What type and guage wire fits in KID battery out terminal?

[NewSolarUser]

I've spend some time on this and I'm running in circles chasing my tail  now. What I've got is a vehicle-mounted portable solar panel array feeding a KID, which in turn feeds four 104Ah 12V AGM batteries in a 2x2 24V array. I run an IRIS 1800/2800W inverter off of that. My question is, what wire should I run from the KID to the battery array? I temp wired it with 12ga THHN for the initial tests because it's what I happened to have on hand. The cable run from the KID to the battery posts is about 5'. I didn't run much in the way of loads for the test and the batteries were already fully charged so I didn't see any heating of the wire. I did a quick calculation using the panel array's theoretical max output of 520W and assumed 100% efficiency in the KID to get 21.7A as the max possible current to the batteries. Looking at http://www.powerstream.com/Wire_Size.htm I see that I need to run around 6ga wire, but that wire size doesn't physically fit in the KID's battery out terminals. So, what wire do I need to use? My only requirement is that it be flexible. This system will be installed and run maybe 4 times a year for two to three days each time.

[NewSolarUser]

I'll try over in the Kid-specific forum.

[TomW]

I'll try over in the Kid-specific forum.

OK, I will lock this.