I use that (female) plug to hook up the mini battery charger.
Bought a couple of male brass plugs (I think they are some kind of phono jacks) from Radio Shack, one red and one black, that were slightly too big and "whittled" them down until they made a good (tapered) fit (one is larger than the other), then tossed the alligator clips and wired them to the charger.
I thought that the red wire on the bike (which on my bike is the larger hole) would have to go to black on the charger, but I was wrong.
Of course, it's possible that at some point the wires to the plug were switched and I just got lucky. But polarity is obviously important if you wanna do this...