There should be no reason for two house banks separated by a switch that can or does get used as your main ON/OFF.. If you want a permanently ON hidden parallel switch, (pretty much just a nuisance IMPH) then you can add one in the battery compartment but it just adds to voltage drops etc..
I just went through this similar scenario on a J-42 that was using two 4D's the same way and trying to get accurate readings from an Ah counting battery monitor. IT DOES NOT WORK.....
We wound up permanently paralleling the 4D's and then adding a small "aux" battery, an Odyssey PC1500 which is nice and compact but has gobs of starting current..
Parallel the two 8D's and add a second small "reserve" battery to the #2 position on the battery switch. If batteries ever go south you simply take them off-line and switch to the reserve bank. Now disconnect the negatives and let the batteries sit for 6-10 hours and watch which bank drops voltage fastest. That is your bad or poor performing AGM.. Often times if one battery in a bank has failed the other battery is not far behind.
I would also recommend keeping your Ah counter but jut use it for amps or even cumulative Ah consumption but don't try to translate it to the SG for SOC unless you've painstakingly capacity tested and gone through all the motions with your Ah counter to get it accurate...
Nice to know the Ah counter function on the Victron monitor would still be accurate enough to be useful, although I've been dubious whether I have it correctly wired & programmed given my somewhat unusual battery set-up.
Speaking of, thanks for the helpful info. I've been trying to get a handle on this since acquiring the boat in 2007. I don't want to improperly hijack MS' thread, but to complete the picture my boat also has -- for better or worse -- a 3rd LifeLine 8D with a separate ON-OFF switch that is used for engine starting only. This has always seemed wasteful to me, and my goal has been to wire this 3rd 8D into the house bank.
My only hesitation thus far has been that this eng start batt is charged via the primary engine alternator, whereas the two existing house batts are charged by a second alternator on the engine. The primary alt/eng start batt is wired through the engine's starter, so if this start batt is paralleled into the house bank, then the entire, newly expanded house bank (3 8D's) would be used to start the engine. From the testing I've read from MS in this area, this doesn't seem to be a problem. As I understand it, having two alternators (both internally regulated) simultaneously charging the same bank of batts. is also OK; they will both be fully utilized during bulk phase but one will kick off as the batts. start topping off. If necessary, Balmar makes an external regulator designed for this situation.
I hope I'm making sense describing my system here, and again, I can take this discussion elsewhere if more appropriate. My boat was built in 1986, and I suspect the current set-up reflects the notion at that time of a large, separated battery that is devoted solely to engine starting, and doesn't account for the higher demands on house banks from modern electronics, etc. As far as a reserve, I should add that there is a 4th battery -- a LifeLine G24 that is used solely to start the 8kw generator -- and this is kept charged via a BEP/VSR echo-type charger, along with an alternator on the genset. Based on the specs in my engine manual, this could also be used (theoretically) to start the main engine (80hp, 4-cyl. Westerbeke) in a pinch.
There are many options here, but it would be nice to get this done the easiest way possible, especially since the current set-up as a whole has worked well thus far. (And I don't want to screw it up!). Thanks for all the help. Dan
