Every car I have owned since radios became integrated with the dash rather than just being a slot in DIN type the radio has turned itself off after about 1/2 an hour when used without the engine running to protect the battery. You simply have to press the on/off button to turn it on for another 1/2 hour. Even after a couple of hours waiting for the wife when she goes to the dentist I have never had an issue starting the car.

I suspect that the battery was already cream crackered before this incident. If the battery was under warranty they should replace it, if not its down to the owner. But noot all manufactirers warrant the battery for the full 3 years and some (I have read) refer the owner back to the manufacturer (like they do with tyres).

The OP has not mentioned the cars mileage, think Audi have a 60,000 mile limit and if exceeded the warranty has ended.

Lack of use during lockdown could also have caused the battery to discharge and eventually fail. Improper use could then be blamed for failure and even though its hardly the fault of the owner its not Audi's fault either.

But has the OP told Audi they left the radio on. If they told them they have simply given Audi a potential get out of jail free card.

As is always the case, too much information can be a problem.

Audi warranty for 12v batteries is 3 years or 60,000 miles whichever occurs first - on VW Group cars with stop-start, it's a scheduled replacement at 5 years anyway.

£400 for a battery A6tdi dealer price what do you expect, probably needs coding to the car, another money maker. Couldn't the RAC supply a battery and code it, my independent garage can code them. I would expect a longer battery life on a premium car.

Twin batteries on my Landcruiser cost the previous owner £160 the pair from the Toyota dealer plus about £50 to fit some 4 or 5 years ago, i shall be both surprised and disappointed if they don't last at least another 5.

£400 is laughable you can get a top range Varta lorry battery with 1150CCA & 225ah for £245.