Easy to do. First buy this device from our host ($58.75):
Pelican Parts - Product Information: 980-611-981-00-OEM
Then, plug one end into a 110V source and the other into your cigarette lighter outlet and you have 12V feeding into the 996 electrical system. When you swap batteries, the 12V remains on and no codes ar lost.
You should use one of these battery maintainers/tenders anyway unless you are driving your car regularly or you will find that the battery drains down remarkably fast because of all of the systems that remain on "standby" in these cars even when the ignition is off and the key removed.
Good luck.