Dr. Raymond Soneira of DisplayMate Technologies analyzed the performance of the new iPad's battery when it says it's 100 percent charged versus when it's actually 100 percent charged. Unsurprisingly, he found a discrepancy; it's already been widely reported that the new iPad's charging indicator is bugged somehow and doesn't correctly report when the iPad is fully charged. Though the new iPad may report 100 percent charge capacity, it's in fact only around 90 percent charged at that point. It's a simple bug to fix, and it'll likely be one focus of a 5.1.1 update to iOS.
Where Dr. Soneira's analysis and CNBC's reporting on the issue both go off the rails is when they both say that the iPad is actually overcharging its own battery and thereby causing damage to it. CNBC's analogy is that a battery is like a glass of fruit juice: it's meant to be filled to below the "rim," and if you overfill the glass the juice can spill out. This is an insipid analogy that demonstrates ignorance of how lithium battery technology has worked since... oh, the mid-1990s or so.
Like all modern computers, the iPad's charging circuitry automatically stops charging the battery and puts it on a trickle charge once it reaches 100 percent of its capacity. This maximum capacity decreases over time as the battery accumulates charge cycles, but it's quite gradual; iPads and most other modern Apple gear contain lithium batteries designed to retain 80 percent of their initial capacity after 1000 full charge/discharge cycles. That's 1000 times taking the iPad from 100 percent charged down to zero, or the equivalent.
It's worth restating that the iPad's charging circuitry automatically stops charging the battery when it senses the battery reached its maximum capacity. This is why you can leave your iPad plugged in overnight without worrying about the battery getting overcharged and exploding in a gooey, hot mess of chemicals and fire. The same is true of iPhones, iPods, and Macs -- once the battery is fully charged, the device throttles the charging circuits down to maintain a slow trickle charge that keeps the battery at or near 100 percent. Or, if you want to go with CNBC's dumb analogy, the glass is actually designed to be filled to the rim, but the juice dispenser is smart enough to stop filling it automatically when it gets to that point.
However, Dr. Soneira and CNBC both seem to think that the bug pertaining to the iPad's charging indicator means the iPad is getting overcharged. They both argue that the iPad's battery is actually at 100 percent of its charge capacity when it says so, and leaving the iPad plugged in after that 100 percent overcharges the battery and causes damage.
As evidence, Dr. Soneira notes that allowing the iPad to charge for additional time after it reads 100 percent charged gives the iPad an additional 1.2 hours of running time. His analysis correctly shows that the 11.6 hour runtime he got by "overcharging" the iPad's battery is likely in line with Apple's officially-stated 10-hour battery life. Apple has a history of being optimistic with its battery estimates for Macs and conservative with estimates for iOS devices, so the 10.4 hour runtime Dr. Soneira achieved when he stopped charging the iPad's battery when it said it was at 100 percent is right in line with what we already know: the iPad isn't actually fully charged when it says it is.
When Dr. Soneira and CNBC both leap to the conclusion that the iPad is overcharging and therefore damaging its own battery, however, they both get it wrong. "Apple has put forth a rather shocking reverse perspective that the on-screen battery indicator is instead the correct one," Soneira claims, and CNBC says "Apple is saying... if you charge it more than [when the battery indicator reads 100%], you could actually harm the longevity of the battery."
The problem with those claims? The problem with those claims? Apple didn't say either of those
things. CNBC said Apple said them. Apple's page on the iPad's battery says nothing of the kind. It's common knowledge that if you leave a device with a lithium battery plugged in literally all the time without ever discharging it that you can do some harm to the maximum capacity, but that's not a symptom of overcharging the battery. Instead, it's a consequence of never discharging a battery that's designed to be discharged from time to time.
"According to Apple the new iPad is configured to damage the longevity of its own battery if it isn't manually disconnected from the AC charger when the 100% indicator appears," Dr. Soneira says, without providing a link to a page proving that Apple actually made this claim. "Anyone that recharges their iPad unattended, especially overnight, will be doing this."
In a word: no. This statement demonstrates a fundamental misunderstanding of how modern lithium battery technology works.
We reached out to Apple for comment, but we haven't heard back. We don't expect to, because this is a non-issue. The iPad may be telling us tall tales about its charge state in the status bar, but it's not destroying its own battery.
Update: In a statement to AllThingsD, Apple confirmed the iPad's battery behaves exactly the same way as all previous iOS devices. Battery charging 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," Apple VP Michael Tchao stated.
It turns out that displaying 100 percent charge in the status bar before the battery actually reaches full capacity is also "normal" behavior. The device actually does continue charging for awhile after displaying 100% charge, discharges slightly when put on "trickle" charge, then charges back up to 100 percent. Rather than confuse users and have them see the battery charge oscillate between 90 and 100 percent while the device is plugged in, the iPad will simply display 100 percent charge status.
So, not only is the iPad not damaging its own battery, the "bug" in its charging status isn't a bug at all.