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What do Macs do differently so its USB ports can charge my iPad, but if I plug into a USB port on my PC I get the "not charging" message?

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up vote 14 down vote accepted

Because the iPad needs more than the 500 mA that the USB 2.0 spec says computers need to provide. Macwork has more information.

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Apple's newer machines (and some newer PCs I believe) have got USB ports that are able to provide more current. The following Apple KB article explain better:

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