I found the place where you can attach a battery tender to. Located on the firewall on the drivers side. It's the same terminals you would use to jump start the car. On my prior car, C7 Z06, there was a 12 volt outlet in the back trunk where you would just plug in a battery tender. The S3 also has a 12 volt outlet in the trunk but not sure that is a good spot for the battery tender. So far have been using the engine compartment firewall spot and it's working fine.