before they rise to that point.
Mex touched on this but this super slow voltage rise while the VRs are asking for "max output Scotty, we need all the juice you can muster", in a modern vehicle could trigger the check engine light and a limp home mode where power and MPGS take a big dump.
Some time spent on a vehicle specific forum might clue one in on the behavior of their vehicles voltage regulator, but don't expect most to have any idea how this relates to charging a large additional bank of depleted AGM batteries.
