Fitbit One - Manual (Page 17)

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A battery indicator will appear when your tracker is first plugged into the charging
cable. Press your tracker’s button while it’s charging to view its current battery level.
A drained tracker should be charged for 1-2 hours for maximum performance and
battery life.
Memory
The One tracker stores minute-by-minute data for one week. After 7 days, that data
is converted to a daily total, which is stored for an additional 23 days.
When you sync your tracker, its data is uploaded to your Fitbit.com dashboard, and
securely stored on Fitbit’s servers. As long as you sync your tracker within thirty days
of activity, you’ll be able to transmit that data to your Fitbit.com dashboard.
Note: Every night at midnight, your tracker will reset itself. This means your goal
progress and daily data will begin at zero again. This does not delete the data
stored on your tracker. That data will be uploaded to your dashboard the next
time you sync your tracker. The time this reset occurs is based on the time
zone set on your Fitbit.com profile.