Thanks alot for the feedback; food for thought. Here are my immediate thoughts. I like the concept of direct connecting all charging devices to house bank. I like the wiring of the "Off, 1, 2, All" as mentioned in my original post which provides for emergency engine start from house bank (and combining if combining device fails). Of the 3 devices mentioned, it seems that two (ie, Balmar and Xantrex) follow the same philosophy of charging start battery from house battery when charging is detected. The Xantrex is substantially cheaper and seems to do the job (ie, $126 vs $225). The BlueSea ACR seems to be a different device which doesn't follow the above philosopy. It appears that the ACR does not provide direct connection of charging devices to house battery and, therefore, seems inherently more complicated and less reliable. Guess my immediate reaction is to select the Xantrex Echo-charger. Comments please.
On other point I should mention is that it is a good idea to add a fuse/breaker within 7" of the battery for the direct feed alt wire and I sometimes add a "service switch" into the alt feed wire so when working on the engine you can simply disconnect the alt from the batts. This is usually a hidden switch not highly visible to guests. It is also a good idea to fuse your banks.
As always there is no one right way to wire banks, unless you do it dangerously, but on small boats it makes little sense to not a perfectly good 1/2ALL/OFF, if you already have one, when a simple re-wire will give you everything you need with simplicity and total redundancy should a combiner fail..
#1 Assign house bank to batt switch position #1
#2 Install reserve battery to position #2
#3 Wire alternator output directly to the house bank with sufficient size cable & fuse.
#4 Install ACR or Echo Charger between banks.
#5 Use boat in position #1 for starting AND house loads. Switch to OFF when you leave it's that simple.
So why do I want to wire my alt direct to the house bank??
I prefer to feed the alt directly to the house bank and not the reserve bank as the start bank is for back up or emergency, in this situation, and would usually be at or near 100% SOC anyway.
1- No more worries about fried diodes.
2- Accurate voltage sensing of the discharged bank & less chance of voltage drop over the generally very small factory wiring.
3- If you use an Echo Charger you need to do this anyway
You should NOT however wire the alt directly to the house battery unless you have either a VSR/ACR (voltage sensitive relay/ automatic charging relay) or a unit like the Echo Charger or Balmar Duo Charger.
Follow me on this one.
If you sense/wire directly to the house bank, bank 1, but do not have an ACR or Echo type unit installed, this means you could physically select the battery being used via the 1/2/BOTH/OFF switch.
If you select bank two as your source and the alt is connected to bank 1 you will draw down bank two and not replenish it unless the ALL function is selected during charging. In short, without an ACR or Echo type charger I would not wire directly to the house bank and instead would leave it wired through the common post of the battery switch.
With an ACR/Echo you can still select bank 2 but it will automatically be getting charged via the combiner or Echo even with the house bank being direct wired...
So what is a battery combiner / ACR / VSR??
Blue Sea ACR Si Series (LINK)
An ACR is an automatic charging relay. ACR is Blue Sea Systems trade name. They are also know as VSR's or voltage sensitive relays. A VSR is essentially a high current automatic relay that senses voltage and combines banks when the voltage rises. If it senses 13.6 volts for more than 30 seconds it combines and if it sense more than 13.0 volts for two minutes it also combines. When the VSR senses a drop in voltage below roughly 12.35V for more than 10 seconds it disconnects or when it sense 12.75V or less for more than 30 seconds it also disconnects the banks. These devices are very simple and fully automatic requiring no human intervention at the battery switch. Truly "set it and forget it"
Start the engine and when the charge voltage begins to rise the VSR/ACR automatically energizes the relay, closes it, and "combines" the banks for charging, just like if you flipped the 1/2/BOTH/OFF to BOTH. The best feature is that it NEVER forgets to disconnect the banks when a charge source is not present. When the batteries are not seeing a charge source they are disconnected from one another. These devices are relatively inexpensive $65.00 - $150.00 depending upon model. They are also very easy to wire. The Blue Seas ACR has three wires to connect, ground and house and start bank jumpers. Yandina also makes a battery combiner/VSR. The Blue Seas models should have a fuse installed in the ground wire as well.
The unit labeled ACR is a Blue Seas ACR:
So what is an Echo Charger?
Xantrex Echo Charger (LINK)
The Xantrex Echo Charger is an electronic 12v to 12v charger. The banks and the banks remain isolated but you don't have the voltage drop issues associated with diode based isolators. It simply takes or "bleeds" it's power from the charge source or the house bank and when it senses over 13.0 volts energizes and begins charging the reserve bank. When it senses below 13.0 volts it turns charging off. It can supply up to 15 amps of charge current to the reserve battery while borrowing/bleeding it from the house bank. Normally that battery will be at or near full and you'll rarely ever see it pull more than an amp or two anyway so 98% of the time your alts full output heads directly to the house bank. The Echo Charger is also a voltage follower and will follow the incoming voltage to the house bank. When the house goes into float mode so does the reserve bank.
The Echo Charger is also a simple three wire hook up but needs to feed the reserve bank, and pull from the house bank. All charge sources should be fed to the house bank, solar, alternator, wind and shore side charger.
The Balmar Duo Charger is similar to the Echo but more expensive and can supply up to 30 amps of charge current. It does have a few flaws so I'd stick with the Echo or ACR.
By simply adding an ACR or ECHO only, you can still go to your boat and select HOUSE and when you leave select OFF. That's it, simple...
Some folks complain of voltage drop out for electronics when starting off the house bank. I have never experienced this in any of my installations but my battery cables and system wiring are of sufficient size and my house bank even at 35-40% SOC never drops out the electronics during engine starting. I can start my engine and not reset any of my electronics even down to about 35-40% state of charge. You should not normally be dropping below 50% SOC anyway. If this is a concern then your batts may be ready for replacement or you have some bad connections. To avoid this you can start the motor, then flip on the instruments.
You can direct wire a dedicated start battery but it will be more complicated if you want to retain the ability to start off the house bank in an emergency. This is why the 1/2/BOTH/OFF is such a good tool despite it often being incorrectly maligned.
I still wire the alt direct to the house bank even with an ACR for the reasons mentioned above.