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From: Jochem Huhmann on 31 Jan 2010 19:28
I noticed that the iPad has no charger and gets its power via USB.
Great, I thought. No more chargers to lug around, just an iPod cable and
you're done.

Hmm. This thing is said to run for 10 hours. It has a 25Wh battery. This
means it draws 2.5 W on average. This seems incredibly low, but let's
assume this is correct for a moment. Now, USB delivers 5 V at 0.5 A, which
means 2.5 W. A tight fit: Run the thing off an USB-port and do something
heavy on it and it will start to draw power from the battery since the
USB connection will not be able to supply it with more than that meager
2.5 W.

Charging the thing while it's off takes 10 hours then and this even does
not take in acccount that the charging tapers off towards the end. 12-14
hours seems more likely. And when it's on while connected, it will never
even start charging...

Did I miss something here? The USB standard talks also about "powering
modes", in which there may be as much as 1.5 A, which would mean 7.5 W,
which helps quite a bit. The USB power adapter from Apple even delivers
10 W. But of course this does not work on a USB-port on a computer,
which still is limited to 500 mA, you need some dedicated device again.

Looks very tight, all this. It might be wise to order the USB-charger
right from Apple with it, because you won't be happy with charging it
from your computer and other chargers may not work at all (or not
deliver enough power).


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery
From: David Empson on 31 Jan 2010 21:21
Jochem Huhmann <joh(a)gmx.net> wrote:

> I noticed that the iPad has no charger and gets its power via USB.
> Great, I thought. No more chargers to lug around, just an iPod cable and
> you're done.
>
> Hmm. This thing is said to run for 10 hours. It has a 25Wh battery. This
> means it draws 2.5 W on average. This seems incredibly low, but let's
> assume this is correct for a moment. Now, USB delivers 5 V at 0.5 A, which
> means 2.5 W. A tight fit: Run the thing off an USB-port and do something
> heavy on it and it will start to draw power from the battery since the
> USB connection will not be able to supply it with more than that meager
> 2.5 W.
>
> Charging the thing while it's off takes 10 hours then and this even does
> not take in acccount that the charging tapers off towards the end. 12-14
> hours seems more likely. And when it's on while connected, it will never
> even start charging...
>
> Did I miss something here?

Apple's iPhone power adapter (and similar) is able to supply up to 1A
via USB when it recognises a compatble device is connected. That will
roughly halve the charge time compared to using USB from a computer.

The Apple power adapter for the iPad must be able to supply 2A, since it
is rated at 10W and USB is 5V. It may require the use of the cable
supplied with the power adapter, with thicker wires.

Or has Apple reintroduced support for the Firewire power pins on the
iPad's dock connector, which would allow a lower current and higher
voltage with the power adapter? (e.g. 20 V at 500 mA).

> The USB standard talks also about "powering modes", in which there may be
> as much as 1.5 A, which would mean 7.5 W, which helps quite a bit. The USB
> power adapter from Apple even delivers 10 W. But of course this does not
> work on a USB-port on a computer, which still is limited to 500 mA, you
> need some dedicated device again.

I've seen suggestions that some recent Macs have the ability to supply
more than 500 mA via USB for certain recognised peripherals, but haven't
seen anything definite to confirm this. (Apple hasn't published
developer notes for new Mac models in the last couple of years, which is
where this sort of thing would be documented.)

This was mentioned in the context of aluminium USB keyboards, and is
probably in the order of 700 mA (giving the keyboard enough power to
supply 500 mA through one USB port), so that isn't a huge increase for
charging an iPad.

Similar issue with the MacBook Air SuperDrive, which may require more
than 500 mA (2.5 W).

> Looks very tight, all this. It might be wise to order the USB-charger
> right from Apple with it, because you won't be happy with charging it
> from your computer and other chargers may not work at all (or not
> deliver enough power).

They should drop back gracefully, but might not be able to supply more
than 500 mA.

--
David Empson
dempson(a)actrix.gen.nz
From: Dorian Gray on 1 Feb 2010 09:30
In article <m2k4ux3ikf.fsf(a)revier.com>, Jochem Huhmann <joh(a)gmx.net>
wrote:

> Hmm. This thing is said to run for 10 hours. It has a 25Wh battery. This
> means it draws 2.5 W on average. This seems incredibly low, but let's
> assume this is correct for a moment.

2.5 W consumption makes sense:
No hard drive
No optical drive
No fan
Low power system-on-a-chip
LED 9.7" screen
Only selective background apps
No Flash!
From: Ric on 1 Feb 2010 10:44
On Feb 1, 12:28 am, Jochem Huhmann <j...(a)gmx.net> wrote:
> I noticed that the iPad has no charger and gets its power via USB.
> Great, I thought. No more chargers to lug around, just an iPod cable and
> you're done.
>
> Hmm. This thing is said to run for 10 hours. It has a 25Wh battery. This
> means it draws 2.5 W on average. This seems incredibly low, but let's
> assume this is correct for a moment. Now, USB delivers 5 V at 0.5 A, which
> means 2.5 W. A tight fit: Run the thing off an USB-port and do something
> heavy on it and it will start to draw power from the battery since the
> USB connection will not be able to supply it with more than that meager
> 2.5 W.
>
> Charging the thing while it's off takes 10 hours then and this even does
> not take in acccount that the charging tapers off towards the end. 12-14
> hours seems more likely. And when it's on while connected, it will never
> even start charging...
>
> Did I miss something here? The USB standard talks also about "powering
> modes", in which there may be as much as 1.5 A, which would mean 7.5 W,
> which helps quite a bit. The USB power adapter from Apple even delivers
> 10 W. But of course this does not work on a USB-port on a computer,
> which still is limited to 500 mA, you need some dedicated device again.
>
> Looks very tight, all this. It might be wise to order the USB-charger
> right from Apple with it, because you won't be happy with charging it
> from your computer and other chargers may not work at all (or not
> deliver enough power).
>
>         Jochem
>
> --
>  "A designer knows he has arrived at perfection not when there is no
>  longer anything to add, but when there is no longer anything to take away."
>  - Antoine de Saint-Exupery

Same as many smartphones. The Palm Pre, for example, takes forever to
charge from a USB sync lead, and the N900 uses power at nearly the
same rate as charging from a PC, but both charge quickly from a mains-
powered USB charger. The USB device can "ask" the charger if it can
take more than minimum standard - I think this is done by jumpering
pins 2 and 3 together.
So in summary: yes, it'll probably take forever to charge off a Mac or
a PC, but it'll charge fine from a wall socket.
From: Jochem Huhmann on 1 Feb 2010 11:53
Ric <infobubble(a)gmail.com> writes:

> Same as many smartphones. The Palm Pre, for example, takes forever to
> charge from a USB sync lead, and the N900 uses power at nearly the
> same rate as charging from a PC, but both charge quickly from a mains-
> powered USB charger. The USB device can "ask" the charger if it can
> take more than minimum standard - I think this is done by jumpering
> pins 2 and 3 together.
> So in summary: yes, it'll probably take forever to charge off a Mac or
> a PC, but it'll charge fine from a wall socket.

OK, what I missed is that the power adapter is actually included in the
package. Not different from any laptop, then. Just that you *can* charge
it over USB if you have to (and if you're not in a hurry).


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery
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