The short answer is to ditch the batteries. They're not designed for cold weather.

The longer answer is a three-step process:

First, and most important, check with your camera's manufacturer to make sure the body will continue functioning in the cold if it has a good source of power. You may have to write and ask this specifically, because the published specs will likely assume there's a battery involved.

Second, identify a suitable power adapter for your camera. Canon and Nikon have AC (mains current) and DC (12-volt) adapters for most of their DSLR models. More point-and-shoot cameras are able to be charged or powered via USB, and adapters and cables for that are cheap and plentiful. All of the above make their own heat.

Finally, find a power source. I recommend going with a 12-volt system and adapter because it gives you a ton of options, most of which aren't very expensive:

• Mains Power. If there's electricity available nearby, an extension cord and a 12-volt power supply will keep you in power indefinitely.

• Battery Clamps. A cable with battery clamps on one end and a cigarette lighter socket on the other will allow connecting to any 12-volt supply you can reach.

• Car Jump Start Box. Many of these have a lighter socket that will power your adapter. Better models will have batteries designed to live in the trunk of your car and work in all kinds of weather.

• Deep-Cycle Marine Battery. If you don't get a hernia carrying it, a full-sized battery will power your camera for a few days and won't even blink at doing it in very cold weather. Smaller deep-cycle batteries for motorcycles, snowmobiles and ATVs will also do the job and won't break the bank. If you go this route, get an Absorbed Glass Mat (AGM) battery, as they don't contain liquid electrolyte that can spill if tipped over. Make sure you understand what you're doing and use care working with these batteries. They're capable of delivering a lot of current, and shorts in unprotected circuits can cause fires. One other tip: Igloo makes versions of its flip-lid Playmate cooler that make excellent carrying cases for batteries of all sizes. The insulation is just as good at keeping the contents warm as it is keeping them cold, which will buy you some extra run time. You may have to cut a notch to get the wiring to the outside, and some foam should be added to keep the battery from moving around.