Lol, yeah. Dr. Seuss wrote a book about this 🤣 Fish Out of Water
https://www.amazon.com/Fish-Out-Water-Beginner-Books/dp/0394800230
Then Mr. Carp told me: "When you feed a fish, never feed him a lot. So much and no more! Never more than a spot, or something may happen! You never know what."
The most important factor here is the the battery manufacturer's charging specs. Find and read those first.
Sizing with lithium (which OP appears to have) is pretty straightforward assuming alternator is capabale. There are no charging rate minimums for lithium, and the most common max current recommendation is 5C (50A per 100Ah) for battery longevity.
So if it were mine I would size a 40A DC-DC to charge 100Ah of lithium. (Actually I would do 20A + solar but OP specifies non-solar charging for this thread).
Sizing for lead-chemistry batteries (AGM, Gel, FLA) is weirder. Here are some generic patterns:
Putting it all together I'd size a 20A DC-DC to charge 100Ah of gel, FLA, and most AGM (Renogy, UB, etc). I'd probably size a 30A DC-DC to charge 100Ah of quality AGM (Northstar, Lifeline, Odyssey, etc).
(In real life I charge 220Ah FLA with an isolator + solar, and they pull right at .2C when discharged).
Counterintuitively, wIth lead chemistries the time-to-full-charge at both maximum and minimum recommended charging rates appears to be nearly identical. In practical effect there is a minimum required charging time: quicker Bulk means slower Absorption and vice versa.
If we are talking about stuffing in partial charges, I haven't seen any info on whether higher or lower current is preferable. If we suspect higher current is better for short trips we just could use a $25 solenoid instead of a $250 dc-dc.
Reminds me of my favorite book when I was a kid. Anyone remember this one?
Wait - I read about this in a book a long time ago - you fed it too much, didn't you? That fish is going to get HUGE!