Apple in iOS 14 added an Optimized Battery Charging feature for the AirPods, which is designed to preserve the battery of the AirPods to maximize longevity.
The new AirPods charging feature lets the AirPods learn a person's daily charging routine, and will wait to finish charging past 80 percent until they're needed.
So, for example, if the AirPods are charged at night while you sleep, the optimization feature might charge them up to 80 percent right away, but wait until an hour or so before you wake up to charge the remaining 20 percent.
Apple uses a similar battery health optimization system for iPhones and Macs to extend the total battery life of the devices. Avoiding topping up a lithium ion battery continually while it sits on the charger reduces the amount of time that a device spends at maximum capacity, and over a long period of time, this better preserves the health of your battery.
Optimized Battery Charging was added to the iPhone in iOS 13 and a similar Battery Health Management feature was added to macOS in macOS Catalina 10.15.5.
