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Because I have a tablet which drains faster, than it charges on the USB 3.0 port, I looked for some USB 3.0 hubs. Indeed, an external power supply helps here, but the charging ports are always separated from the data transfer ports. In my case I need both (data transfer and charging) and 900 mA are too few.

Now I found that there is USB Battery Charging standard for this. Do you need a BC 1.2 compliant device for this (in this case the tablet)? So if the device doesn't support it, I'm stuck with 900 mA? Or are there solutions, which would help me here? I don't want to ask for a device, rather I'd like to know if this is possible in general and under which circumstances this would be possible.

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USB-A

There are two different modes for USB BC 1.2 & BC 1.1 that supply over 900mA.

  • Dedicated charging port (DCP) - 5v@1.5A, without data. D- & D+ used to indicate the amount of power. This is why data can't be used with DCP.
  • Charging downstream port (CDP) - 5v@1.5A, D- & D+ used to handshake before the device starts taking 1.5A. After the handshake is complete data can be used over D- + D+.

References

I have only seen a few USB hub that note they support "Charging downstream port" (CDP). If hubs state they support BC 1.2 this mostly means they just support the DCP (no data) mode.

USB-C

USB-C supports at least 5v@1.5A = 7.5Watts but in most cases you will get 5v@3A = 15Watts from a port on a desktop, notebook, or hub. USB-C can go up to 20v@5A = 100Watts via the USB-PD standard.

USB-PD Reference

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Actually this is quite possible, if you are comfortable with a soldering iron (or can find a cable as decribed below somwhere) - the point is, that the power and the data lines in USB 3.0 can come from seperate sources without one of the bus devices know about it.

Take a look at the wikipedia page for USB 3.0 for a table - power is provided to the bus via the VBus line. You can now create a cable, where the VBus coming from the host is terminated to ground (GND/Drain) via a 1KOhm resistor and the VBus line going to the device (the tablet) is fed from a charger capable of providing more than 900mA.

I have made and used such cables quite often to cater to host/device combinations where the current needed by the device was higher than the current provided by the host (i.e.: Slim notebooks and high-capacity spinning harddrives - the disks would fall off the bus under heaviy load, and a multimeter showed this being due to power starvation)

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  • Do you have a name how this type of cable is called? I searched for Y/splitter cable, but didn't found the right one.
    – testing
    Sep 11, 2017 at 17:15
  • You might remeber similar cables from USB 2.0 times with two plugs for the host and one for the device - ment to accumulate the power of two USB ports for one device - they were called Y-Cables. I have not yet found the USB 3.0 version readymade - else I wouldn't touch the soldering iron myself. Sep 11, 2017 at 22:46

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