VIEWSONIC VS12835 - Manual (Page 43)

Enjoy Life, Enjoy ViewSonic
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XI. Use of USB and battery charging
There is no need to slide the power switch to the ON position when
connecting a USB cable to the player for file transfer.
The VPD400/VPD500 battery can be charged with an AC adapter or
connect to the USB on the computer. The power switch must be set to the
ON position when charging the unit. However, the player does not have
to be powered on.
Five hours are required for a full charge. It is recommended that you
charge the player eight hours before the initial use for best battery
performance.
ViewSonic VPD400/VPD500