ground - Y USB Cable with external 5V supply

I just got a brand new Nexus 4 phone and am a bit dissappointed that it doesn't support USB OTG. One reason is that is doesn't supply 5 V out of its USB port, as opposed to other Android devices I tested.*

Since I have an extra USB OTG cable lying around, I am thinking about modding it to supply the 5V from somewhere else (e.g. from a USB power supply adapter). How would I do that?

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  I think I should connect Vcc from the power cable to Vcc from the child, and also connect all the grounds together (so that host (the Nexus 4) and child have the same 0V potential, relative to which D+ and D- are measured). (Is that sufficient to pull host and power supply to the same potentials? What potential do I end up with anyways? I find that a bit confusing.)

  
  



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  Do I connect the host Vcc with the power supply+child Vcc, or do I leave it dangling? It seems there is no power on it anyway, so I can regard it as not connected (in USB mode).

  
  


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  Is there anything special I have to do with the Sense, D+, or D- pins, or can I leave them connected as they are?

  
  

Sorry if this is a stupid question!

Here is a schematic of a regular USB (OTG) cables for reference: ^{From http://tech2.in.com/how-to/accessories/how-to-make-your-own-usb-otg-cable-for-an-android-smartphone/319982}

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*) There might be other problems related to its Slimport feature, though.