You'll need to order part no's (I got these from Listers Seat for around the £30 mark):
So all the info is in one place, my current battery is slightly higher CCA than the factory Varta that came with it:
(096 (278mm x 175mm x 190mm) same as recommend for an Audi S3).
Note that if you have some types of map (I have Revo), you will lose some settings. So I took my SPS and noted them down first so I could put them back on later.
Old (above) new (below)
Before you start, disconnect the negative terminal and tuck it out the way (so it doesn't spring back and touch the terminal). You can then go ahead and remove the positive.
Then locate and remove the bracket holding the battery in place (13mm socket).
You can then go ahead and heave the battery out (it's quite heavy) and throw away the cloth cover (we're never putting it back right?).
I thought I'd got the wrong part, but those 3 square hooks at the bottom go behind the current battery base and there are hooks for it to connect to firmly.
At this point, I actually put the battery back in (cause it's heavy). You can see the channel at the back where the negative cable will run (but don't connect it just yet).
Now slide the front piece of the box over the top (it slots in either side and clips at the front). Make sure the cables are routed with the red at the side and the black at the front.
Black cable routes through here.
At this point you can replace the bracket and tighten it down.
And reconnect the positive, then the negative terminals (note the negative cable in place now).
Then just hook the lid on it's hinge, click it down to go from this.