The iPad is a first generation product. And you know what that means – we’re going to see some bugs and glitches hidden throughout the tablet’s hardware and software. The first of those not-fully-ironed-out bugs seems to be related to the way the device charges. According to reports, and confirmation on our own fleet of iPads, the Apple tablet has issues when trying to top off its integrated battery. Instead of charging the battery, certain charging scenarios will result in a “Not Charging” indicator being displayed in the top-right corner of the display.

It turns out that the new Apple gadget won’t charge through USB ports on non-Apple computers. Here’s what we know so far through reports and our own tests:

• iPad charges just fine through the wall charging brick and accompanying USB data cable
• It charges just fine when connected to USB ports on iMac, MacBook Pro
• It does not charge correctly when connected to a PC
• It does not charge correctly when connected to an older MacBook Pro (pre-Unibody)
• It does not charge through USB hubs, regardless of what the hub is connected to (PC or Mac)

What’s going on here? Well, Apple apparently designed the iPad to be charged through its own wall charger. It was designed to be charged only through high-power USB 2.0 ports, but Apple also mentions in their tech notes that some USB 2.0 ports do not provide the necessary power to charge the battery.

This may be the reason why:-

Ok, let’s do the math:

The iPad has a 25Wh battery, it can run for about 10 hours, to it draws about 2.5 watts.

The iPad charger can provide 10 watts of power, so the iPad has its 2.5 watts for running and can put 7.5 watts into the battery. If it’s sleeping it can put, I’d guess, about 9.5 watts into the battery.

The iPhone charger and newer Macs can provide 5 watts of power. That’s 2.5 resp. 4.5 watts for the battery.

A standard USB port as it can be found about anywhere can provide 2.5 watts. So a running iPad has what left for the battery? Um, that’s hard. What is 2.5 minus 2.5…?