AllThingsD's Ina Fried published an article today indicating that the observations of unusual charing behavior with the iPad 3 aren't so unusual after all.
Yesterday I noted that if you unplug your iPad 3 as soon as the battery indicator says "100%" you’re actually missing out on as much as 1.2 hours (10 percent) of additional run time. But Apple says that this is the expected behavior and that the iPad battery is designed to work this way.
Fried explains it this way:
Apple does, in fact, display the iPad (and iPhone and iPod Touch) as 100 percent charged just before a device reaches a completely charged state. At that point, it will continue charging to 100 percent, then discharge a bit and charge back up to 100 percent, repeating that process until the device is unplugged.
Soneira, President of DisplayMate Technologies Corporation, first discovered that the iPad 3 experienced a net loss of 1.2 hours (around 10 percent) of battery run time if unplugged as soon as its battery indicator displayed 100%. His essential point was that if the iPad 3 is fully charged (i.e. overnight) it will run 11.6 hours, which is 1.2 hours longer than if it is only charged to 100% (10.4 hours).
Apple VP Michael Tchao tells AllThingsD that Apple decided not to keep changing the battery status "so as not to distract or confuse users," explaining:
That circuitry is designed so you can keep your device plugged in as long as you would like, It’s a great feature that’s always been in iOS.
So there you have it. The iPad battery displays "100%" a little prematurely because Apple didn't think that it would be wise to show the battery constantly fluctuating between 90 and 100 percent as the battery goes through its normal trickle charge/discharge cycle to keep it topped off. Makes sense to me.