iPad Charging – Why Mac Can Charge but Other Computers Can’t

From the article, The iPad's USB charging caveat:

[Originally posted on Neowin.net by giga on 04 April 2010 12:26]

The iPad specifications page clearly state the ability to charge with USB (with no mention of OS requirements) but Apple have additionally provided a support document disclosing that "some USB 2.0 ports and accessories do not provide enough power to charge iPad."

Providing some further clarification to Macworld, Apple asserts that USB charging is supported but that it varies according to the state of the iPad and the power output of the USB hub:

• For the fastest charging, use the iPad’s included 10-Watt USB power adapter. This will fully charge the iPad in a few hours, even if you’re using the iPad at the same time.
• When connected to a high-power USB port—such as the ones on recent Macs and the iPhone Power Adapter—the iPad will charge, even during use, but more slowly. (We haven’t yet determined how much more slowly.) Some third-party powered USB hubs provide higher-power USB ports, but many don’t; similarly, the USB ports on most Windows PCs don’t provide this additional power.
• When connected to lower-power USB ports—those on older Macs, most Windows PCs, and most USB hubs (powered or unpowered)—the iPad’s battery is not charged while the iPad is awake, but is charged (again, slowly) when the iPad is asleep. What’s confusing here is that the message "Not charging" appears in the menu bar when the iPad is awake, which might lead you to assume that the offending USB port can never charge your iPad. But rest assured, Apple says: once you put the iPad to sleep, the battery will indeed charge. (If you could see the screen while the iPad was asleep, it might even display the charging icon. It’s the modern-day “Does the refrigerator light stay on when I close the door?” mystery.)

Further analysis from Apple's statements indicate that the iPad can only be charged off of USB connections that provide near 1 amp of current. The majority of USB ports on most older Macs and PCs only provide 500 mA and thus can't be used to charge the device.

The USB 2.0 specification provides 500 mA of current to high-powered buses and 100 mA for low-powered buses. But in April 2009, the specification gained a new "Battery Charging Specification" which provided additional current to high power devices and it seems that Apple has started to incorporate it in their newer Macs.

The iPad uses a 10W charger. The iPhone has a 5W charger. Most older computers only support the original USB spec of 2.5W USB port power support. Your iPad, when on, uses more than 2.5W, so it will not charge unless it's turned off, and when off it will charge at less than 1/4 the speed of the normal iPad charger. That can be up to 16 hours for a fully depleted iPad.

Newer Apple computers, and computers that support USB3.0, support higher USB power standards, but if your computer doesn't then you'll need to invest in a new charger for your iPad.

Note that the iPad will generally charge off other USB chargers from other manufacturers, but only at a slower rate than the original iPad charger, unless the charger you use is specified to charge the iPad. When the original iPad came out 10W USB chargers were not common.