Last week I was out in -10°C or so for 1-2 hours, kept the camera under my jacket when I didn't use it and took it out for shots when I saw something I wanted to capture. It was snowing little flakes that for the most part I could just blow away with my breath, but some liquefied. The worst was my breath fogging up the viewfinder, or rather the O-ME53 magnifying eyecup I have installed, so I used Liveview for some shots where I couldn't see anything otherwise. It was just a short outing, but the battery was not bothered by the cold. I did take extra batteries that I kept in my trouser pocket, but didn't need them. I blew away some snow before zooming my 18-135 back in. I had the lens hood installed, I don't know if it's thanks to that that the front lens looked pristine all the time, or if the coating did its job as well.
I did make the mistake of bringing the camera back in with me afterwards, which I only realised when I wanted to take a selfie with the snow in my face after passing a mirror and the front lens was all fogged up... After wiping away some moisture from lens and camera I put the camera down near a window with the lens still all zoomed out from outside and let a fan blow on it so that the outer condensation would at least be reduced or swept away a little (I also do that with newly acquired lenses I took out of the box too fast if they're still cold from the delivery and I see them fog up
). Took off the O-ME53 and cleaned it for the first time, it was a little disgusting how much gunk had settled behind the rubber cup... did not take long until the camera had acclimatised, which is when I opened the battery flap, took out the battery and let the fan blow onto the camera for another while (that was most certainly unnecessary as there was never really any wetness on the bottom plate of the camera).
At least camera and lens still worked like a charm the next day, but I made a big mental note to try to prevent that by thinking ahead about how I want to bring the camera back in when taking it out with me (I hadn't even taken a bag with me when I started that walk, so that wasn't an option when coming back in.... )