This morning I ran into a battery issue with my Canon 5d mark III, and in the interest of being thorough, I wanted to do a full system flush on the camera. You know the flush I mean – where you pull the battery, card, and even the clock battery, then let the camera sit for 30 minutes to completely “flush” the system. Think of it as a thorough system reset.
Anyhow, I quickly discovered that the clock battery is not stored in the same location as it is on my older 5d mark II. A quick Google search turned up plenty of references to performing the process, but nothing that indicated just where the clock battery was located or how to remove it. Since I was on my own, I decided to go exploring and post images of the process.
You’ll need a very small Philips-head screwdriver; something you might use for repairing glasses. There’s only a single screw to remove, but be very careful not to lose it.
Edit 4/18/2015: The replacement button clock battery for the Canon 5d mark III is a CR1616.