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Ipad doesn't charge while in use and connected to the usb port. It definitely charges when connected to the supplied 10 watt power adapter.

The USB port, as far as I know, supplies 5 watts. Can we modify it by simply adding an extra usb adapter to the cable of ipad to draw the extra 5 watt? something which the older external hard-drives used to do?

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You may find this thread over at adafruit of interest. Some people have stripped down various chargers for the iPad (including the Apple one) and published the schematics for everyone to analyse.

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USB supplies per specification only 5V * 0.5A = 2.5 Watt maximum. Even my mobile phone (Sony, not Apple though) does not charge to 100% on an USB port.

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    \$\begingroup\$ Apple violates the spec for their own products, though. The MacBook Air supplies 1.5 A to a SuperDrive, for instance, probably after some proprietary negotiation. Also, USB 3.0 ports can supply 900 mA instead of 500. \$\endgroup\$ – endolith Oct 7 '10 at 13:49
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No. The ability to use the USB interface for data while charging requires some negotiation WRT current capabilities when the USB device is connected to the computer.

Basically, the computer has to tell the iPad that it can source 2A once it is connected. The charger does this by pulling some of the USB data lines high or low. However, this precludes data transfer.

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    \$\begingroup\$ When plugged into a computer, the power is negotiated over USB signals, not electrically. When plugged into a charger, the amount of current a device is allowed to draw is determined electrically, either by applying specific voltages to the D+ and D- pins (Apple) or shorting them together (USB.org Battery Charging spec). Otherwise a good device will only draw 100 mA. But it's really just the honor system, and many devices will draw more without asking and just hope that the computer's port doesn't shut down. \$\endgroup\$ – endolith Oct 7 '10 at 13:57
  • \$\begingroup\$ Isn't that what I said? \$\endgroup\$ – Connor Wolf Oct 8 '10 at 2:38

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