SoftBank 815T User Manual page 168

Note
To charge using the USB cable provided, turn on the PC and the
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handset.
Charging may not be possible depending on the connection settings of
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the PC.
If the handset and PC are connected with a USB cable, either the PC
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battery or handset battery is consumed depending on the battery
setting, regardless of whether they are communicating data.
ON : PC battery is used
OFF : Handset battery is used
Tip
Compared to using the AC charger or In-Car charger (optional), it may
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take longer to charge the battery through a USB cable.
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