After startup, after the car has been sitting for a few days,,the red battery light stays on for about 15 seconds. Then it goes out. Repeated startups do not cause this, until car has been sitting for a few days. Cold weather here may have something to do with it. The alternator seems to charge normally. When the red light goes off, there is no sudden increase in the brightness of lights.

Do you think this is an alternator problem or a battery problem? Or something else?
Or am I worrying too much?
Thanks for your help.

 

Battery is most likely beginning to fail. Does The Owner Manual describe this?

Do a load test on both the left and right batteries to determine the culprit.

 

I leave my car at the airport for days at a time. During a particularly low temp cold spell about a month ago here in Chicago, same thing. Car in an exposed parking garage for for days, went to start, battery light stayed on, I freaked the hell out, then it went right out.

I would guess the number of cranks lowered your battery voltage by a tiny amount, tripping the red battery light, but then it charged right back up. While it could mean the end of your battery, it' store likely just a normal reaction to an extended cranking time. Mine never came on again since then.

But...I will tell you that at the first sign of battery end-of-life, I'm changing both out! Too many horror stories of electronic weirdness and fault codes being thrown just because of unstable voltage.

In this case, you might be fine though.