I know I need to get it somewhere to get properly diagnosed, but I haven't had a chance to do that and the problem seemed to "go away" for a bit. Jotting this down here in case it helps someone who finds this thread in the future once I finally figure this out.
Throughout this, the radio fuse is not in the car.
About 2 weeks ago, I took the car off the charger (battery fully charged) and ran some errands. It did
not complain about high discharge while being turned off after parking it at the store. Cool, I think maybe I've solved it with the radio fuse even though it wasn't drawing very much current.
Right after that, I leave the car OFF of the charger, and it sits for a
week in my garage while one of the wheels gets refinished/repaired. A full week, not on the charger, not being driven. I get the wheel back, go for a drive...and the car is fine. It is not in "low power" mode when I go to get in it, it does not complain about discharge while off. After sitting for a
full week. I'm sure I've solved it now.
A few days later I get in the car to go to dinner just a little over a mile from home (hey it was 22 degrees and snowing, I'm not walking in that!). I start it up, no battery message. Car's fine. I'm convinced I'm in the clear.
I get in after the car sat in 20 degree weather for over an hour at dinner...no battery message. I applaud myself for my troubleshooting skills. I drive 2 miles to pick my kids up from my mom's house. I'm inside for 10 minutes, go out to get in the car, and....
BAM excessive discharge while car is off.
And it did it again the next day during a couple of errands.
So, I don't know what to make of this, because for awhile it seemed to be fine when it just sat. And yes I know I need to go get ISTA diagnostics, but that doesn't stop me from gathering data.
Why would it be intermittent? My leading theory now is that is has something to do with the fact that most of my trips are very short...1-3 miles at most. I live in a downtown area, a few blocks from many things I need, and within a few miles from everything else I need. The farthest I go from home in a typical month is making the arduous, epic 4.5 mile journey to Costco, and for that I'll often take my wife's SUV. I have no commute, so most days the car sits, and then I'll take it for a 2-4 mile round trip twice a week.
I know that's not great for cars but that's my lifestyle, and we've been that way for years, so I don't know why it would suddenly be a problem now. Also, if the problem was simply not driving the car enough to charge the battery, I thought that would give a different message than the one I'm getting (which is specifically about discharge while the car is off).
Going to try to find an indy mechanic for BMWs. I've never had to take it anywhere since the warranty expired, I've done all the work myself.