The Ray has a built-in solar cell that harvests energy from both indoor and outdoor light.
What kind of light works best?
Ray charges fastest in direct outdoor light, but shaded outdoor light and bright light near a window also work. If you want to top it up intentionally, leave Ray on a windowsill or anywhere the solar face points outside. In good daylight, that can charge it fully over the course of a day.
Indoor light works too, but it is much weaker than outdoor light and takes longer.
What if I don't get any light at all?
Ray can last up to about 2 months on battery when sampling about every 5 minutes. Battery life is shorter if the app or firmware samples more often, if step counting is enabled, or if Ray is spending extra energy reconnecting over Bluetooth.
Do I ever need to plug it in?
No. There's no charging port on the Ray. It's completely self-sustaining.