The OEM battery from 2004 died sometime during the winter storage.
I did not have time to wait for a new OEM battery from the US so I opted for a cheap Biltema Battery that would fit inside the battery well.

Biltema advertises their batteries as "dry" and service free units.
After some serious track driving later this season I found this not to be true as there were battery acid all over the trunk.
As far as I can tell this was not an issue on my first trackday or my annual Nürburgring trip. It might be the new suspension setup and higher cornering G's that has made the battery spill its guts. Shite..
I managed to clean up most of the acid before it did any harm but I can still se some white dots in the paint by the battery and on the car jack.

To fix the problem once and for all I ordered one of these babies:

Braille B14115 No-Weight Lightweight Daily Use Battery

Braille B14115 No-Weight Lightweight Daily Use Battery.

These are completely sealed and weigh only 11 lbs with lots of cranking power.
Since they are sealed they can be mounted in any directions.
This is what I ended up with: