Just changed the battery In my Holeshot because I noticed it was losing time. It was pretty easy and I learned the movement is ETA 4-jewel. Battery is Renata 394, 1.55 volt. I used this tool to remove the back with ease. Here's the inside There is a plastic spacer that I already popped out to access the battery. The O-Ring is also removed as it kept sliding around while I was working with it. Just be sure to line the O-Ring back up when screwing the back case back on.