What SOC should I use during winter?

[mayo]

What state of charge would it be good to leave the car in during midwest cold nights around 15 to 32 degrees. Don't intend to charge to 100 percent unless needed the next day.

 

[Stoaty]

Same as for warmer temperatures--30-50% is probably optimal, but there isn't much evidence that higher SOC up to 80% or even higher makes much difference when the temperature is that cold. See link in my signature for everything we know about the batteries.