Hi Trouble (I always start humming Cat Stevens... "Trouble..." when I type that
),
I totally get where you are coming from, and I think your line of thinking is sound. You understand what the WFCO type charger cannot do, and for now can work around it. Having a reasonably inexpensive source of flooded cell batteries means that - for now at least - you can just replace them as needed and that's part of the plan.
So just to go over your concepts:
1) Keep the three-stage WFCO charger for now. You understand that even though it's supposedly a smart charger, it's really only smart in comparison to the quite stupid original one. The issues with it are that you cannot set the charging voltages for the stages (and the pre-set ones are not necessarily correct), and that without temperature compensation, those voltages are incorrect anyway unless the batteries are at 77ºF. But this fits in with your plan now, and you can get another one if/when you upgrade batteries to AGM (or heck, maybe lithium by then).
1a) is that you *could* choose to upgrade the battery to charger/loads wiring now, which would mean you'd at least GET some voltage to the batteries. I should add a caveat: I don't know how the wire run works in the Destiny. In the Concourse/Premier, they use the same wire for both charging to the batteries, and loads from the batteries, and it's an 8AWG wire that's around 30' long. Any voltage drop calculator can do the math, but..... there is a lot of voltage drop. So once again if you were to set your precious voltages (as AGM likes) on a smart charger..... they'd change by the time they got to the batteries anyway. And another problem with this is that the fusing has to be very low (50 amp) because the wire can't be fused any higher (no problem with small charger, but maybe a future thing.
The wire run (if yours is similar) is so long because on the 21-footers it starts out in the ell under the sink, goes to the outside wall, then goes UP over the window, all the way forward up there by the roof, then all the way back down the pillar behind the driver, out through the floor, and then over to the battery compartment.
What I did was run a new wire that simply runs along the outside wall under/behind the couch (behind the water tank on 21-ers). Nothing tends to get near it anyway, but I also have it in protective loom, etc. This run is less than half the length, and the wire is also larger, so double win. I now have an 8AWG wire running up to the overcab and/or the base of the pillar that I can use for something else if I want to. I also separated that wire from the wire that feeds the load center, but that was obvious for me as I no longer have the all-in-one load center and charge wire. (That all in one wire is what necessitates the "buzz, buzz, hey, you have charger on but the coach switch on store!" alarm.) But you could run a new, larger/shorter wire and still have it do the dual functions, to keep it more stock like.
2) Alternator charging.
So first of all, pardon me if this is obvious review, but we'll back up and talk about charging in general. And also we're talking about lead-acid batteries (flooded cell or AGM) not lithium which have some different characteristics. But the way the charging works is that the batteries will take all you can throw at them (which is a fair amount for flooded cell, and even more for AGM) until they get up to around 80% charged. This is what would be called the "bulk" stage on a good charger or solar controller. The voltage will start out at whatever your batteries are at (say 12.6 or so) and all the amps possible will be plowing into the batteries as they make their way up to around 80% charged. The charge voltage will keep creeping up. When it gets to whatever is set as the "absorption" voltage (say around 14.4 volts) then it goes into the "absorb" stage. In that stage the voltage is held constant, and the amps slowly taper down to around 2 amps (for a pair of batteries). This is the stage where the battery goes from around 80-85% charged, to 100% charged. This can take a long time (relative to the bulk stage) because the amps are limited by design. The last stage is actually after the batteries reach 100%, and is the "float" stage. Basically maintenance, in the low 13.x volt range.
So, the alternator has no stages and no smarts. You get what you get. In my case, watching my Scan Gauge, it generally puts out around 13.7 volts. Well, you can see why this never gets the batteries more than about 80% charged: it never reaches the absorb voltage (much less holding it there for hours). And of course no temp comp, and probably plenty of voltage drop with the original wire (however you have lost the voltage drop that the original separator type induced). I suppose maybe the batteries could get up to 100% charged at 13.7 volts if you went on and on and on. But in general it's somewhere between a bulk and float voltage (and really a bit high for float on my AGM batteries.)
What the "B2B" (same as smart alternator charger or whatever you want to call it), such as the Sterling ones do, is take that "unsophisticated" charging coming from your alternator, and make it three stage smart charging. I have not had one, so not sure if they have temperature compensation, or how much you can set the voltages, but the basic ideas is they mimic a smart charger. The reason I have not looked into one is that with 200 watts of solar, I don't really need any alternator charging. Very occasionally I could use some, and that's when I manually connect my alternator/start battery to the house bank. But that's been like three times in the past year, so not a high priority for me. Of course this may differ for others.
3) Solar charging
Presuming you buy a decent solar controller (which does not have to be super expensive; I have a Morningstar Sunsaver MPPT 15 and it's $225 ($315 total including remote display and temperature sensor cable). It's not even necessarily the cheapest one. It is a good controller though. So with this controller (or many other similar ones), I can set the exact voltages of the stages, and it's temperature compensated. So since/if/when you'd be adding solar (the stock system doesn't really count as it's more of a battery maintainer when rig parked and not in use), there'd be no reason for you to get anything but a good/smart solar controller. That will take care of AGM's, flooded cells, or lithium even.
So to recap, here is what I would probably do if I were adopting your "I'm going to stick with flooded cells and the WFCO charger for now" and had your scenario.
1) Consider upgrading charge battery cable, presuming it's anywhere near as small/long as the one on the 21-footers. True your charger is still not the greatest, but at least you'd be getting the volts it does put out (this would hold for shore or generator charging.
2) Upgrade battery combiner (you have already done this with the 7622).
3) I forget if you said that you upsized the cables from start to house battery. There are two good reasons to do it. One is for less voltage drop, and the other is that you can't fuse that cable at the stock size. Too many amps go through it and the wire can't handle the correct size fuse.
4) Add solar charging system. Reason I put this before B2B addition is that solar will charge your batteries well and smartly when you are camping OR driving. Enough so that you may very well not need any smart charging from the alternator. In fact you may not need alternator charging of house bank at all, and/or only for bulk charging (which is all it can do now anyway) and solar takes care of the rest.
5) B2B type charger (but I'd do step #4 first as you may find you don't even need this. I don't.
For me solar was the BIG upgrade, and I don't really need any other charging. That would not hold for everyone, but I think it might cover you as well.
