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Official website of Apple explained in detail that "battery life 18 hours" is based on such usage frequency: checking the time version 90 times, receiving notifications 90 times, using the application for 45 minutes, exercising for 30 minutes, and listening to music using Bluetooth. Usually this frequency can guarantee at least 18 hours of battery life.
It can also be seen from the figures of "using the application for 45 minutes" and "exercising for 30 minutes" that Apple roughly simulated its usage and used it as a test benchmark for battery life. In actual use, the battery life will be affected by usage and other practical factors.