What is Meant by Tough Solar (and Eco-Drive) in Casio (and Citizen) Watches?

For those who are unfamiliar with the term, I explain what "Tough Solar" means as a feature in Casio watches. The same concept is used in Citizen watches, where they call it "Eco-Drive". Basically, there is a solar cell built into the face of the watch and it charges the watch's power supply automatically when the watch is exposed to normal lighting conditions. I found a well-made video showing how someone changes a battery in a Casio G-Shock "Tough Solar" watch. This features the same model of watch I purchased in 2006 (and which is still running on its original battery!). So, while I haven't personally opened up any of my Tough Solar watches for battery maintenance, this video shows you how it's done on one of the older Tough Solar models.    • How to Change a CTL1616 Rechargeable Watch...   I caution anyone who attempts to change the battery on a Tough Solar watch to be sure that the replacement is the proper item. Even if you have a professional change the battery, be sure they have the right replacement item.