Depends on what type of battery you're talking about. If you mean Lipo, then yes, 45F is a bit too cool to be charging them to full capacity. Some of the better lipo chargers will reduce the max charging voltage in colder weather to prevent damaging the battery.

It's best to keep Lipo batts above about 65F minimum (and about 140F max), especially when fully charged. They don't like the cold at all...

In cold weather, I charge my lipos indoors where it's warm, and don't take them outside until I'm ready to fly. You can also keep them in a pocket close to your body to keep them warm until ready to fly.