

IPad Doesn't Charge from Windows PC USB Port? - aresant
http://blogs.consumerreports.org/electronics/2010/04/charging-the-ipad-hit-or-miss.html

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jws
To put real numbers to this:

The iPad battery stores 25 watthours of energy.

USB 1.0,1.1, and 2.0 devices may draw a maximum of 500mA from the 5v lines of
the connector, this is 2.5 watts.

USB 3.0 will up this to 900mA for 4.5 watts, but it isn't here yet.

Actual current available at a port depends on topology. If I plug my keyboard
in to my USB 2.0 computer, then plug my iPad into the keyboard it will only be
authorized for 400mA, because the keyboard claimed 100mA. 2 watts.

So, best case, charging an iPad from USB 2.0 will take 10 hours, if it is off.
Notice this is suspiciously close to the runtime on battery, i.e. a running
iPad consumes ~2.5 watts, your whole USB power allotment. Hence you can't
charge while running on a stock USB port.

So, what is with the 10 watt charger? Apple, and probably others, got tired of
how long it took to charge and have deviated from the USB spec to allow larger
charging currents. 1A chargers are common for iPhones, and at least some
Macintosh models have higher current USB ports on board.

I don't know how they negotiate the higher current, but a 10 watt charger
could theoretically charge your iPad in under 3 hours.

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PStamatiou
USB 3.0 is here. Lots of motherboards are shipping with it already:
[http://www.newegg.com/Product/Product.aspx?Item=N82E16813128...](http://www.newegg.com/Product/Product.aspx?Item=N82E16813128434&cm_re=USB_3.0_motherboard-_-13-128-434-_-Product)

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troubledwine
It'll only work on USB ports that support 10 watt charging.

It's explained pretty well here:
[http://www.macworld.com/article/150356/2010/04/ipadcharging....](http://www.macworld.com/article/150356/2010/04/ipadcharging.html)

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naner
The article states that it will charge from lower power ports if it is asleep.

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jsz0
High power mode was added to the 2.0 spec as an addendum only about a year ago
as an optional feature. As more devices require high power for charging it's
going to be a very confusing situation for consumers. They could have at least
bumped the spec to 2.1 to identify high power capable ports.

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Groxx
If it becomes a big deal (pretty likely, now), they'll probably start slapping
"High Power!" on systems that pump out more juice. Simple spec or not, if it's
a selling point, it _will_ be made known.

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gojomo
Someone should make a little dongle/tester. (A quick search of newegg and
monoprice did not suggest one yet exists.)

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ars
I think the port reports it to the computer, so you should be able to find
some sort of USB utility that will tell you.

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NateLawson
This is merely an issue of not enough power available from certain ports, not
a Mac vs. PC thing. You'll notice one of the ports they tested was on a Mac
keyboard. The keyboard is self-powered and has a built-in USB hub for the
mouse.

Those ports have less current available because the keyboard, mouse, and hub
consume power in addition to whatever the iPad uses. The host machine has to
supply power to all of those.

This is why some USB hubs have a power injection connector. It allows them to
supply the same current to downstream devices as the host PC.

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seattleslow
A few points to clarify. In USB 2.0 and earlier, there are two kinds of hubs:
self-powered and bus-powered. Self-powered hubs must provide 500 ma at each
downstream port. Bus-powered hubs must provide 100 ma at each downstream port.

There are also two kinds of devices: high-power and low-power devices, meaning
that they consume from their upstream port more than 100 ma or 100 ma or less,
respectively.

A bus-powered hub is a high-powered device, as from a port point of view, 500
ma can go down the cable to it (and devices connected to it).

The maximum number of ports that a bus-powered hub can provide is 4 as 400 ma
are required for each downstream port and the hub functionality uses some
power.

You cannot plug a bus-powered hub (a high power device as it can consume 500
ma from its upstream port) to a bus-powered hub. You can of course alternate
between self- and bus-powered hubs.

If the iPad can operate on 100 ma, then it can be connected to a bus-powered
hub. If it requires more power, then it must be connected to a self-powered
hub.

Hope this helps.

On a historical note, Apple has built systems that provide more than 500 ma
from the root (the ports in the system itself). I am not an expert on if they
still do. I'd hazard a guess the answer is yes.

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makecheck
Not all USB ports are powered. It is often necessary to connect certain
peripherals directly to a USB 2.0 port on the computer (as opposed to, say, a
USB jack on the keyboard itself).

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NateLawson
That is incorrect. All USB ports are powered, it's just the amount of power
available varies. The USB spec says all ports must provide 5V. The second part
of your comment is correct.

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makecheck
My mistake. There are USB _hubs_ that are classified as "powered" or
"unpowered", but I guess in the unpowered case it's relying on the computer's
supply.

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weaksauce
It charged fine on my MBP 2009. But only after it was activated through
iTunes. (9.1 is crap by the way consistently crashes when trying to play
music.)

