[designerX]

This round threaded port is about 1" in diameter and has 2 prongs. It's wired directly to the battery. I'm hoping it's a tender/trickle charge port.

Happen to recognize it?

 

[Mike S]

Looks like an audio plug, that has been re-directed to another use.

 

[N282S]

I think I know

Looks like the 12v port found on my bass boat used for fwd and aft (ahem, bow and transom) push in lights. You can go to Walmart, watch the fat people yell at their kids, and then pick up a spare light in the boat/aviation section. Cut off the light and there you go.

 

[xavierm]

Trickle charge port

I have the exact same thing on my UTV I use as my airport runabout. A charger connects to it to charge the battery.