I've just ordered a Concorde SunXtender PVX-1040T AGM battery. Concorde also makes the Lifeline batteries. I wanted to use the Lifeline, since the ratings are published in automotive terms and easier to compare, but the largest that would fit without battery tray modifications is the Group 24. By going with the SunXtender model (different case) I'm able to fit a Group 27 (barely). I may have to add sturdier t-bars since this battery weighs 66!lbs <img src="/forums/images/graemlins/lol.gif" alt="" />

The PVX-1040T is rated 100AH @ C(20) rate - compared to an Optima Group 31 Blue Top Marine at 75AH @ C(20) rate. The PVX-1040T has the same specs as Model PVX-12100T in this link:

Concorde Specs

For heavy users of electric winches/auxiliary lights, it's probably easier/better to install the second battery in the engine bay, but the tradeoff with the driver side headlight location is high heat (bad for batteries). Air from the engine fan spins off directly onto that location at idle.

I don't plan to install an electric winch, so if a second battery is used, it'll be a duplicate and I'll probably locate it behind the passenger seat in the footwell. Turned on its side, the battery should fit in a box and locate easily for wiring to a charger/inverter.

Another option is 2 smaller batteries turned sideways in the regular battery location. For maximum capacity, some "massaging" of the inner fender and heat shield will be necessary.

FYI, here's the main link to Concorde

Good luck,
Frank