2010 S6 V10 voltage issues

[TravisV21]

Recently bought a 2010 S6 V10 and it's been nothing but problems since the day I bought it. Chased down misfires for weeks and finally got it running smooth after doing coil packs, spark plugs, valve cover gaskets, cam solenoids, oil change, air filters, and a new battery. Now the problem I'm having is with voltage. While running, it holds a constant 12.5v at the battery and 12.0v at the engine bay posts. The battery is a Super Start AGM, and I did code it through OBD11 when I installed it. Didn't have any issues with it before I fixed the misfire. I hooked it up to a charger overnight then drove it for a while and it was just fine, but eventually the battery drains down to 12.5v and I get the usual limp mode, EPC, Traction Control, and ABS lights. Seemed easy enough, I just need a new alternator. But while it was running I disconnected the battery and the car runs fine, and measures at about 13.8v. I do have other electrical gremlins that I'm assuming are unrelated but I'll list them just in case.

Driver's side motor mount shows open circuit plugged or unplugged

Passenger side motor mount shows no faults plugged or unplugged

Driver's seat lumbar works, but no other controls work. It also shows a parking brake malfunction if I try to adjust it while driving

Steering wheel adjustment only works one way (either in and out or up and down, can't remember which). Scan shows no communication to seat control module

Daytime running lights show as open circuit, but they come on when exterior lighting is on

Occasionally, the needle backlights on some of the smaller gauges will flicker momentarily

The console control buttons for MMI will occasionally flash what seems like randomly

All of these issues never seemed to effect the car running before I fixed the misfire. I'm way out of my element here so any advice is appreciated. My other vehicle is a 1970 Cabover firetruck so all this technology is a bit over my head. Anyone have any ideas?

[Avantly]

You are using VCDS for checking codes, correct? Do you have any codes for the alternator or battery monitor module?

[TravisV21]

Was actually just about to jump on here with an update. All seems fine now. I was pulling fuses to check for a parasitic draw in the trunk fuse block. Not sure what I did but the issue seems to have gone away. Drove for about 50 miles today with no issues, running a constant 13.9v. I use OBDELEVEN to check codes and no, no codes for anything directly related. Just a code for the seat control module, the driver's side motor mount, and the daytime running lights. I get an occasional "MMI malfunction" code but it's very nondescript and it seems like it's working fine. I'm concerned because of my experience with the car so far that my fix may be temporary. If the issue comes back, what should I be looking for?

[Avantly]

Typically when you have an issue with the battery monitor module, you will get a code. The primary issue I've seen with those is water getting into the back edge of the trunk one way or another, running down and getting into that connector that plugs into the monitor module because the harness for that connector tends to be pressed right against the back of the spare wheel well and if water runs down there it just goes right to the connector. That can cause the module to go offline and the alternator will then stop regulating.

In your case, the symptoms sound strange, I wouldn't expect the voltage to go up to 13.8V with the battery disconnected (and at ~12.5 before i.e. right at battery voltage) unless 1) You had a really high impedance between the alternator and the battery, which seems very unlikely or 2) There is something strange going on with the battery monitor module though it seems this would set a code or 3) The alternator is acting "weak" for some reason, maybe the brushes in the alternator weren't making contact and now they are....

If the alternator stops working, or is extremely weak, you should get the battery light. Was the battery light on when you were having the voltage issue?

MMI can be a lot of things but in general that generation isn't too hard to work on and you can use junkyard modules if you get someone to remove component protection from a junkyard module (or do it yourself online with ODIS and the VAS dongle).

Since you haven't owned this car for long, you need to check all the common water leak locations as these have caused a large percentage of my troubles with C6. You need to make sure that the cowl piece is snapped down against the windshield all the way across, and not cracked. You need to make sure the drains down in the cowl are clear, there are some threads on this, just make sure that water drains quickly out of there. And you need to make sure that no one has had the ECM box cover on passenger side and relay/fuse box cover on drivers side off before and incorrectly reinstalled them, or worse yet cracked them. Every C6 I've purchased has had either damage or incorrect installation on one of these two covers. They aren't hard to fix, are readily available from junkyards if damaged, if either cover is damaged or not correctly installed then water that drops off of the front edge of the cowl cover can run down these box covers, right into the box and into the cabin. Common failure is the bluetooth module, it is on the drivers side floor, and one of the above mentioned leaks will flood the floor under the carpet and kill that bluetooth module. Fortunately the bluetooth module is plug and play, no coding nor component protection required, so that's an easy fix, but regardless you just don't want water coming in the cabin from the cowl there are too many wires and grounds that will become miserable when wet.

[TravisV21]

Second update, it started throwing those codes again as I pulled into work this morning. I forgot to add onto my post yesterday that I've been leaving it hooked up to a battery tender overnight for the past few days. Up until yesterday, it would leave me with enough charge to drive for about 20 minutes. But yesterday I drove for about an hour and a half with multiple starts and it was still at 13.9v when I got home. I figured I was good so I didn't hook it up to the charger last night. Sounds to me like the battery may just be bad, yeah? I didn't find a parasitic draw, and even then it should charge while running I would think. It seems strange that a battery that new would have problems already. The previous battery is from 2021 too, so it seems even more unlikely that two in a row would be bad basically off the shelf. They are, however, both Orielly batteries and I don't think I've ever gotten a part from Orielly for any vehicle that wasn't junk. When I bought the car, the guy had the new battery in the back seat and just said it needed to be replaced so I didn't do any kind of testing on the old one. I would have never bought an Orielly battery otherwise. As far as water goes, I haven't found any sort of leaks anywhere. There are some leaves in the bottom of the front cowl, so I suppose I should probably clean those out but the trunk is definitely bone dry and the front cowl is snapped down and not cracked. I'm at a loss here. I may have to just pony up the cash for a new battery. Orielly won't warranty mine because I didn't purchase it

[Avantly]

Can you post the results of a full scan? The battery should not go dead without some codes being set first.