Nokia 9 - Manual (Page 10)

You do not need to charge the battery for a specific length of time, and you can use
the phone while it is charging.
When the battery is charging, the charging indicator flashes slowly. When the battery
is full, the charging indicator is continuously on.
If the battery is completely discharged, it may take several minutes before the
charging indicator is displayed or before any calls can be made.
Charge the battery over USB
Battery low, but you don't have a charger with you? You can use a compatible USB
cable to connect your phone to a compatible device, such as a computer.
You can use USB charging when a wall outlet is not available. Data can be transferred
while charging the device. The efficiency of USB charging power varies significantly,
and it may take a long time for charging to start and the device to start functioning.
You can use your phone while it's charging.
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