Juice Watch

Monitor your iPhone battery level from your wrist.
Juice Watch is a simple app designed to do two things well:
• Show the battery level of the paired iPhone in a complication on Apple Watch
• Deliver a notification on your iPhone when your Apple Watch is charging and gets above 90% (or whatever level you choose, from 50% to 100%)
Just glance at your wrist to monitor the charge of your iPhone throughout the day. Or, keep tabs on the battery level of your iPhone while it is plugged in and charging in another room.
You can see both your iPhone and Apple Watch battery levels on the same complication.
You can also get a notification on your Apple Watch when the paired iPhone is charging and reaches at least 80% (or whatever level you choose, from 50% to 100%).
You can set up low battery notifications, to get notifications when your Apple Watch or iPhone battery is low.
Juice Watch is available for a wide variety of complications, so you can use it on your favorite watch faces.
You can also use this app to check your Apple Watch battery level from your paired iPhone. This can be useful while your Apple Watch is charging somewhere else.
Keep in mind that the iPhone battery level shown on Apple Watch is not real-time. It is refreshed periodically, about every 5–60 minutes. The Apple Watch battery level shown on the paired iPhone refreshes at approximately the same rate.
As of iOS 17 and watchOS 10, Juice Watch reports battery levels rounded to the nearest 5% since that's now the granularity that Apple makes available to apps.