What should I be careful about when charging the solar-cell?

You can charge the cell without any difficulty in places where you usually use the watch/clock (where your watch/clock receives light from the sun or a fluorescent lamp).
However, avoid charging or leaving your watch/clock in high-temperature environments as shown below. Heat generated in such environments may cause damages to your watch/clock, and overheated watch/clock can cause you burn injuries.

• Places where temperature becomes high with direct sunlight received for a long time
• Places extremely close to a light and heat source such as an incandescent lamp and halogen lamp
• Dashboard of a car parked under a hot sun