Arccos sensors use replaceable, CR-2032 batteries. These batteries typically last 30-40 rounds, depending on use and brand of battery, but they eventually will need to be replaced.
To access the battery, remove the rubber cover. The best way to do this is by peeling the green edge back with your fingertips while pushing the top of the cover with your thumbs. If possible, try not to push directly on the sensor button in the middle.
Use something like a golf tee to push the battery out of the sensor. When inserting a new battery, make sure the positive side faces the same way as the sensor’s screw.
Once finished, replace the rubber cover. Make sure the cover is completely on. The edge of the cover should overlap the base and fit snug and secure. (See the "Changing Batteries" video)