The battery life indicator you see in the exercise start display or the battery mode settings estimates how long your watch's battery will last during the exercise. The value is always rounded down, which means that, e.g., 20.9 or 20.1 hours of calculated battery life will be rounded down to 20 hours.
The following settings and conditions may impact the actual battery life:
GPS conditions: A low number of satellites and surroundings with obstructions like trees, buildings, mountains, etc., impact the GPS signal strength. Thus, the GPS will consume more power to track your exercise.
Backlight is turned on: To save battery life and keep it according to the estimations, keep the Backlight toggle normal (toggle is grey) in the Options menu below the exercise start display.
Browsing through data displays during the exercise: When switching between displays, the display backlight consumes the battery. To save battery life, keep display browsing to a minimum.
Using the navigation display during exercise: When using this display, the GPS accuracy is switched to Best to provide the best possible track for navigating. This means that the GPS setting of the battery mode will be overridden as long as you are in the navigation display. The GPS accuracy will be switched back to the GPS accuracy defined in the battery mode when exiting this display.