Thanks. Apologies, as necessary, to OP if my Q is too far off. While my setup is maybe a little more advanced, both of us are asking the same general thing about systems interfacing. If someone searches and finds this thread someday, it'll save them lots of invented wheels.
The reasons I'm trying to get solar to the starting battery are: 1) currently all my 12V lighting is hooked to the OE system, 2) if I leave my inside light on (even after switching the, over to LED) or run my campsite outdoor LED lights off the OE, I will draw the Diehard down, and 3) if I do a winch recovery the starting battery will need more than the alternator can give, at least right away. I just thought there might be times when I use the starting battery for camp things, if solar could refill it, it'd be full the next day.
Having said that, with my concerns about disparate batteries, and those ya'll have mentioned I'd not thought about, I think the best approach is to set the house bank and solar up separately. I might move some lighting over there, too. HOWEVER, if A. I find myself with a starting battery too low too often, or if B. my house bank can't get recharged due to no sunny weather, then I'll design something then. That's the simplest approach.
I've studied the Blue Sea smart isolator and relay before and for reasons I can't remember, discarded them I will revisit.
A "dual sensing" ACR is good because it will tie the batteries whenever the voltage on either side rises (charging is going on). So when the engine is running, the voltage on the engine side rises and the house bank gets tied in so both charge. When the engine is off, but the solar is putting out power, the voltage on the house side rises and the batteries get tied, and the engine battery gets charged as well.
The gotcha in your situation is that you are drawing down the engine battery. So if you hook the solar to the house bank, then the ACR will not tie in the engine battery until the house bank voltage has risen to the tie-in set point. Depending on how big the house battery is, how far it's drawn down and how much solar you have, the house bank voltage might never rise high enough for the engine battery to get tied in and recharged. Even if it does, it might happen so late in the day that the engine battery doesn't get much of a charge before the sun goes down.
So, in your situation; hook the solar to the engine battery instead. That way, the majority of the solar power goes to recharge the engine battery first, and then when the engine battery voltage rises high enough, the ACR will tie in the house bank. If the house battery draws enough to bring down the engine battery, the ACR will untie them until the engine battery voltage comes back up, then automagically tie them together again.
So, at this point, you are thinking, "Right. That's why I don't want my alternator to charge the house bank until AFTER the engine battery is charged." Understandable. Fortunately, that's how the ACR works. It won't tie in the house bank until the engine battery voltage first gets high enough (13..0v for 90 seconds, or 13.5v for 30 seconds).
Of course, at 13.0v or 13.5v, the engine battery won't be "fully charged" if you've drawn it down a good ways. BUT, it will be "mostly charged" and at that point, it's going to be drawing a lot less amps off the alternator anyway. Whether or not the house bank is tied in, the engine battery is only going to absorb at whatever rate it chooses based on its own internal resistance. So you might as well go ahead and tie in the house bank and let the alternator feed it some amps WHILE the engine battery also absorbs whatever it can. With any lead-acid battery, It's that long-assed time required to absorb that you gotta worry about, not the relatively short time spent spent getting "bulked up".
For winching, you want the ACR with the manual over-ride. The smaller Blue Sea ACR doesn't have that, but the larger one does. That way you can force-tie the batteries when needed.
You can also force un-tie if you want to make all the solar go to the engine battery and none to the house bank. That wouldn't nomally be needed with the solar hooked to the house bank, but in your situation, you want to take care of the engine battery first, so you hook the solar to the engine battery instead of the house bank, and by forcing the ACR off, you can make sue ALL of the solar goes to the engine battery when you need to.
You certainly should move as many loads as possible to the house bank.
Different batteries is an issue if they are tied into a full-time bank. If they are only tied while charging, then it doesn't matter. Both your engine and house batteries are AGM. The Diehard/Odyssey wants 14.8v bulk/absorb, and the Lifelines want 14.6v bulk/absorb. Close enough for government work. All lead-acid batteries are nothing more than sloppy chemistry experiments in a plastic box. They are not at all precise electronic components. Get the voltage up high enough and hold it there long enough for the chemistry to get fully saturated and you're done. Both of your batteries could be fully charged even at 14.0v - but it will take longer to get fully absorbed/saturated.
Nations Alternators in Missouri suggested I put a Balmar regulator on my alternator. It acts like a multi stage charger and is designed to send voltage at the three different levels to whichever load is calling for more. It also protects the alternator from working itself todeath trying to get 150A back into my house bank. That's all good; but I still need the smart solenoid switchover to get the alternator's juice into both systems. I'll get the house bank and solar operational and see how things go.
If you've got a modern vehicle and the voltage regulator is part of the computer control system, you might not be able to simply "swap out the regulator". You also probably don't need to, since a modern computer controlled voltage regulator is going to be pretty smart to begin with.
