Parasitic Draw?

Yesterday afternoon I went to start my 2010 MS3 and it would not crank over. I assumed it was the cold (I live in Alaska) and had the car jumped. The car started fine when hooked up to another vehicle, but would die immediately when disconnected. I jumped it again and this time read the battery voltage on my Accessport, and the battery would only charge up to 10.7 volts, and the last reading before the car would die was 7.8V. At this point, I thought maybe the battery could have been frozen so I took it inside for the night to thaw and charge. In the morning, the car still would not crank over, so I thought one of the cells on the Walmart battery could have shorted and I bought a brand new one from Napa (the interior also smelled like battery acid). The car started fine with the new battery and charged up from 10.4 to 11.5V at idle. After about an hour I went to start the car again to drive it around the block and see if higher RPM would allow more alternator output and the car was dead to the point I had to manually unlock the door with the key. I've looked around online and found a few different parasitic draws, most having to do with the BCM and the "Main" relay in the engine bay fuse box. I don't think the "Main" relay is at fault because the car has started just fine in much colder conditions than today, but I pulled it and have in inside for the night just in case. It was too cold and dark to pull the BCM tonight, but I plan to tomorrow once the sun comes up. With all that being said, has anyone experienced this issue? I haven't heard of a parasitic draw that can completely kill a brand new battery in an hour.

 

I would look into ground issues. Any changes to your car recently, aftermarket lighting /stereo stuff?

 

Are you sure the alternator is actually working as designed? A good battery should be between 12.2-12.7 volts KOEO. KOER it should be between 13.5-14.7 volts depending on the loads your using such as headlamps, HVAC etc.

You could also get a battery blanket for the winter time. It's kinda like a foam surround that keeps heat in the battery. But I would do a trickle charge inside first to get the battery back up to 80-100% health then reinstall and check running voltage after that.

 

No mods in the past 4ish months, and nothing to do with wiring or electronics. I’m wondering if the alternator could be bad because the battery never charged above 12.6V.

 

I didn’t think to check voltage KOEO, but the battery read 12.5V at Napa, so I’d like to think the battery is good. I’m charging it indoors today. I’m trying to get my hands on a multimeter so I can get a more accurate reading with the vehicle off, but I’m thinking there must be a short in either the starter or alternator for the battery to drain completely flat as fast as it did. I’m going to pull the alternator sometime this week, but unfortunately the vehicle is parked outside and I’m not comfortable jacking it up on snow, so it’s a matter of getting the vehicle somewhere dry at this point.

 

I would make sure all the consumers are off and see if it's charging at idle around 13.5-14.5 as stated above.
If the battery is good and not to cold outside where you are at and have a multimeter. What I do at work is set it it in amps. Take the negative cable off the battery terminal. Then hook the positive multimeter lead to the negative cable and hook the negative cable of the multimeter to the negative terminal on the battery.
On the Mercedes I do this test with they are usually around 30 milliamp draw or less which means there are no consumers on causing a draw. If your looking at the meter and open the door it should jump to around 4-7 amps or more. Close the door and give it 2-5 mins for the modules to go to sleep and it should drop back down to 30 milliamps or less.
If you have more then 30 milliamps with all door closed then start pulling fuses one by one until you find the circuit or relay.