Are you talking about the one down by the coils?
BMW makes a battery charger that fits it. I have also seen an aftermarket set of jumper cables that will plug in as well. Do not use the jumper cables or the charger on the outlet by your key switch.
In the past, BMW has made a heated vest that would work on that plug. It was like the one made by Eclipse but sported a different plug. I think Eclipse made it. Do not know if it is still made. If you want one, buy the Eclipse version and save yourself some cash. If you still use the old System series helmets from BMW, the heated visor will plug in too. If you use that helmet we need have a little talk about a new skid lid.
An adapter is made so that you can run a normal phone charger like you would use in your car. Any BMW motorcycle dealer should be able to get it.
Try A&S, I think it runs about $15.