AirPods wireless Charging case

Hello, well I get as a present a new AirPods with wireless charging case, the thing is that the charging case is 100% charged same as my AirPods, but when i leave them into the case, even when they are fully charged the battery of the case decrease slowly, it's seems like the AirPods always are charging inside the case using the charge of the case, and when I check them in my iPhone, the AirPods shows the light when they are getting charge, even when they are fully charged, that is normal? Thanks!