When shooting snow, it's best to set exposure compensation to something between +1 and +2. - Gabe

Thanks for the tip Gabe. Clare was shooting that day, and I think she was just using fully auto. - Robert Felty

What makes you think cold drained the battery? Usually keeping a battery cold keeps it charged, but using it while cold drains it faster. - Gabe

While hiking in yosemite five years ago, my camera battery was dead after the first night. Remarkably, it worked again later that day as it warmed up. That was a different camera, but I thought that was universal. In this case, the battery was at 0 for the rest of the trip, including when we got home, and it was fully charged when we left. I am normally very impressed by how long the canon batteries last. - Robert Felty

Camera manuals often include battery life at different temperatures (say, 500 frames at 25C and 400 frames at 0C) because batteries don't work as well when cold because the chemical reactions that generate power slow down. If I'm shooting outside in the winter I keep a spare battery or two in my pockets to keep them warm, and exchange them regularly. Electric cars often have heaters for the batteries for cold starts. - Gabe