Hi all! Here is my personal experience on this subject. Hope it helps!
The Porsche® Battery Maintainer (that the oficial name) can be used with ANY recent Porsche model up to the 911-996 and 986 Boxster.
It is used to charge the battery when the vehicle is left standing for long periods and it connects easily via the cigarette lighter. I keep mine always connected to prevent the battery from discharging. It is harmless to the system since it was designed fot this purpose.
I think the oficial Porsche reference is WAP 00004320256 and you can get one from Porsche (I did!) , but you can get one over from "Porscheshop", "Performance Products", "Suncoast Motorsports.com" or "Pelican Parts" for example. It costs around 60 Euro/US$70.