iPad update via windows PC
What happens if the iPad shuts down due to low battery during the initial phase of update download (not update install) ?
iPad Air, iOS 8
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The USB ports on your computer are unlikely to be able to provide adequate power to charge your iPad - as, unless the USB port is designed to charge external devices, they are generally limited to 0.5-1A @5V (i.e., 2.5-5.0W). Most models of iPad require at least 2.0-2.4A to reliably charge (10-12W).
As such, you should ensure that your iPad is fully charged prior to attempting the update - or that the computer can provide sufficient power. Allowing the battery to fail during critical phases of the re-flash process can leave the iPad unable to boot - and possibly unrecoverable.
If the battery becomes fully depleted prior to completing the update, recharge fully using the iPad Power Adapter prior to trying again. If the iPad is not subsequently recognised by your PC when connected, you will need to try a “DFU” restore.