"If you stop charging the iPad when the battery indicator says 100% you won't get the maximum running time," said Ray Soneira, president of DisplayMate Technologies, in an updated paper on the new tablet.
According to Soneira, when the iPad's battery meter first shows 100%, the tablet is actually charged only to 90% capacity, shorting users by more than an hour on the device.
That may jeopardize Apple's claim that the iPad can keep going for over 10 hours, said Soneira.
If the iPad remains connected to an outlet, the remaining 10% of running time -- about 1.2 hours -- is eventually added as the charge is slowly trickled to the tablet. All the while, said Soneira, the meter continues to read "100%."
Although the iPad's software -- which monitors the charging process to insure that the battery is not overcharged, and thus at risk of damage -- is generally working as designed, Soneira said there is "something wrong with the battery charge mathematical model on the iPad" that results in the inaccurate charge status.
"It should not say 100% until it actually stops recharging and goes from the full recharging rate of about 10 watts to a trickle charging rate of about 1 watt," said Soneira. "Otherwise the user will not get the maximum running time."
Soneira's tests showed that an iPad that was turned on while charging reached 100% capacity 2 hours and 10 minutes after the indicator prematurely read full. If the iPad was off or asleep, the additional time needed to completely charge the tablet ran just over an hour.
Soneira also noted that a video report by CNBC last week claimed Apple said users could damage the iPad's battery if they continued to charge it after the meter showed 100%.
"Apple is saying... if you charge it more than [when the battery indicator reads 100%], you could actually harm the longevity of the battery," said CNBC's Jon Fortt in that report.
There is no such warning on the Apple website, where the company offers battery longevity and charging guidance. Advice like that would also run counter to the long-standing practice of leaving portable devices unattended while charging.
Apple did not immediately reply to a request for comment on Soneira's findings or CNBC's claim that the company has advised users not to continue charging the iPad.
The new iPad sports a lithium-polymer battery rated at 42.5-watt-hours, a 70% increase over the iPad 2's 25-watt-hour battery.
A bigger battery is necessary, said Soneira, to drive the tablet's higher-resolution display and its backlight.
Apple may be able to tweak the battery indicator software with a future iOS update to make it more accurate. The company has made similar moves in the past, such as in July 2010 when it revised the iPhone 4's signal strength meter during the brouhaha that then-CEO Steve Jobs called "Antennagate."
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