These watches have their power replenished continuously by exposure to light, so they never need new batteries. This saves the wearer the trouble of bringing the watch into the repair shop and eliminates the risk of environmental pollution from discarded batteries.
The light enters the watch through the watch’s crystal and dial. It then hits a solar cell underneath the dial. A light-powered watch will run indefinitely in normal lighting conditions, i.e. sunlight or indoor lighting. The energy it stores up during the day will keep it running through the night.
When fully charged, the watch will run from 40 days to 6 months.