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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.
There was a time when I assumed everyone owned only one watch. But when the battery needed replacing it took me so long to get around to a replacement that i started buying novelty watches on sale, usually for $50 or less. This works well, because when the batteries go dead, I switch to a different watch. When you spend $50 or less on a watch, it hurts a little to pay $8 to $10 for the battery/labor. My editor goes to Kohl's, where the replacement is $5 as long as you purchase the watch from Kohl's.