[JulianHicks]

I've had what appears to battery drains for over 4 years with my 2005 3.0TDi A8. Mine's still not fully sorted but my advice would be to :

1) Clear out all the faults/DTCs so you know what is actually currently faulty on the car.

I'm not sure much reveals itself with a scan when there's a battery drain anyway, but you've got too many faults in your scan to see what might be going on currently! Not sure why you haven't got ECUs 1 [Engine], 3 [ABS], or anything above ECU 37 in your log. The most useful to see would be the Battery Manager [BM] ECU - 61. BM DTCs might be useful and some SW versions of the ECU are buggy and should have an update. [BTW; as well as missing some key ECUs, your logs seem to incorrectly identify Software levels as Hardware levels].

2) Fully charge you battery with a smart charger.

You can do this on the car without disconnecting anything but do it across the battery positive and the negative 'bar' connected to the BM. Will take about 10 hours - best done overnight. When charged fully, that is with the smart charger 'bulk' and 'topping' charge, battery volts with a voltmeter should be about 12.6V. Note - I'm not sure the BM that all A8s have correctly allows the battery to be properly 'fully charged'. Audi don't publish any specs. for the BM, so it's difficult to determine if it's charging with the 'bulk and topping' charge that lead acid batteries need now and then.

3) Check the battery volts with a voltmeter now and then.

I bought one of those LED display voltmeters that plug in to the cigarette lighter socket. These aren't super accurate but you're checking voltage changes rather than actual values. Leave it in the front socket all the time and you can check the battery voltage by just looking through the window. [I left it in while I was away in Italy for the week and got my neighbour to look each night when it was dark so he could see it easily. On that ocassion I got virtually no voltage drop all week - went down about .1 Volts.] Bear in mind that approximately, 100% is 12.6V, 50% is 12.2V and 0%[flat] is 11.9V.

Alternatively, just check the battery voltage with a voltmeter across the battery terminals every now and then. Either of these techniques will at least indicate what the degree of 'voltage loss' is.

And then all that work won't necessarily show you what's going on! It's easier if one circuit is causing the drop; the recommended technique is to measure voltage across the fuses. The voltages should all be zero as the circuits should be 'dead' but a fuse with a small millivolt drop indicates a 'live' circuit. [But this won't normally spot the device that comes on and goes off again later].

Finally if/when replacing your battery make sure you get a battery with an appropriately high CCA [cold crank amps]. This needs to be higher for diesel engines as they turn over less easily; the A8 diesels should have at least 850A CCA.

Julian