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Subject: Re: [LML] Re: 10 Tips to Conserve iPad Battery Life
From: Colyn Case <colyncase@earthlink.net>
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thanks Hamid - useful.

On Oct 31, 2010, at 9:41 PM, Hamid Wasti wrote:

> JAMES COLLINS wrote:
>> Quoting from the Cardinal list:  I had an aftermarket iPhone charger =
that
>> caught on fire in my car; that's a bad thing.
> That is a really vague piece of information and not really useful =
without some more detail. What kind of an aftermarket charger? What was =
he doing with it?
>=20
> Not all aftermarket products are the same. Not all aftermarket  =
chargers are the same. There is some really bad no-name stuff from China =
as well as decently designed & manufactured stuff from reputable US and =
EU companies (though still manufactured in China). Lumping them all into =
the same category is not reasonable.
>=20
> I can not tell you what is safe to use, but I can tell you what is NOT =
safe to use. If you buy a charger off eBay for $4 including shipping, =
chances are that what you are getting is neither safe nor capable of =
meeting all the specs.
>=20
> Here's a brief tutorial about this subject, for those that are =
interested. I have to start with a bunch of seemingly random background =
information that will all come together in the end:
>=20
> The USB 2.0 spec specifies that a USB device can draw up to 500mA =
(5W). However, it must start off drawing no more than 100mA (0.5W) and =
ask for permission from the hub to draw more than that. In real life, =
many devices, especially ones that are using the USB port just for power =
or charging, blatant ignore this and just start drawing 500mA (5W) or =
sometimes even more.
>=20
> The USB 2.0 spec specifies that a USB hub be able to provide at least =
100mA on every port and up to 500mA on some or all ports at the =
manufacturers' discretion. In real life, most SUB ports found on =
computers and hubs are capable of providing quite a bit more than 500mA.
>=20
> After the USB2.0 spec was finalized, a Battery Charging spec was added =
to it in 2007. This defined, among other things, a "Dedicated Charging =
Port" that can deliver up to 1.8A or 9W. According to this spec, when a =
USB device detects that it is plugged into a Dedicated Charging Port by =
looking at the state of the two USB data signals, it can start drawing =
up to 9W of power without talking to the hub.
>=20
> Most typical cell phones require about 5W to operate and charge their =
batteries at the same time. I suspect that many of the really cheap =
aftermarket "USB Chargers" intended for use with cell phones are =
designed to provide about 5W of power.
>=20
> According to the iPad spec from Apple =
(http://www.apple.com/ipad/specs/), the iPad has a 25W battery which =
gives it about 10 hours of use which comes to a power consumption of =
about 2.5W  This means that when an iPad is connected to a standard USB =
2.0 port, it can operate normally but not able to charge its battery.
>=20
> According to Apple's website, the iPad comes with a "10W USB Power =
Adapter"  charger. This suggests that the iPad is capable of detecting =
when it is connected to a "Dedicated Charging Port" and can draw up to =
9W.
>=20
> I suspect that if you connect the iPad to a charger that advertises =
itself as a "Dedicated Charging Port" on the USB data signals, the iPad =
will draw close to 9W until its batteries are charged. It will also draw =
its operating power from this port. If you are using a properly designed =
charger from any manufacturer that lives up to the spec, you should not =
have a problem. If you use a cheap aftermarket charger that is designed =
to provide 5W because that is what most cell phones need, you are likely =
to have problems. I have no idea how these problems will manifest =
themselves. It could be no charging at all, it could be a failed charger =
or it could be a fire.
>=20
> I hope this clears up some mysteries about this subject.
>=20
> Regards,
>=20
> Hamid
>=20
> --
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