Not to scare you, but Airpods are basically designed to kill the battery ASAP. Once you attach them to an Apple account, they drain constantly whether they're in their case or not. In the case they're supposed to be in sleep mode, but if you charge the case and AirPods to 100 percent and leave it over night, you'll notice the case dropped in percentage and that's because the AirPods are constantly draining. If you pull the AirPods from the case and leave them out they'll drain from 100 to nearly 0 over night but the case will remain at 100.
In other words, dont sweat the battery. Just know that within a year or two, you'll be getting about half the advertised battery life out of them.