Stop iPhone From Charging

So for the first time ever I've plugged my iPhone into my MacBook Air today to use iTether. Despite my MacBook Air running on battery I notice that my iPhone believes it is getting charged. This is going to drain my MacBook Air's battery . I haven't plugged my iPhone in to be charged, I've plugged it in for data.

Is there anyway I can stop this? All I want the USB connection to be doing is transmitting data to/from my iPhone. I want to turn off the 500mA of power that the USB port has, make it unpowered if you see what I mean.

Any ideas?

maflynn said:

No, that's how most if not all cell phones work. Plug them into a USB port and they'll draw power to charge the battery.

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I know that's how USB works .

What I'd like to do is tell my MacBook Air "You're running on battery FFS. Don't waste your battery charging the iPhone. Disconnect the +5V pin of your USB port when running on battery."