Image size affects battery life

I did a quick search to see if anyone had spoken of this before, but didn't see anything. Maybe it's a well-known fact, and probably the time-lapsers already know it, but here's the story. I set up my 7D with a fresh battery to do some time-lapse shooting. As it would only be used for making a video output I set the camera to shoot small jpegs (still higher resolution than HDTV), and used the timer-controller to take a single exposure every 5 seconds. When I came to stop the camera 3 hours later, it had taken 2,294 exposures, and still had plenty of battery left according to the display.

I would have thought that file size wouldn't make much impact when compared with the draw on the battery of firing the shutter and processing the image, but the 7D normally reckons to manage 1000-1100 shots when shooting RAW. I guess that compressing down to jpeg doesn't use much juice, but writing to the card must be quite an effort, which is then much reduced when the file is so much smaller (each image was about 1.2 MB.

Although I've been shooting Canon digital for the past 7 years I've never come across this fact. Has anyone else?

I would think that file size has nothing to do with battery life as all the processing needed to make either a large or small image is the same. There may be a slight savings due to the fewer memory writes but that has to be counted in a handful of electrons.

What I think you are seeing is that you are saving a lot of electrons because you are not focusing between each shot. Moving the lens takes a lot of power and as you are time lapsing, I'm guessing you focused and left it be. Also, you are not re-metering between each shot either. That has to save some power. And lastly, though you don't mention it, you probably had the rear LCD turned off (just guessing) as there is no need to review each shot. The LCD consumes loads of power..

Good point about AF. For the first few shots I forgot and left it on, but saw it was adjusting so switched it off. I also realised too late that I should also have switched off the instant preview, so that was still left on, but it's only set to 2 seconds. The mode was left to Av, so it was actually metering each shot. I had assumed the difference was due to memory writes, but maybe it's lens focusing. The lens didn't have IS either.