As your Apple Watch's battery level indicator is not behaving as expected, your watch may require service.
Before contacting Apple, it may help to follow these steps, which have previously resolved battery-related issues for other users, including after software updates, when usage has not otherwise changed (check after each):
If you have not yet done so, then update your iPhone and Apple Watch to the latest compatible software versions, starting with your iPhone:
Close certain apps and then restart (normally) both your iPhone and your Apple Watch:
If you have ever (not necessarily recently) previously used the Workout app and/or any third-party fitness, sleep or heart rate tracking apps on your watch, then check on your Apple Watch to see whether there is an active or paused session within any of them.
- If there is, then end that session from within the app on your watch (not your iPhone).
- If in doubt, remove any such third-party apps from your Apple Watch, at least temporarily.
Unpair and re-pair your Apple Watch:
Some users have previously experienced battery life issues in connection with using corporate profiles and/or Exchange Calendars. If you are using an Exchange Calendar, one or more of these steps may help:
- Archive your Exchange Calendar for all items before today.
- Remove / delete all old inbox messages relating to calendar invites (accept and decline notifications).
- Create a rule in your Outlook Mail account that moves all future accept and decline replies, immediately upon receipt, into a separate mail folder that does not sync with your iPhone and Apple Watch.
- Check your Exchange Calendar for errors.
Test by changing notification settings and/or by removing watch face complications and third-party apps:
- Turn off notifications for third-party apps and test performance after re-enabling them one at a time:
- Remove watch face complications and monitor performance as you add them back:
- Remove third-party apps from the Dock and/or remove them completely from your Apple Watch and then monitor performance after adding them back, one at a time.
If the issue persists, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:
Apple Watch - Battery - Apple
Batteries - Maximizing Performance - Apple