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...or, how not to charge my phone when I'm using it as a GSM modem in USB tether mode?

I can use it as WiFi tether modem, but it wastes battery both of the netbook it supplies connectivity to, and on the phone side. In USB tether mode I would avoid the power-hungry WiFi connection, but instead the phone wrecks the netbook battery by charging from it. I'd like to be able to maintain the data connection while disabling or restricting the current fed over the power lines; let the phone and the netbook use their own batteries without draining each other.

I'm using Kubuntu 14.10 but any system solution is okay, I'm sure I'd be able to adapt it. Even hardware solutions would be okay - cut the power line in the USB cable or solder a large resistor in maybe? Would that work?

  • i think this looks like a electronics.stackexchange question . – 把友情留在无盐 Jan 31 '15 at 14:27
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reading through multiple posts of this conversation https://www.spinics.net/lists/linux-usb/msg61106.html , it seems the conclusion is NO , we can't . the question in that conversation is exactly the same thing we want here .

i come to this understanding . in usb the amount of current is determined by the device , not by the host . if we want to limit a phone's usage to 100 mA , we have to do it by telling the phone . the host can only answer a yes-or-no here , and if no is the answer , the device is to be shut down

The device can, optionally, advertise multiple configurations which use differing amounts of power. However, very very very few devices actually do this. I do recall hearing about a Blackberry which did this 3-4 years ago... but I think that was the only one. Most devices advertise only one configuration which draws the maximum power the device will ever draw.

  • 1
    That's why/how I hoped this could be done: I read there's a 'power negotiation' phase between the device and the host, and the host can decline high power requests. But I never learned what happens when it does. I hoped the phone would advertize at least two configs: 'charging' and 'communication'... I still wonder what happens when the exchange goes like: Device: "I demand 500mA". Host: "Okay, here's your 5mA" (e.g. host agrees to exorbitant request, then fails to deliver.) – SF. Jan 31 '15 at 14:54

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