If I'm not mistaken, there is an adapter that allows you to do just that. There is also a Battery Tender that is for use on a boat that is water proof. Actually the Battery Tender Jrs. are pretty much water proof if you wrap a plastic bag around the plug part. And let me repeat myself, as I have said this before, and I'm sure I will say it again, Battery Tender is a brand name for a battery maintainer. There are others on the market, and I'm sure some of them are as good as the Battery Tender. I have about 20 or 25 years of experience using the Battery Tender, a couple of the different models, and they have extended the life of my batteries by almost 100%. Right now I have them on my boat, my garden tractor, my Spyder, and my 2 automobiles. If I'm not driving the vehicle, it's connected to the Tender. Winter, Summer, it doesn't make any difference, if I'm not driving it, it's connected. Poo poo all you want, I get 7 to 8 years from a battery, that's good enough for me.
David