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All-day battery life is based on 18 hours with the following use: 90 time checks, 90 notifications, 45 minutes of app use, and a 30-minute workout with music playback from Apple Watch via Bluetooth, over the course of 18 hours.
Apple says that it takes around an hour and a half to charge the battery from zero to 80% and about 2.5 hours to get to full. The company also noted that battery claims are based off tests performed on the 38mm Apple Watch and that the 42mm model typically experiences longer battery life..