OK don't disconnect the battery,if you look at the negative terminal you will see the short earth lead goes to the battery manager control unit,there is a negative post there and that's the charging point as the manager measures the amps also if you replace the battery it requires recoding to calculate charge rate.As i mentioned A8 is a very complex car and the battery manager will slowly shut the cars systems down on the returning amps calculation to the battery.The battery needs to be charged correctly and in circuit to prevent further red hearing fault codes.I cant offer a full answer about the how to and how it works as there's not enough space on the forum but if you have the battery display in the mmi functions it will offer a fairly accurate consumption level.If you don't have the display i will tell you how to adapt the mmi set up to show the battery state.The dash communication fault also concerns me but it will become clear when the fault codes have been erased.Also always attach the battery charger when carrying out any electrical work as the battery drain on a8 is very quick and you may be draining the battery with out noticing by having the door open whilst using vag com