With the passing of time, the battery on the Audi A4 will drain out and will need to be recharged; however, if the battery was recharged several times and has come to the end of its life cycle, it must be replaced with a new one. The battery on the Audi A4 lasts about 73 months and is located underneath the hood, on the passenger's side of the vehicle. Replacing the battery takes about 20 minutes to complete and should be done when the engine is cool.
Open the bonnet of the Audi and pry the rubber seal strip to detach the strip from the firewall. The firewall is located on the passenger's side, at the far end of the engine, near the windshield.
Remove the tray (located by the firewall) by pulling it upwards.
Remove the three Philips screws located on top of the plastic passenger windshield cover, using a Philips screwdriver, then remove the bolt found near the cover, using a ratchet wrench in a counterclockwise motion. Pull the plastic cover upwards, away from the Audi, in order to gain access to the battery.
Remove the two bolts located at both sides of the battery, using a ratchet wrench in a counterclockwise motion, in order to remove the bracket that is attached to the battery.
Clean out any dirt around the drain hole located underneath the battery, in order to properly install the new battery.
Pull and remove the two clamps located at each side of the battery. Detach the drain hose from the battery and detach the battery cables from the battery terminals in order to pull out the battery from the Audi.
Repeat the removal steps in reverse to install the new battery.