I had been researching this for the past few months, and had settled on a setup that I ran across online from Ken Wilford (you can find a video he made on Youtube). He utilized a few smaller 12v, 22Ah wheelchair batteries that fit perfect in the box beneath the drivers seat, in fact, you can fit 4 of them in there. These are a sealed, lead acid battery and you can find them on Amazon for about $35-40 each and free shipping. I chose to start with 3 because that affords you a little extra space under the seat to mount your relay and run wires a bit easier. I figured that I could always add one more later if my energy demands are too high for this setup (see next paragraph). I also have an empty spot under the passenger's seat for yet more batteries in the future because my engine battery is now located under the rear seat.
|UB 12220, 12v 22Ah.|
I bought mine on Amazon
The smaller batteries are wired in parallel, which keeps the same voltage and adds the amperage, so 3 of these 12v, 22Ah batteries wired in parallel, will equal 66Ah, but remain at 12volts.
Here's a quick reference to how they "should" be wired to help equalize the load and charging on the batteries.
|Located under the driver's seat|
There are many different ways to go about this and you'll read about folks who run utilize huge solar panels and enormous battery banks. It's really up to you and your needs. For what we do, this works great.