Sender: <marv@lancaironline.net>
To:  lml@lancaironline.net
Date: Sun, 31 Oct 2010 21:41:01 -0400
Message-ID: <redirect-4549616@logan.com>
Received-SPF: none
 receiver=logan.com; client-ip=206.130.116.53; envelope-from=hwasti@lm50.com
From: Hamid Wasti <hwasti@lm50.com>
User-Agent: Thunderbird 2.0.0.24 (Windows/20100228)
MIME-Version: 1.0
Subject: Re: [LML] 10 Tips to Conserve iPad Battery Life
References: <list-4548270@logan.com>
In-Reply-To: <list-4548270@logan.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

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?

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.

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.

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:

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.

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.

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.

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.

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.

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.

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.

I hope this clears up some mysteries about this subject.

Regards,

Hamid