As WatchFaceFormat can only display data provided by external applications named complication provider, I created an external application that will allow you to display your phone’s battery level on your watch.
First, you will need to install it on both your watch and your phone:
https://play.google.com/store/apps/details?id=fr.thema.wear.watch.complication.phone
Once installed, you need to go into the watchface settings on watch, on the complications screen. Select the dedicated area to open the available datas list, and select Thema: Phone battery (and not Battery, as this is the watch battery level).
The phone battery complication data is meant to be displayed on the phone battery dedicated area of my watchfaces. But you can display it on the other complications too, and on any watchfaces, not just mine 🙂
N.B.: The phone application is required to send the data to the watch application. Do not uninstall the phone application. If uninstalled, no data will be received by the watch app.
N.B.: On some phones, application are put by the system in hibernation when not periodically started manually. I would recommend to check the battery optimisation applied to the phone battery complication app, in the android settings of your phone. Deactivate the battery restrictions to allow the app to run in the background.