A few questions.
What is your alternators output?
beside the vehicle and batteries, what else do you have drawing power?
Do you have solar?
Alternator damage is primarily from overheating. A general rule is not to exceed 75% of total out put for long periods of time. 90% for an hour is pushing it.
Typical L/A and AGM batteries will draw a max of around 40 amp's each. This is based on a number of factors. As they charge, the amount they draw is less. The amount a lithium "Can" draw is based on what type, how many cells and BMS design. I have a lithium pack that can draw 250 amp if I chose.
You also stated your going to run a DC-DC charger rated at 50 amp. These are nice because they limit the amount of current flowing to the batteries. But, you need to choose carefully and hook them up accordingly.
The advantage with this is, your only flowing 50 amps to your batteries. the disadvantage is your only flowing 50 amps to your batteries.
If you have two low batteries (80 amp) and give say 20 amp for your vehicle, that totals 100 amps. If you have a 100 amp alternator, you take the chance of burning it out. If you have a 150 your good.
With a 100 amp alternator and the 50 amp charger, your drawing 70 amps (50 batteries, 20 vehicle). Your 100 amp alternator is fine. Your just going to take much longer to charge your batteries.
If you have solar or keep your batteries over 70% your draw will be less.
As you can see there is no yes/no answer. Its all based on your setup and usage.
I have 3 batteries (4 soon). My vehicle with my fridge on is about 35 amps. 100 watt solar, two agm's in front and a 100amp lithium in the trailer. 120 max on batteries and 35 max on vehicle = 155 amp. My alternator is a custom 250 amp. Even when I add in my 4th battery later, i'm good.
The solar keeps everything topped off somewhat so I never see max load anyway.
Again, as you can see, there is no correct yes/no answer without more info. With the DC-DC charger odds are your good. Most modern alternators are over 120 amp with towing packages running 150 ish.
I would not get a high output aftermarket before you figure out if your already good where you stand now. I had to because my stock alternator was 80 amp.
