This was the most insanely difficult battery replacement ever. I can't believe how hard VW made it to get at the battery on this car! I figured, "hey, how hard can this be?" Then I started reading a few other posts on this topic and figured I had better proceed with caution.
A few caveats about doing this replacement that I learned from other threads on this forum:
1. If you disconnect the power to the ECU, the car loses track of an adjustment for the throttle body and the car runs like crap, if at all. You will then (most likely) need a VAG-COM to reset the adjustment. Moral of the story: Provide another power source to the car while replacing its battery.
2. Make VERY sure the battery you get fits properly. Some other posters have reported that the replacements they got were slightly too small and/or lacked the hydrogen vent hole (charging batteries give off hydrogen...hydrogen + spark = BOOM)
So, here is the detailed writeup on how to perform this task:
1. Locate the correct size battery. The proper size is 94R. Accept no substitutes; many sources have the wrong size code in their books.
2. Peel back the weather seal that helps retain the plastic cover over the battery, master cylinder and ECU area.
3. Remove the plastic cover. This reveals the battery but does not provide enough clearance to actually remove it.
4. Remove the metal retaining clip that holds the battery in place. There is a hard-to-see bolt (13mm) that has to come out; it's just forward of the battery. You'll need an extension for your socket wrench.
The "water deflector" (a.k.a. the cowl, attached to the bottom of the windshield) has to come off to provide full clearance for the battery to be removed.
5. Remove the wipers...not the blades, the wipers themselves! This is done by popping the plastic bolt caps off with a flatblade screwdriver and then removing the 13mm nuts that hold the wiper arms in place. Mine were stuck pretty bad--I had to really work at them to get them to come off. A bit of penetrating oil and a rubber mallet are your friends here. Wiggling them back and forth finally got them loose after pounding on them for a while. Make note of where the wipers line up against the windshield before removing them completely.
6. Remove the "hinge covers" at the edges of the water deflector. This will require a Torx T-30 tool. I had to move the hood up and down a bit to work them loose and remove them.
7. Remove the two metal clips from the leading edge of the water deflector. This can be done by hand.
8. Remove the water deflector.
9. While the battery terminals are still connected to the battery, attach a set of jumper cables to them while leaving room to still loosen the terminal nuts. Connect the jumper cables to another power source, such as an extra battery or another car. This is crucial since if you disconnect power from the car, you will lose crucial ECU settings for the "Throttle body adaptation." Search the forums for this term to see what I am talking about.
10. Carefully remove the battery terminals...WITHOUT interrupting the connection to the external power source!
11. Disconnect the vent hose from the side of the battery.
12. Now, after all of that, you can FINALLY remove the battery. Be careful not to knock the jumper cables off the battery terminals! Having someone hold the positive terminal out of the way is a big help here. The sucker is HEAVY, watch out!
13. Put the new battery in the car and carefully reconnect the terminals. Again, use caution not to knock the jumper cables loose.
14. Reassemble all the crap you had to take off the car. A few choice curse words for the German engineers that made this task so difficult are a fine companion to this task.
FYI: A few posters on this site have mentioned that the area at the bottom of the windshield must be VERY clean before the water deflector will reattach properly. One even mentioned that a small stone got trapped beneath the deflector and cracked his windshield! As such, I cleaned mine pretty thoroughly and had very little trouble getting it reattached.
I thought it was absurd to have to take the back seat out to get at the battery in my Audi, but this makes the Audi look like simple and elegant--at least that only took two screws and then the battery was easy to reach!