I have a problem in temperatures below 0 C with batteries also, so I have bought a variety of back up power including usb power, dummy batteries to ac and cig lighter (to be powered by car battery), even battery holders that take AA batteries.
I good trick is to keep batteries in a pocket close to your body to keep them warm, and insert them just before you use the cameras. Still, in very low temperatures batteries can cool down quickly. And LCD displays might become so sluggish that they're useless. Also avoid taking cameras indoors and then outdoors again, since frost and dew easily form on and in the camera when you take in indoors. Once indoors the camera should be left unused until it was fully warmed up. These things are usually not a big issues down to -10C or so, though sometimes batteries are so bad that even around 0C there could be problems. But at -20C and colder these things definitely become problematic. And cameras become problematic to operate because you need to take off the mittens and it's easy to get frostbitten skin.
Good points, Mid! We see these recommendations made for telescopes and eyepieces too. I wonder if dew and frost can damage optics or electronics or do they return to normal if left unused for awhile until they warm up like you said?
Also, I am considering putting a lot of my excess equipment that I don't use very frequently in my garage, which has no heating. Would that damage any of my extra cameras, lenses, eyepieces, batteries, binoculars or telescopes do you think? That's at my other house where it can get colder than it does here (it's in the Poconos mountains where it can get down to -30 C infrequently.)