Power Monitor - with this app, you can easily check your iPhone power; If it’s not convenient to open your iPhone, or your iPhone is in another room, you can check it's power through you Apple Watch! It can notify you if your iPhone’s power is pretty low or high.

Power Monitor will check your iPhone power every half hour, and will send you a notification when your iPhone power is within the following range:
- Less than 10%
- Less than 20%
- Less than 30%
- Less than 50%
- Higher than 50%
- Higher than 80%
- Higher than 90%
- Fully charged
And you can select in which situations the notification will be send.
(NOTE: For notifications to work, you must have Power Monitor running in the background, and have both Background App Refresh with Power Monitor enabled: Settings > General > Background App Refresh.