Q: I’m charging my iPad by connecting the cord to a PC. Though my battery seems to be charging, the status bar displays “Not Charging.” Why? Should I continue charging it this way or stop?
– bhimarao, via comment on Best Practices for iPad Battery Charging
A: Due to its larger battery, the iPad requires a higher current—specifically 2.1A (10W)—for optimum charging. The wall charger included with the iPad provides this, however most computer USB ports and third-party chargers do not. A computer’s USB port typically provides only 0.5A (2.5W), as do almost all USB hubs; some newer computers include USB ports that can provide up to 1.5A (7.5W) expressly for charging purposes, and modern Macs can even provide the 2.1A (10W) required for iPad charging on at least some of their USB ports. Note that third-party USB chargers generally provide anywhere from 0.5A to 1.5A, with most of the inexpensive generic chargers on the 0.5A end of that scale.
The current provided by a computer’s USB port or USB hub can also be affected by how many devices you have connected and what order you connect them in; most PCs and Macs allocate USB power on a first-come, first-serve basis, and even the 2011 Macs which provide 2.1A of current will only do so when this power is not already being consumed by other devices—see Charging your iOS devices on a Mac or PC for more information on this.
When the iPad is connected to a USB hub or standard, low-power USB port, it will only receive 0.5A of charging current, which triggers the “Not Charging” display in the status bar. Even in this state, however, the iPad will still charge—albeit it slowly and provided that you’re not subjecting it to heavy usage such as leaving the screen on at full brightness or running power-intensive apps such as games or GPS navigation apps. Note that you’re not harming the iPad in any way by connecting it to a lower power source—you’re just not charging it nearly as fast as you could from the wall charger or a higher-power USB port.
You’ll definitely want to ensure that you’re connecting your iPad directly to a USB port on your PC rather than a USB hub, since USB hubs typically only provide 0.5A of current. Further, because USB port power can vary, you may want to try different ports on your PC to see if that makes a difference, as well as disconnecting other devices that may be consuming the extra power that your iPad could otherwise use. Further, if you have devices such as external hard drives with the option to connect to external power, doing so instead of relying on USB power for these devices will help to ensure that any additional current is available for charging your iPad instead.