The Audi RS6 is a high-performance luxury vehicle and along with that status comes a lot of weight! Even though 500hp+ sounds good, the weight of the car is over 4,000lbs. The car is still quick, but handling, performance, and fuel economy could be improved through weight reduction. The general rule of thumb is that every 10lbs lost, is equivalent to gaining one horse power from the engine. Not only do you get increased performance, but you also get increased fuel economy.
My previous Audi was a S4 and at one point I had an Odyssey PC680 battery installed. The PC680 weighs less than 15lbs., and with mounting brackets, it weights 17lbs. The custom aluminum PC680 bracket for Audi's can be purchase from a person known as Jet Jocket in Canada. When I sold the S4, I kept the lightweight battery so that I could install it in the RS6. The only negative side to a battery this size, is that you could accidentally drain the battery very quickly. You do not want to sit in the car with the radio playing or lights on, for more than a few minutes. It drains as quick as it charges! Don't dispose of your stock battery, as you will probably want to put it back in during the winter.
You will first need to remove the battery cover in the truck by removing the two 1/4 turn fasteners.
Next, with a 13mm wrench, remove the two black brackets on the front and side of the battery.
You will also need to remove the negative battery terminal, as you will have to later relocate it. The negative terminal needs to be relocated, because the PC680 is much shorter and the cable won't reach in the stock location.
Disconnect the terminals from the battery, and you should now be able to remove the battery.
How much does the stock battery weigh compared to the PC680 (with brackets)?
That is almost a 40lbs weight savings!
Next, slide the PC680 battery into the battery tray with the positive terminal closest to the back seat. Slide it all the way up and toward the outside of the car. The custom aluminum bracket I am using is the same width as the stock battery, so it should fit snug.
Take the black side bracket, remove the rubber washer on the bottom, and loop the negative terminal through the bolt. Screw the bolt, negative terminal, and bracket into the threaded hole near the corner of the battery's aluminum platform.
You will want to angle the bracket as seen above and then tighten down. After this bracket is installed, the battery should not be able to move in any direction.
Reconnect the battery terminals, charge the battery, and enjoy a nearly 40lbs weight savings!