I've got a duo-Optima (yellow and red) setup routed through a Wrangler battery isolator. I also have a Mean Green 200 amp continuous duty alternator.
I couldn't find an XJ kit anywhere, so I finally had a custom installed performed. It was expensive, but I've been very happy with the results to date. He's how I handled it.
1) Optima yellow deep cycle (secondary battery)
2) 25 feet of 1 gauge high quality fine wire cable.
3) Military connectors.
4) Wrangler battery isolator for battery management with LED switch on console
A) Runs from primary only (charge and usage).
B) Runs from primary but charges secondary.
C) Runs from both.
5) Swapped out the stock air box for Rusty's air tube (makes room for the extra battery).
6) Custom made secondary battery box.
It was a time consuming but fairly straight forward install. The custom wiring was time consuming aspect of the job.
I still need to change the main junction of the battery and electrical system lead which is only a 8 or 10 gauge wire to 4 gauge, beef up my ground and beef up my alternator to relay wire. All my above work, is still being clogged by not finishing the job.
Here is a link to a great article on electrical upgrades that piaa_nissan initially found.
Hope this helps,
94 XJ (purchased July 00)
Skids, XD9000I, Lights, Bumpers, CB/Ham/Scanner, Due roof racks, ARB compressor, Duo-Optima's, 200 amp alt, and too much more to list!
These trucks rock!