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Normal computer USB max current is much lower than needs of modern smartphone to charge while running, commonly needing ~2 amp chargers.

I want to be able to have my phone connected to my computer via usb for data purposes, but it maintain it charge.

Is it possible to do something like this to a usb cable, so instead of the 5v coming from computer, it comes from phone charger instead ?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ How about just buying a USB hub that supports charging? \$\endgroup\$ – Nick Johnson Jul 17 '15 at 12:12
  • \$\begingroup\$ Didnt know they exist ? I had looked into prospect of a powered usb hub, but came the conclusion that even then you where limited to about 500-700ma whatever the usb spec is... \$\endgroup\$ – Hayden Thring Jul 17 '15 at 12:14
  • \$\begingroup\$ You could do that, but it likely wouldn't help you. Your phone will still detect that its plugged into a PC (since that's where the data lines are still connected) and it will still decide to only 'slow-charge', even though your 5V supply is capable of much more. \$\endgroup\$ – brhans Jul 17 '15 at 12:42
  • \$\begingroup\$ A quick search for "charger hub" turns up lots of results on Amazon. Besides which, most computers and laptops offer "charging device ports" now too. \$\endgroup\$ – Nick Johnson Jul 17 '15 at 12:44
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While it is possible to do something like this, there are 2 immediate problems I see:

1) Your device no longer has a common ground with the computer. This will likely result in your computer not 'seeing' the device, or complaining that it cannot communicate with the device.

2) Assuming the above was not an issue, certain phones (Apple products for sure), have a unique method to detect if the charger they are plugged into supports high(er) current charging. As you mentioned, the USB standard officially supports up to 500mA charging. The way Apple handles this is by adding a resistor divider on the D+ and D- lines (in their charger accessories) to bias the DC voltages to different values. The phone is able to detect the voltage ratios and then determine the type of charger it is plugged into, activating the supported charging mode.

Here is an image of the resistor divider that was used in a nifty product:

https://learn.adafruit.com/system/assets/assets/000/005/971/thumb160/adafruit_products_usb4res.png?1396830863

To find out more about, check out the project on ADAFruit: https://learn.adafruit.com/minty-boost

So in summary, it's not as simple as your schematic. As others have suggested, your best bet would be to look into USB hubs that support high-current charging.

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    \$\begingroup\$ I believe that the resistor network is only detected by the phone if data is not being used. If connected to a USB host (computer), the phone negotiates power with the host and so will not try to draw the extra current, even though the power is available. \$\endgroup\$ – DoxyLover Jul 17 '15 at 17:42
  • \$\begingroup\$ Hmm, so in the case where you connect an iPhone to a Macbook (which supports 1A charging on its USB ports), are you saying that the negotiation is made directly over USB? Macbooks will charge iPhones at 1A - but I wasn't aware that the resistor network isn't used. \$\endgroup\$ – KnightsValour Jul 17 '15 at 18:43
  • \$\begingroup\$ The resistor network is a "hack" designed for dumb chargers so they don't need to implement USB host to do power negotiation. \$\endgroup\$ – DoxyLover Jul 17 '15 at 18:57
  • \$\begingroup\$ What about the OEM charger bricks? I believe they use resistors as part of the original design. \$\endgroup\$ – KnightsValour Jul 17 '15 at 19:36
  • \$\begingroup\$ What I ment is that it's a "hack" by the phone manufacturers. Without it, each charger brick would need to have USB host hardware and firmware to do power negotiation. \$\endgroup\$ – DoxyLover Jul 17 '15 at 20:41
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You can do this provided you

  • keep a common ground between both ends of the cable and the power injector.
  • disconnect the 5V line from the computer and route it to the power injector.
  • don't damage the data lines while doing the splicing.

The device may not draw a higher current as discussed in the other answer, but this does allow you to power it (and turn it on and off) seperately.

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  • \$\begingroup\$ thanks, i tried it and it behaves as suggested, not engaging fast charging \$\endgroup\$ – Hayden Thring Jul 22 '15 at 12:43

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