Auto V regulators BRAIN
Many folks have a stock auto or truck style alt on their propulsion engine.
They are cheap so and work very well for a car , but are a problem in a cruiser IF used for house battery charging.
The auto brain expects to have to replace 1% or 2% of the start batts charge , used in starting and to simply keep the voltage up during operation for lights , wipers and HVAC blowers.
AS a charger it will attempt to provide 1/2 of what the batt needs to reach full charge every hour..
So your 120A house batt is 60A low , the V regulator will provide 30A of charge plus about 4-8A more to push it in.
The second hour you are 30A shy , so 15A plus a few more will be provided, the third hour 15A shy and so on.
With a large bank of 4 or 8 batts you can see the car profile will work OK till the tapering starts , IF it doesnt burn out charging the big bank from overheating.
This seldom happens as the HOT rating of most auto alts is not at all high , why folks see 50A from an 80A alt .
If more rapid house batt charging is needed a switch to a smarter V reg is in order.
IF FAST charging is wanted a larger alt with a robust HOT rating will need to be installed , along with usually dual belts.
Unless folks have big buck fast absorbing batts instead of std Lead Acid , the charge can only be so fast , and for most a 135A truck alt with 3 stage regulator is the modest buck solution .
Folks that anchor out a great deal may be better served with the BIG alt belted on the noisemaker , as that would be the most likely engine operating.