Porsche 2003 C4 battery hold down bolt won't come out.

Try spraying a lot of penetrating fluid eg Liquid Wrench on it and leaving it a few hours. Then pull the nut up to the battery tray and try to lock it in place with the force on the battery tray as you undo the bolt. If that doesn't work, can you get thin pliers to the nut? Failing that, you will likely need to cut the bolt off with a dremel or similar.

Where did you purchase the battery? Auto Zone or O'Rileys, install for free. And, probably are use to seeing your problem on a daily basis. Let us know how you did. Good luck.

Using a small pry bar under the head of the bolt while exerting upward pressure on the bolt head, use a 6 point socket with extension if necessary, and then turn the bolt counter clockwise. Keep the strong upward pressure on the underside of the bolt. This almost always works. Good luck!

I finally got the old battery out. It was bought from and installed by a local Mobil station. Turns out that they ordered the wrong battery for my Porsche and decided to cram it into the car. It was too long by 1.5" so the hold down was relocated to a hole outside the normal, with a simple nut and bolt. I had to remove the plastic cover on the passenger side of the battery in order to get a wrench on the nut. The new battery is the proper size and fits easily into the intended position. Thanks for all the help.

As usual, it's do it yourself if you want it done right. You've got a Porsche, never trust it to any shop that doesn't specialize in them. I wonder how many batteries were installed in Porsche's by that station??