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I have an HP Envy M6 with a USB 3.0 port next to its charging port, doesn't matter what state the Laptop is in, Sleep, Hibernated, and completely powered down. It never stops charging whatever phone is connected to it via that particular USB port.

I can't seem to get around the physics behind what really happens, but it does look like that port holds some amount of electricity to charge the phone no matter what state the laptop's in and this only works for the port next to the charging port.

What's the phenomenon behind what happens?

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    Phenomenon and advanced principle of physics: The designer was drunk and/or stupid and either figured out how to save a few cents on every unit or overlooked a flaw that allows the port to be powered from the battery when the power is off. If or when it's discovered, it will be labelled a design feature that enables you to charge your phone when the power's off. – fixer1234 Oct 14 '15 at 8:18
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Modern computers often use electricity, even if the motherboard isn't supplying power to the CPU, internal fans, hard drives, etc. Perhaps one of the original motivating technologies for this was WOL (Wake-on-LAN for network cards). On some systems, pressing a certain keyboard combination would tell the BIOS to "wake up". The computer needs to use a little bit of electricity to do this.

In short, if you ever open up a computer, be careful: don't touch things, counting on the idea of there being no electricity flow just because you don't see lots of lights and hear lots of fans. Make sure power all power sources are removed.

Taking that into consideration, your observations are not all that surprising.

  • That's their story and they're sticking to it. :-) – fixer1234 Oct 14 '15 at 8:21
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This is a common feature of laptops that makes it possible to charge your laptop and your phone at the same time, even if you don't have a phone charger, even if you only have one outlet.

Because some people don't like it, it can usually be disabled. Usually the setting is in either the BIOS or the manufacturer's hardware control program.

The power to the USB port comes from the same place power to things like the power button comes from, the phone's standby power. This ultimately comes from the battery if the laptop is unplugged or from the wall if the laptop is plugged in.

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    Usually USB charge ports only work if the computer is plugged in or in standby; hibernate or off + no external power usually turns them off. – CBHacking Oct 14 '15 at 8:29

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