Batteries are though... NiMH's loose their juice pretty bad when it's below 32 out. NiCad's not so much, but they still drop about 25% vs 40%...
That's what I've read anyway... I've experianced it somewhat, but that's with receiver batteries. I have 4 NiMH packs made and 2 NiCad packs made. I just keep a close eye on them and swap them out if they get weak before I get cold.