If you are using the newer NiMH slow discharge batteries like the Enloops they will last a long time without freezing them. They also survive freezing just fine (at least down into the -15C range). If they are the original NiMH batteries (no claims of year long storage) nothing I have seen will keep them from being self discharged in a month or two.

I have switched over to Enloops entirely. I use them in trail cameras in the Winter and they last fairly well, and definitely survive and function in cold weather.