The time required for solar cells to be full should consider many factors, such as the amount of the original power of the watch, light intensity, light time, and conversion efficiency, etc. So it's not clear how long Casio's solar charge will be full.
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Light energy watch is also called solar watch, as long as the surface is exposed to light, it can walk around, and it can persist for a period of time in a place without light. The use of light energy can avoid the trouble of regularly replacing the battery, is more environmentally friendly, and the average life span is more than 10 years, so it is the first choice for young people.