Use this PPC-019 adapter to plug a battery charger (Polaris, Accumate or Optimate) directly into a Powerlet socket.

Works with BMW, KTM, Triumph, Victory, and Ducati motorcycles equipped with the smaller accessory outlet.

Aboiut 6" long.

You can also power Oxford HotHands grip heaters with this handy adapter (they use the same type connector)
However, always check polarity before connecting other accessories. The plug is easy to open in case you need to reverse polarity