The battery in the A8 can be found in the trunk on the passenger side. Remove the cubby vertical (if equipped) and pop up the cover, you'll find the battery. A 10 mm opened ended wrench (most likely in with the spare tire) is needed to unbolt the terminals.
Posted by "1998 A8" on Audiworld
My OEM Autobahn battery has not been performing very well - slow cranking, clock resets itself...so while at AdvanceAutoParts I decided to see if they had the correct battery (type 49, 850 CCA...) and they did (made by Exide). So I went out to my A8 in the parking lot took a 10mm wrench and removed the + & - terminals...then used a 13mm to remove the hold down bracket. I carried the battery in and they tested it and it indeed failed. They tested the new battery just to make sure and it showed 12.8v and a little over 1060 CA...since the battery is rated 850CCA (ColdCrankingAmps) this sounded about right for room temp.
I purchased it for $64.99 + 10.00 core but this was countered by my OEM core so It cost just under $68 total with tax.
I carried it out and proceded to install it, I agree with some other poster in an earlier thread that stated that the + post should have the small 2ndary wire which is removable with another 10mm nut taken off to allow room to manuver the wires as you place the battery back in...this worked just fine...
Immediately after connecting the - terminal ( I connected the + terminal 1st) I heard the radio come on to my surprise...there was no key in the ignition the car was locked...I had left the radio in the "on" position when exiting the car but that was it...I was happy this was the case so It did not require resetting the radio code...I guess the radio had some form of short term memory??? Although the station memory had to be reset so it noticed that there was a power disconnect...Go figure...
Anyway, the battery works just fine and the engine cranks over much easier...I very pleased with my purchase vs. going to Audi and getting robbed. I also saw some AOA service flyer that stated that some batteries in 2000 were made by Exide to begin with.