Charging the Apple iPad via USB

After forgetting to bring the iPad’s wall charger, I got a bit frustrated when I tried to use the USB (Universal Serial Bus) cable and connected it to my laptop for charging. The iPad showed the “Not Charging” message beside the battery indicator on the top right of the screen. 😦
How could that be? 😐  The same USB cable, when connected to the same laptop, can charge my iPod touch, my nephew’s iPod Nano and my fiancée’s iPhone 3G. After some slight reading on the internet I learned that apparently, I am not alone. 😀
When Apple released the iPad last April 3, 2010 many early adopters encountered then what I’m writing about now. They were having problems getting their new devices to charge via USB. Some users had luck while using a Mac, but the success rate is not a 100%. Other users speculate that it has something to do with the OS that is running on the machine which added more fuel to the OSX-vs.-Win debate. But from what I’ve learned, it’s really more about the power that comes out of the USB port. ;))
The most common type of USB port is the “Standard A.” It is the flat-type of plug as opposed to the “Standard B” which is a boxy-type of plug (different from mini-USB and micro-USB). Both of these plugs types follows the USB standard specifications which states that it should provide around 5 volts (+5v with more or less +0.25v) to power a device connected to it. A USB controller or hub should at least provide power to one unit load or 100 milli-Ampere (100mA), which is considered a low power load. It can optionally power up to five unit load or 500mA, which is considered a high power load.
To give you an idea on the discrepancy of the power supplied between charging the iPad via USB connected to a computer versus charging it with the supplied USB wall power adapter, let’s me show you some computations:
Power or wattage (watts) is computed as P = (I*V) where I is Current and V is Voltage. It is read as Power is equal to the Current multiplied by the Voltage. So, if we do the math, (5v*100mA) = 0.5Watts for low power load and (5v*500mA) = 2.5Watts for high power load. Still with me? 😀
The supplied USB wall power adapter is… Wait for it… 10W. :))
I am guessing that there is programmed logic in iOS that checks for the supplied power for charging and if it doesn’t reach a certain value, say 5.5v (1100mA, 5v), it displays “Not Charging”. x_x
But don’t worry if the iPad displays “Not Charging” when connected to a computer via USB because IT IS charging, I tried it. Although much slower due to the big difference in the power supplied coupled with a much bigger battery and a big ass screen compared to the iPod/iPhone. 😛
It also helps if the screen is turned off or the device is put on stand-by mode as the screen really eats up a lot of juice.