The Casio CTL 1616 battery is manufactured for Casio by Panasonic for use in the Solar G-Shock line of watches. These watches use a speciality battery, as they continually recharge the battery using the face of the watch as a small solar panel. Removing the back of the watch and replacing the battery is a tricky process, and must be done carefully.
Remove the watch band from the watch case. Push a small screwdriver into the hole on the edge of the link closest to the case to depress the spring bar. Pull the band gently away while depressing the bar to remove the band.
Remove the four screws holding the back of the G-Shock in place. Slide the back off slowly, being careful not to disturb the small springs underneath it.
Use the plastic tweezers to remove the springs, noting their locations. They will need to be returned before closing the watch. Remove the rubber cover after removing the springs. Some models will have another white plastic cover under this---remove it as well.
Use the plastic tweezers to detach any latch that is holding the battery in place. Latch types vary, depending on the model. Remove the battery from the watch.
Insert the new battery in place with the plastic tweezers, being careful not to touch the battery to other metal components inside the watch. Close the latch to secure the battery.
Use the metal tweezers to link the hole indicated (AC) inside the watch to the point indicated (-). This will restart the watch functions.
Install the removed springs, then insert the back plates and close the watch. Screw each side of the back plate down evenly.
Your local jeweller can replace your battery for a small fee, and avoid any potential accidents while working with the watch. Your watch manual is helpful when working with your watch.
Be careful when using metal tools inside your watch; causing a connection that can ruin the watch is easy.