Bingo you are 100% correct regarding keeping the batteries isolated, and charged separately. The latter being very important.
the OP was merely asking about mixing types so when replying I'm trying to provide good info without going into information overload. Thats why I offered up some of the info in 2 posts rather than one.
I'd also venture a guess that the average person is not going to want to spend the time and effort isolating and then charging separately so in my 1st post I didn't get into that. I personally don't see the value in a single AGM being so great as to want to go thru that effort but that is a personal choice and there's nothing wrong with it if its installed properly.
My replies are always coming from the perspective that perhaps the person is going to want to create a larger bank for extended dry camping at some point in the future so any suggestions I offer would be to try start them on a path that would be scaleable. Again, mixing types would not be the most advantageous towards that end. Likewise the slow degradation of a single battery is not as expensive as the slow degradation of 4 or 6.
Any advice that I offer is geared towards trying to create a system that will be as efficient electrically as possible allowing maximum power, while making it as hands off as possible so as to eliminate the possibility of operator error or mistakes. (why I favor a sub-panel on inverter installations vs manual power management) Just think of all the dropped trailers and un-lowered tailgates and you can see why I suggest the hands off approach.
My sailboat had 6 lifeline deep cycle and 2 optima starting batteries. Although all AGM - they were still isolated. Not only by an Battery Switch, but with a
Balmar Digital Duo Charger which automatically supplied alternator power to the Start Bank once the House bank was fully charged.
A battery switch is probably the single most way that people fry their alternators by inadvertently turning it to off while charging. By using the Duo Charger it eliminates the need to ever touch the battery switch for power management. When you're running 180amp 4 stage alternators it can get expensive and more importantly downright inconvenient if you are offshore.
Unless you have a battery monitor (like a Trimetric or some such item) you are probably not going to notice that your bank is only supplying 80% vs 100. And if you are spending most of your time plugged in it probably wont matter.
Another reason why I'm not a big fan of AGM's in our Monty's without installing a different charger:
AGMs are very charge sensitive. (why? thats another post) Over charging will kill them a lot quicker than just about anything. The Progressive Charger supplied with our rigs is geared towards wet cells and does not allow for modification of the charging profile. The charge voltages are generally a bit too high for AGM's. Way to high for some brands. They do not have temperature compensated charging (very important) , and they have this storage mode thing that boost voltage to 14.4v every 21hrs when the batteries are fully charged. This is simply because they didn't want to actually build a proper equalization cycle into the charger so they did this "electrolye stirring" function. This electrolyte stir is beneficial for a wet cell, but damaging to an AGM. Once fully charged, boosting the voltage back up to 14.4v every 21hrs is going to put one more small nail in the coffin of the AGM by overcharging it a just little bit every 21hrs.
Now that's not to say that the PD is going to kill them tomorrow. There's some nuance here. It's just not the best situation, and in some cases without the temp compensation its worse.
Add up all the nails we've put in the coffin of the AGM's (in this particular instance) and you'll see why my advice to the OP was to just go for another wet cell. don't get me wrong.. I love AGM's and wish I had 6 of them in my Monty. Maybe when I replace the Trojans.
Sorry for the length of the post... I've actually left a bunch out to try and keep it short.
Linear Mode
