The battery used by Porsche is Moll.
http://www.moll-batterien.de/ Porsche does not make batteries.
I do not know of an aftermarket source in the US for this brand. For a while Porsche was using Douglas batteries for warranty replacements. Douglas is a US company but their batteries could be made in Mexico for all I know. Porsche switched back to Moll a few years ago. Guess Moll was the lowest bidder.
I have been doing this Boxstering stuff since 1999. Been to a lot of dealerships and independent shops. I see the defective Moll batteries stacked outside on pallets. I would not buy an overpriced Moll battery when there are better alternatives out there.
http://www.renntech.org/forums/index.php?showtopic=2950
If you have money to burn give some of it to me and I will put in you Box a better battery at a fraction of what the dealer will charge you. There is nothing special about about a 12 volt lead acid automotive battery called Moll.