Body Battery data field. Simple as that.
You can add it as a Connect IQ DF to your activity layout:
1. From the watch screen enter the activity: Start - choose your activity
2. Up (Options) - {activity} settings - Data Screens - choose screen - Start
3. You can change the Layout and add a new field or you can change an existing Data Field.
4. In Data Fields - Start - ConnectIQ Fields - Body Battery
5. Exit the {activity} settings.
You can start now your activity.
If you like this DF, send me a beer: https://paypal.me/GavrielF