Oh, sorry, lost track of my subject. On the subject of cold weather and batteries, I was poking around on google scholar and found this article, which seems to be exactly on point:

http://www.aes.nu/publications/2002-01-1975.pdf

The conclusion is that internal core heating, using resistance heating, is by far the best way to warm a hybrid battery in cold weather. It's a 2002 article, so surely it's been implemented by now. Anyway, if you wanted to read about the scientific details, here is some background for you, although I suppose it may not be up-to-date, as it doesn't involve the most recent hybrids.