A8 Parts Forum - weird one total electrical failure

OK, so this is a weird one.

Drove my car into dundee for work, then various trips around the city and back home. Total of about 40-50 miles. After I got home, I needed to get the shop so went out, five miles or so. No problems at all. Car working as normal.

Came out of the shop maybe ten minutes later. Car opened as normal with keyless. Sat down and pressed start. Nothing. Absolutely zero happened, nothing lit up, made any noises or anything. Tried a few more time but got the same. Tried key in ignition slot, absolutely nothing still, except that it wouldnt give me the key back. There was total electrical failure, not even the hazard lights would work.

I got the OH to come down with a spare battery for the car and a pin to let me get the key out of the ignition slot!

When I got into the boot, i disconnected the battery and reconnected after a few minutes, still nothing, but when i was undoing the positive lead, there was a fair bit of sparking going on. Something was clearly drawing current - enough to heat the cable up to maybe 40 or 50 degrees (at a guess). When I reconnected the battery, there was still nothing.

Put the spare battery in and it all came back to life. Inside the cabin, the dash screen was solid red and every single warning light was on. MMI screen was up but blank. Tried the start button a couple of times but nothing happened. Tried tentatively in the ignition again. Turned to left for a few seconds. The speaker made a really loud buzzing noise (like when you accidently touch wires together when wiring up hifi speakers) for a few seconds, everything flashed then went off. car now worked as normal with no apparent problems.

When i got home the battery was totally empty. My DMM has stopped working so couldnt check voltage but dial on battery charger showed zero.

Last wek I got into the car in the morning and the battery was flat and struggled to turn the engine over. i had to change it for the one that failed in this instance. The new battery is about a year old and is known good. It has been in my s3 land rover where it only has to start the engine. This happened the day after i got the car as well about 6 months ago and occasionally sounds like its struggling to start.

My laptop has stopped charging so I cant do a VCDS scan for now. Hopefully the IT department (my better half) can sort that for when I get back from work later today. Will update when I can get a scan done.

Anyone got any initial ideas? the only thing i can think that could drain a 900AH fully charged battery in ten minutes would be a short somewhere but how the hell do i go about finding that before the car goes on fire is anybody's guess!

cheers, gill