With a new battery my A6 will occasionally go completely "dark". No dash lights, nothing when key turns, nothing when I try to remotely lock/unlock. If I unhook the battery and then hook it back up everything works. It has done this off and on for a year. Recently I went six months with no issues - now I have had to do my "reset" three times in a week!
At first I suspected the security system - but now I don't know.
As an aside... The positive battery terminal post seems to have more corrosion on it than I'm used to.
Any thoughts from you all?

 

If you have a voltmeter, when the fault occurs check the battery voltage across the actual battery terminals and then across the positive and negative leads. You should have about the same reading, otherwise it is a connection fault (corroded terminal).