Ok, here is how the setup works:
Costco: Batteries (and other questions!)
The main battery reamins in its normal position under the hood, and the alternator reamins connected to it like normal - you don't have to do anything to these items.
I connected a 4 gauge wire to the positive terminal on the battery. This runs to a 200 amp circuit breaker, and then to the battery isolator. The battery isolator is essentially a giant relay - it has a low-amp positive input, a low amp negative input, and 2 high-amp (200 amps in the case of this one) terminals that become connected inside the isolator once there is power provided to the low-amp positive input. I connected the low-amp positive terminal on the isolator to a fuse tap that is drawing power from a circuit in the fuse box that is only on when the key is in the "on" position (I don't remember which specific fuse I tapped in to, but it really doesn't matter because the current draw to switch the isolator to the "on" position is like .3 amps).
So the 4 gauge wire continues on after the isolator and goes through the firewall, where it is connected to a "distribution block." The distribution block is not shown in the schematic above, but it's the point where the red positive wire splices off and runs to the aux fuse box. So the 4 gauge wire then continues on to a second 200 amp circuit breaker and then to the positive terminal on the Optima battery. The negative terminal on the Optima battery is simply grounded to the body of the Xterra using one of the seat bolts.
The wire that runs from the distribution block to the aux fuse box is 8 gauge. I mounted the aux fuse box behind the glove box (sorry, don't have any photos of that). It has 10 fuses and 5 relays, and is powering the Ham Radio, Inverter, amber strobe lights, LED light bar, LED side lights, LED rear lights, etc...
With the setup wired like this, both batteries are connected in series when the Xterra is on/running. This allows the alternator to charge both batteries, and allows every single item in the Xterra to run off of both batteries as if they were one giant battery. Once the key is switched off, the isolator separates the circuits, which basically disconnects the Aux battery from the rest of the car. The aux fuse box is connected to the circuit AFTER the isolator, so it's basically only connected to the Optima battery when the key is off. This allows me to run all of my lights, inverter, etc with the key off and doesn't drain the starting battery.
If I completely drain the Aux battery, I can simply reach under the front seat and flick off the circuit breaker that is mounted near the positive terminal on the Aux battery. This ensures that the dead battery won't immediately drain the main starting battery under the hood as soon as I turn the key on and the isolator connects the batteries together. Once the Xterra has been started (and runs for a few seconds to recharge the starting battery), I can flick the curcuit breaker back on and allow the alternator to recharge the deep-cycle Optima battery. I try not to make a habit of this, because the amount of current flowing to the aux battery can be pretty high and could damage the battery or put too much strain on the wiring.
One thing I can do is leave the curcuit breaker switched off and put a battery charger on it when I get home. With the Xterra running (or the key "on") and the second curcuit breaker (the one closer to the Optima battery) switched off, the lights and accessories work just fine using the alternator/main battery because the the distribution block that sends power off to the Aux fuse box is located BEFORE the curcuit breaker that is switched off.
If, for some reason, I want to separate the circuit entirely and have the accessories that are connected to the Aux fuse box run ONLY using the Optima battery even with the key in the "on" position, I can switch off the curcuit breaker that is BEFORE the isolator near the main starting battery. If the 4 gauge wire were to short out anywhere between the two batteries (or the 8 gauge wire before the Aux fuse box), it would ground out both batteries and trip both of the circuit breakers. So the breakers are there more as a safety measure, but also add the convenience of being able to control how the auxillary accessories are connected to the batteries.
In this photo, you can see the 4 gauge wire running from the positive post on the battery to the first circuit breaker, then to the isolator, and then on to the hole in the firewall:
Costco: Batteries (and other questions!)
Here is a close-up shot of the connections on the isolator:
Costco: Batteries (and other questions!)
Here is how the Optima battery is mounted under the driver's seat:
Costco: Batteries (and other questions!)
The battery is an Optima model D51. It's the smallest battery they make and the only one that will fit under the seat like this. I could be wrong, but I believe the Off Road and Pro 4X versions of the Xterra already have a bunch of crap under the seats for the audio system. Anyway, I mounted the battery down using two "L brackets" that are screwed to the floor with self tapping sheet metal screws, and then used two regular battery tie downs that hook to holes in the L brackets and run through a long piece of metal with a bunch of holes in it (I think it's called plumber's tape or plumber's strapping, it's commonly used to anchor conduit and pipes to walls and what not).
In this photo, you can see a close-up of the Optima battery and the second 200-amp circuit breaker near the positive terminal. If you look toward the top of the photo, you can see the negative battery wire and where it grounds out to the body of the Xterra when the seat is mounted:
Costco: Batteries (and other questions!)
In these photos, you can see what the battery looks like with the seat mounted in place. The seat slides just fine, and the rear seat passenger has plenty of room for their feet under the seat (the photo from the rear seat has the driver's seat slid all the way forward so you can see the battery). The seat also slides forward and back just fine with the battery under there:
Costco: Batteries (and other questions!)
Costco: Batteries (and other questions!)
I purchased the isolator on eBay for under $50. It is very similar to this one:
http://www.ebay.com/itm/PAC200-High-Current-Relay-Dual-Battery-Isolator-200-AMP-for-Multi-Battery-System-/360432953695?pt=LH_DefaultDomain_0&hash=item53eb7a695fThe curcuit breakers also came from eBay and are under $10 each shipped. They are similar to these ones:
http://www.ebay.com/itm/200-AMP-12V-DC-CIRCUIT-BREAKER-REPLACE-FUSE-200A-12-24V-DC-FAST-FREE-USA-SHIP-/390474856510?pt=Motors_Car_Truck_Parts_Accessories&hash=item5aea1d783e&vxp=mtrI had the 4 gauge wire and distribution block lying around from some old car audio projects, but you can also get these on the cheap online (the wire is the most expensive part, but you can find it for under $.30 a foot if you shop around). The complete setup can be had for WELL under $200 plus the price of the battery. I got my battery free from a buddy of mine, but I believe it runs about $160 retail.
Hopefully this all makes sense, it's a bit hard to explain. I can answer any questions if necessary.
