A recent analysis of Apple’s new iPad revealed something strange about how the device charges its battery. Apparently, the iPad’s battery is not fully charged when the device says it is topped off at 100%. Now, before you stampede back to the Apple store waving the tablet over your head and demanding a refund, it turns out that Apple’s new device is operating exactly as it should, and these weird charging behaviors are actually intentional.
Raymond Soneira at DisplayMate, who originally clued people in as to why the new iPad seemed to run so hot, noticed that when performing some tests on the new iPad that the tablet continued to charge long after the display indicated that the charge cycle was complete. From DisplayMate:
While preparing to measure the battery running time to evaluate the Retina Display’s load on the iPad battery I noticed that the power meter indicated that the iPad AC charger was still continuing to deliver close to the full 10 watt recharging power long after the screen indicator showed that the battery was 100% charged – the additional time is slightly over 1 hour if the new iPad is off or in sleep mode, and slightly over 2 hours if it is on.
Soneira goes on to say that by his figuring, when the battery level indicator reads 100%, the battery is only 90% charged. This, says Soneira, robs users of over a full hour of usable time. In his experiments, he found that by charging the iPad for an hour after it read 100%, he was able to get over 11 hours of use. It should be noted that Apple claimed the tablet would only run for 10 hours after a full charge.
So, what’s going on here? Apparently, all is well in iPad town and the tablet is simply doing what it was designed to do. Speaking with AllThingsD, Apple VP Michael Tchao said:
“That circuitry is designed so you can keep your device plugged in as long as you would like,” Tchao said. “It’s a great feature that’s always been in iOS.”
What the iPad is actually doing is charging itself up to about 90%, but displaying 100%. It then continues to charge until the battery is full, and then ceases to draw power through its charger. During this period, the iPad battery begins to discharge until it reaches about 90% again and then starts charging once more. The iPad, or apparently other iOS devices, will continue to do this until it is unplugged.
The idea, it seems, was to tell users it was fully charged at the point when the battery could run for 10 hours in order to avoid confusing users. If you reached for your “fully charged” device and suddenly saw it drop down to “not quite fully charged” just as you went to pick it up, you might be a little freaked out.
On a technical level, the iPad’s on-again-off-again charging behavior is apparently dictated by a desire to maximize performance. AllThingsD notes that most modern devices have batteries with microprocessors of their own to dictate their charging behavior. “Fully charged” actually has little to do with the state of the battery, and everything to do with when the battery “thinks” it should be disconnected.
While this is a unique little window into the internal workings of modern devices, there are still some confusing points. In his original piece on iPad battery time, Soneira points to a CNBC piece where Apple was quoted as saying that charging the iPad battery beyond 100% could damage the battery. Apple’s current statements would seem to refute that.
Hopefully this will give those newly minted iPad owners some peace of mind. Their device has a great screen and a great battery, apparently one that’s even better than advertised. Just don’t try to think too hard about how it works.
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