This winter I let my o8 e61 sit outside for more than a week under a blanket of snow.

On first start it gave me a low voltage warning but that went away since I drove it for a while.
After the clock would lose time and I would have to reset even after a few days.

Now that is warm I don't seem to have any issues.

My question is :

1) Someone told me that the clock is powered by a capacitor and not the battery in sleep mode is that true?

2) Is this a sign that I need to replace the battery even though I only got the low voltage message once and that was with the car siting for more than a week in the extreme cold?
I would say that the car started was proof that the battery is still strong?

 

I agree, mine was doing this too. If I let it sit for a week or so or if in my garaged and not locked and if I were in and out of car but not driving it, I would get the message. It's signs the battery is weaker. As long as I ran car every day or so all was ok. I did change the battery and no longer have this going on.