Follow-up: new iPad battery takes 5.5 hours to fully charge
updated 09:30 pm EDT, Mon March 26, 2012
Testing shows bad indicator can cheat users
The bad battery indicator in the new third-generation iPad has been further tested by Dr. Ray Soneira, the president of DisplayMate and the leading scientist testing and explaining the technology behind the new iPad's doubled resolution and advanced "Retina Display" screen, as well as one of the first to uncover that the battery indicator falsely reads "100%" more than an hour before it is actually fully charged. He has since clarified some of his findings.
Soneira found by measuring current draw that the battery indicator showing 100 percent was not at all indicative that the charging process was complete, reaching the point where the iPad stops drawing the full 10 watts of power needed for charging and reduces gradually down before finally reaching a "trickle charge" point of around one watt. If the iPad is on during the charging process, it may need just over two hours (at normal brightness) beyond the point where the indicator reads "100%" to actually fully charge; if off or sleeping, it needs slightly over one extra hour beyond the 100 percent mark.
With the iPad off or sleeping, it now takes 5.5 hours from a complete discharge to fully charged, he says, roughly an hour to 90 minutes more than the previous models of iPad required. With the machine on and the display set to maximum brightness, the recharging process can take up to 20 hours, demonstrating how much power the new display draws. He found that a truly "fully charged" iPad battery operated at normal brightness (medium) and very little activity (no apps, Wi-Fi off, Airplane Mode) was 11.6 hours, exactly in line with conditions specified on the Apple website. Maximum brightness cuts that figure in half, but delivers almost three times as much luminance as the middle brightness setting almost all users would use by default.
The biggest problem, Soneira says, is that customers who simply charge the battery until the indicator reads "100%" are getting cheated of the full runtime of the device. His testing showed that unplugging once 100 percent was indicated reduced the useful running time by 1.2 hours to just 10.4 hours -- still significantly more than Apple's claimed "10 hours running time" but only using conditions that most users would find unproductive (the same ones in the "minimal use" conditions detailed above).
Users who let their machines charge overnight, therefore, would not see any performance drop compared to those who pull the plug based on the faulty software-based battery indicator, which uses a mathematical algorithm rather than a hardware sensor to determine when the battery ought to be full (or drained for that matter).
Soneira says an Apple spokesperson has claimed that owners should not allow the iPad to charge beyond the 100 percent indication lest it "damage the longevity of the battery," but this hasn't been independently verified beyond a single report at CNBC. It would also contradict Apple's previous guidance that its equipment will automatically reduce current draw once the device is fully charged, meaning users can leave their devices plugged in as long as they like. Most iPad (and iPhone, and notebook Mac) owners often allow their devices to recharge overnight.
Soneira says Apple simply needs to fix the software-based algorithm for the new iPad's battery indicator to make it more accurate to how long it takes to charge. Thus far, the company has not issued any formal statement to the general media on the issue.
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Joined: Jun 2007
Do the math
The new iPad has 42 Watt Hour battery. Watt hours is a way to measure capacity, meaning if you were drawing 42 watts, the battery would last 1 hour, or if you were drawing 4.2 watts, it would last 10 hours.
The charger included with the iPad is a 10 watt charger. In order to get 42 watt hours of energy out of the charger, it would need to operate for 4.2 hours. Now, any time you convert energy, it's not 100% efficient. Charging a battery does result in some heat being created, so the estimate of 5.5 hours of charging is very realistic. That's about 80% efficiency, not even considering that the iPad is still drawing some power in the process.