So directly connecting two dissimilar battery types, at different stages of their lives, with charging system designed primary for bulk is the way to go.
This thread has wandered a bit, sort of, but electric system is so intertwined.
I'm trying to get a handle on all this especially since I am literally starting from the beginning again. My conclusion was that without a DC to DC charger and having a flooded lead engine battery and an AGM house battery bank:
1- There might be a slight delay in charging the house batteries if they were extremely low, but my thought was on a long driving day that would be moot.
2- Most alternators wouldn't be able to "top off" the AGM batteries, but one could "finish" them off with solar as long as it was sunny.
3- There is the inherit problem of having dissimilar batteries connected together
4- There is the inherit problem of having batteries of different ages.
Given the cost of a DC to DC charger, and the fact that I plan to have 200 watts of solar on the roof and 200 aHr of AGM house batteries. Couldn't I just:
1- buy a new AGM engine battery when I install the batteries in the new camper? and skip the DC to DC charger. My new to me truck is a 2016, so assuming its the original battery, its almost time to replace it anyway. The cost difference between a flooded lead and a AGM starter battery is only about $100. Plus I'd have the benefit of a starter (engine) battery that would handle vibrations better.
2- Bring larger wire to the installer and upgrade the wire from the usual 10ga that FWC does to say 6 or 8 gauge?
Somewhere in one of these threads, someone stated and I agree, that given a large solar array, and proper battery bank, you probably should need charging from your alternator anyway.
Looking forward to opinions.
2016 F150, FX4, XLT super cab
2019 FWC Hawk Shell
2009 Ford Ranger 4x4, XLT, Supercab total loss on 7/30/19
2014 FWC Shell total loss on 7/30/19
Towed behind our home which is a 2005 Kountry Star Diesel Pusher