Battery empty!!! Help!!!

Hello dear Phaeton lovers, I can now announce my first Phaeton as my property for a few weeks now and had no problems at all… until the day before yesterday! Even in advance, it is a V6 3.2 gasoline from 2002. The problem: When I came to my babe the day before yesterday morning, the dear car did not open with my radio key. There was still a very weak light on the turn signal, but that was it. So I went with the key into the keyhole and got manually When turning around in the ignition lock: GAR NO MUX! Then I started the car with the emergency start (key to the left and then to the right), everything worked out immediately. Only all settings had disappeared, long-term statistics, time (the beautiful analog clock turned and turned until I have set the time by GPS again) etc. When I set everything again, I was able to leave normal and everything was good… Then I turned off the car and after 2 S yesterday and this morning then again the same dilemma! From a service life of approx. 5 hours the car is absolutely flat! When looking at the battery (left in the trunk) I saw that there was already a new battery installed there. Gem. of a friend car connoisseur, however, this is an acid battery (white battery), which should not have been installed with me, because of cycles or such things… So I have the car immediately with this b car connoisseurs connected to his analyzer and had the fault memory read out. There was actually nothing exciting except many messages because of too low onboard voltage etc. and once a short circuit or something similar in the memory seat of the driver side. So, now I could of course buy a new onboard battery tomorrow and hope that that was the fault. But I’m also afraid that I will buy the battery and do nothing else! . already experience with such a problem? That was very long now, but I wanted to describe the problem as accurately as possible. I hope you can help me!!! Thanks already for your efforts! Many greetings from Wolfsburg André