Modern Audis (and almost all cars for that matter) use advanced keys that contain a microchip and are battery powered for unlocking the door, turning the alarm on and off, opening the boot and other functions. The battery on the key fob occasionally goes bad and need to be replaced. Replacing the battery is relatively easy to do.
Flick the key out as if you were going to be opening the door. Locate the thin line that goes across the width of the key parallel with the buttons on the key and which is where the two halves of the key meet.
Wedge a thin screwdriver or similar tool carefully into this dividing line. Note that this is not the same thing as the holes or the grooves that are located inside the recess of the key fob.
Twist the screwdriver slightly to separate the key portion from the fob portion of the key fob assembly. Wedge the screwdriver into the top of the fob, the sides and the back and twist until the two sides separate. This may take some force, but be very careful not to break the plastic.
Open the small plastic ringed end of the lid that is located in the side of the key that does not have the button on it. The lid can be gently pried open with your nail or even the screwdriver, but be careful not to break the lid as you pull it out. Do not touch the computer chip under the lid.
Remove the old battery and install the correct new battery, available from the Audi dealer. Replace the lid and snap the two halves of the key back together.