Lurky, here you go:
Amazon.com: Battery Tender 021-0123 Battery Tender Junior 12V Battery Charger: Automotive
They also sell a cigarette lighter adapter on amazon from the same company that's even more convenient. Since our cars have an "always on" cigarette lighter, we can just plug in the charger into the cigarette lighter socket and you're topping off your batter when it dips below the recommended level.
I have used this brand for a couple of years. No need to pay $125 or more for the Porsche one.