Charging varies according to whether you are on battery or AC, whether you are in Standby (which can run PowerShare while on battery) or Hibernate/Shut Down (which will run PowerShare only while plugged into AC), what battery thresholds you've set, etc. It can be complicated to remember it all. This Dell document describes it in detail. Also, on some models PowerShare works only if the external device was connected to the USB port and charging before the laptop was put into Standby, Hibernate or Shut Down.
Hi guys, I have a Latitude 3520 Laptop, and I need to use the PowerShare feature, nevertheless, it doesn't work when the laptop is off (Shutdown), and it only works when it is in sleep mode or when the power adapter is plugged (Whether it's off, in sleep mode, or hibernated).Is it like that how this feature is supposed to work, or something is off.  Note 1: I have this feature already turned on on the BIOS settings Note 2: I had had an older HP laptop that I used to charge my devices via a similar port even when it's completely off, it only needed the battery to be charged. 
Powershare on Latitude 3520