Dell Venue 8 (7840, Early 2015) - Manual (Page 10)

10 | Setting Up Your Tablet
NOTE: A fully discharged battery takes up to 4 hours to fully
charge.
NOTE: You can also trickle charge the tablet via a computer’s
USB port using the micro‑USB cable. When connected to
a computer, the battery charges slowly as the charge time
depends on the power output of the USB port. The tablet may
not charge if it is connected to a computer that is turned off or in
sleep mode, to a USB hub, or to the USB port of a keyboard.