I was getting a "charge battery message" recently, but had used a battery tender so I thought I was OK. After driving about 100 miles earlier this week, I parked the car and came back a couple of days later to a completely dead battery.

I then replaced the battery with a new one from Advance Auto Parts with proper fitment, and everything worked perfectly - car started right up, all electronics good, etc. - for another 100 mile drive home. A day later, I just tried to start the car and it's completely dead again. No electronics, won't start, completely dead.

Any ideas on what is causing this and how I can resolve it? I suspect some grounding issue, since even if my alternator was shot a new battery shouldn't discharge completely in 36 hours when the car was only driven for 2 hours. There were no indicator lights or anything. The only thing that seems to be working is a flashing green light on the N position on the shifter (which is odd since everything else electronic on the car seems to be completely DOA)

I recently purchased an OBD ethernet cable so I'd be able to run my own diagnostics, but haven't set that up yet. But I'm pretty technical, so if any suggestions require diagnostics, I can probably run them soon.

Car is a 2012 Alpina B7 w/80k miles. Thanks in advance.