Cold batteries are dead batteries, so do all you can to keep them warm. Never leave them outdoors, in unheated buildings or in the car for extended periods of time if you can avoid it. If you can’t—when camping, for instance—try to keep those batteries close to your body to keep them warm. Interior pockets are an ideal place, much warmer than a camera bag. If the quantity of batteries means you can’t carry them all in your pockets, try rotating them to warm pockets when possible, and consider using chemical hand warmers to keep those pockets warm. Just don’t stick the batteries and the hand warmer in the same pocket at the same time or you run the risk of overheating or worse.
A wet camera is no good, for obvious reasons. And while you might not think working outdoors in the dry winter air is much of a risk, there are lots of ways this can go wrong. One obvious dampness risk is snow. If it’s falling while you’re shooting, you most certainly want to keep the camera protected with a waterproof camera cover.
Op/Tech USA makes rain sleeves ideal for this, as well as emergency rain covers from ThinkTank Photo, Peak Design’s Shell Camera Covers or the Ruggard DSLR Parka. Even if there’s no risk of snowfall, snow on the ground presents a moisture risk too, as does overnight frost. The safest approach is to ensure the camera bag and its contents are secured in a waterproof dry bag, particularly overnight, but in a pinch, a trash bag or individual plastic zipper bags are a good solution for ensuring moisture gets nowhere near expensive equipment.
Waterproof protection is also important when it comes time to return your equipment to the warmth—and humidity—of indoors. If you carry a cold camera around your neck, mere moments after walking in the door you’ll discover a cloudy, fogged-up lens. Not only is this moisture dangerous, but it also makes your kit useless until it warms up—if it escapes permanent damage.
The best course of action, short of preventing the camera and lenses from getting ice cold in the first place, is to let them come up to temperature slowly in a sealed environment. An airtight hard case, such as a Pelican, can certainly accomplish this, but an all-around easier and more affordable option is to simply use a sealed plastic bag to prevent condensation from forming on the camera and lenses themselves.
To do it right, put the items in the sealed bags before coming indoors, and ideally, move them in stages from the cold to a slightly-less-cold area—a garage, for instance, or an automobile—before eventually coming inside. Should these techniques fail you, in order to dry out equipment, carry absorbent silica packets in your camera bag to remove any moisture that’s trying to infiltrate your expensive equipment.