If you are using the Apple Watch Series 4 or Apple Watch Series 5, watchOS 9 will be adding a new battery recalibration feature.
According to the watchOS 9 release notes, the recalibration feature will offer a more accurate estimate of the maximum battery capacity. This will give you a better estimate on how long your smartwatch will last. The recalibration process will take place during the watchOS 9 update and will be completed upon installation.
Apple introduced the battery recalibration feature for the iPhone 11 with iOS 14.5.