I use the Porsche brand maintainer which plugs into the cigarette lighter. Have done so successfully for two years now. I understand you can get the same maintainer from CTEK. (
http://www.ctek.com/EN-US/home/default.aspx) for less money. It's the same maintainer, but without "Porsche" written on it and we all know what that costs. Do a web search for CTEK and you can find it in many places for less money than the one from the stealership.