What is Battery life in days?

Battery life in days is the longest run time a brand states for its leading model, counted in days on a full charge. It tells you how often you will reach for a charger, which matters most if you sleep in the device or train without a cable nearby. Amazfit and COROS both list 21 days for their flagship, while ERA-FIT gives a range of 10 to 14 days and Fitbit states 7 days. Battery life is measured under set conditions, so treat it as guidance rather than a promise.