Why USB charging is so slow.

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Level 3

887 points

Why USB charging is so slow.

For those who were wondering, here is a probable reason.

The USB ports provide 500 milliAmperes of current or .5 Amp while the voltage is 5 volts. Using the formula Watts = Volts x Amps, we see that the USB port is providing about 2.5 watts.

The iPad charger is rated at 10 watts, so DIVIDING that by 5 volts shows that the current coming out of the charger is 2 amps.

Thus the USB port is only providing 1/4 of the power needed, so it will take almost 4x as long to charge the iPad.

Active: iMac (G5, G4), PB G4, iPad, iPhone, iPod, MP2100 Retired: iMac G3, A2c, Mac OS X (10.5.8), Mac OS 10.4.11, Mac OS 9.2.2, iPhone OS 3.1.3, Newton OS 2.1

Posted on Apr 4, 2010 6:57 AM

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Answer 1

Apr 4, 2010 7:36 AM in response to romad

Please see http://support.apple.com/kb/HT4060 for the reason charging is slower on USB.

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Answer 2

romad Author

Level 3

887 points

Apr 4, 2010 7:52 AM in response to maclover009

That just confirmed what I said. Specifically:

"Note: Some USB 2.0 ports and accessories do not provide enough power to charge iPad. When this occurs the message "Not Charging" appears in the status bar next to the battery icon."

BTW even though USB only provides 1/4 of the power that the charger does, I was able to "top off" my iPad battery using it. Of course the iPad said "Not Charging" but it was brought up to 99% after being on the USB port for 3 hours!

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