ECU went to poop? I have nothing left for ya.

 

alternator

 

Did you figure out what it was? Also what makes you think the alternator went out, just wondering?

 

If you haven't fixed it yet, I have a suggestion. Older A8 have an electrical connection at the front right , just behind where the suspension is fitted to the car. With time it often falls to pieces, and from the symptoms you described, that might be where the problem lies. Let me know.

 

I would get a clamp on amp meter and measure what load you are seeing. Clampthe meteraround the positive wire on the battery. There may be some specified small amount of load, which would be good to know. If it is high, I would start pulling fuses untill it dropps off. To check the alternator, maybe disconnect the lead leaving it to see if the drain is beyond it. If the load is still there, try diconnect the lead to the alt. If it then drops off then most likley its shorted out. Good luck.

 

Have you solved the problem yet?

 

As avant80@2.6 said it could be the alternator. Diodes could be shorted,
also chech the voltage regulator.