The batteries are to be stored in my cap so sealed is a must as I dont want to add venting. I'm looking at 4 of these (
http://www.sears.com/diehard-marine-deep-cycle-rv-battery-group-size/p-02827582000P) Die Hard Marine Deep Cycles with 115AH a piece (12v) for a total of 460AH across the bank. That should be plenty to cover me for at least two days of intermittent usage based on my calculations.
230ah available if you want to keep the bank above 50% DoD (depth of discharge) for long battery life.
The batteries will be in the back so I plan to run some 00 cable to the rear and a BlueSea ACR to mediate the charging of the battery bank.
You don't really need that size cable for *charging*. There is likely a #10 or a #8 from the alternator to the primary battery anyway, so anything bigger than that won't really help.
The ACR will tie the primary and secondary. The secondary will suck amps from the "12v bus" (alternator + primary battery) until the secondary bank reaches a "surface charge" equal to the 12v bus voltage. After that, it'll mostly be a trickle charge effect.
I know...you're thinking of voltage drop. Voltage drop is very important when sizing cable for *loads*, but not very important when sizing cable for *charging*.
(At least, not so important when sizing cable for charging from a voltage-regulated alternator, which is a "constant voltage" charging system. Cable size is a bit more important when charging from a "constant current" charger, but even then, it's only really important in terms of the max amps the charger can supply, and not so important in terms of voltage drop.)
The batteries are what control the voltage of the bus (until they reach the voltage regulator's bus voltage set point, then the regulator controls the bus voltage).
When the secondary bank is tied in to the primary bus, the voltage of the whole bus (primary and secondary tied together into a single bus) will be drawn down by the secondary battery. The voltage regulator will keep the alternator switched on until the bus voltage reaches the voltage regulator's set point.
Then, the voltage regulator will just keep switching the alternator on and off to hold the bus voltage. But by then, the secondary battery will have a surface charge equal to the bus voltage, and there won't be many amps flowing through that battery (because there won't be much voltage differential, or "
electromotive force"), and there won't be enough voltage drop to matter.
The secondary battery's voltage will be constantly trying to drop (and pull down the bus voltage), so the voltage regulator will be constantly switching the alternator on and off to keep bumping the bus voltage back up to where it's supposed to be. But the secondary battery's voltage won't drop much, so again, there won't be any great voltage differential to pump amps through the battery.
Ultimately, as the secondary slowly creeps up toward full charge, the amps flowing to it will be less and less - and any voltage drop will also be less and less - until finally, only an amp or two is flowing, and there isn't any voltage drop anyway.
Voltage drop is important for loads from a battery, as the battery drains and the amp flow increases and so does the voltage drop. It's not very important for charging, where the battery voltage rises, the amp flow decreases and any voltage drop eventually goes away anyway.
I have a 2kw inverter that will be attached to the bank.
Now here you need BigAss(tm) cable!
What is the maximum overload that inverter will handle? Lemme just do a WAG here and say it's rated at 2kw continuous, and can handle up to 3kw surge. Now, factor in the low voltage shutdown set point of the inverter. Let's just say...oh...10.5v.
Okay, so here's the math:
3000w / 10.5v = 286a
For loads, you gotta figure it based on the worst case scenario. So to feed the inverter in my example, you'd need cable (and fuse) rated to handle 300 amps with X voltage drop (say 2%) over Y distance (better not be much...say 3' one direction or a 6' loop).
Now, if you plan to run that (huge, horrible, ginormous, ungodly) load WHILE the engine is running, then yea...you'd want BigAss(tm) cable from the primary to the secondary as well as from secondary to inverter.
1) Are there other, more efficient, battery options I should consider?
Nah.
Lead-acid batteries are not precise electronic components. They are all just big sloppy chemistry experiments in a plastic box. In other words: Junk. Properly cared for, you probably won't kill them too quickly. Personally, I buy CheapAss(tm) batteries, beat the crap out of them and replace them every couple of years. I can't be bothered to pussyfoot around babying some goofy chemistry experiment.
2) Suggestions for an alternative charging relay/isolator?
With that battery bank, you would be far, FAR better off with a good quality DC-DC multi-stage charger. One of these bad boys would do the trick right proper:
http://sterling-power-usa.com/batterytobatterychargers.aspx
No, not cheap. But a hell of a lot cheaper than replacing a quartet of quality AGMs after you kill them by never getting them properly charged from a crappy automotive so-called "charging system".
And keep in mind - even a proper 50a DC-DC charger is gonna take
AT LEAST 5 HOURS of engine run-time just to do the bulk stage recharge on your proposed battery bank.
PLUS however many hours it will need for the absorb stage.
And that's if the bank is only down to 50% DoD.
A stock voltage-regulated alternator setup is gonna take a WHOLE LOT LONGER to get it done.
So you better figure on buying a bloody good BigAss(tm) shore powered charger as well. A Prosine 2.0 inverter/charger that can do 100a bulk stage from shore power would be best (but again, not cheap):
http://www.xantrex.com/power-products/inverter-chargers/prosine-2.aspx
But hell, ANY decent multi-stage shore power charger like an Iota, Samlex or even a Battery MINDer would do the job. And you WILL need it if you don't want to have to replace that battery bank in a year or two.