Power generated by the solar panel powers watch operations and also is used to charge a rechargeable (secondary) battery. The solar panel is integrated into the LCD of the watch, and power for charging is generated whenever the face is exposed to light.
Charging the Watch
When you are not wearing the watch, put it in a location where it is exposed to bright light.
While you are wearing the watch, make sure that its LCD (solar panel) is not blocked from the light by the sleeve of your clothing. Power generation efficiency is reduced even when the LCD is blocked only partially.