For those of you who own or care about cold weather battery performance in the Canon 5D Mark III, I further optimized on the recent cold snap in Fairbanks and did another battery test to see how well the LP-E6 performed in colder weather. At minus -31 degrees F, the camera and battery still held up reasonably well.
Details: The camera was at room temperature before going outside, so performance is probably a little better starting at this temp, compared to replacing a battery in a very cold camera to start with. I put a corded release in the camera and took continuous 30 second exposures, with long exposure noise reduction turned off.
The important thing the graph reveals is that the battery performance percentage is fairly consistent up to about 50% and then the batter just goes dead. So be advised that if you are monitoring the battery performance level in your LCD display, it is not a truly linear representation. The graph shows how after a gradual decline up to 2:20 minutes, it just tanked after that and died completely. The 2-2.5 hour mark seems to be about the performance level of this battery in temps around -20 to -30 degrees below zero (F).