Ipod power supply

I don't know if I'm writing in the right topic, because this question is not connected to arduino.
I would like to make a modify to the cable of the ipod to avoid the charge of the battery and to permit the data receive and transmit. When the ipod is connect to the usb of pc or stereo, I don’t want the battery charge but I want the possibility to sync with iTunes or control the ipod with a compatible product.
I have try to isolate the +5V or the GND of the usb but it doesn’t work. When the +5V are isolated, the ipod say that it is connected with iTunes, but the computer doesn’t recognize it.
What can I do? Is it possible what I ask to do?

Did you try to just not connect power but do connect ground and other 2? If you tried that and it still does not work then I would venture to say it is not possible.

I have tried what you have said.
I have seen some guide that explain different mode of charging of the battery in refer to the voltage applied at D+ D- of the usb.
But I know that some dock station plays music but doesn't charge the Ipod (I think that this behavior is not wanted by the productor). How that is possibile?

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