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If my iPhone 4 is turned off and I plug it in to charge, it automatically turns itself on again. Anyone know why this happens?

It appears to happen with all three iPhones in the house (a 3G, and 2 x iPhone 4). All the chargers are the chargers that were bundled with iPhones.

The cable on the chargers range from the original iPhone cable, the original dual charger cable for the iPhone and Apple Bt headset, and the newer low profile connector cable that came with the iPhone 4.

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up vote 18 down vote accepted

When the phone operating system is off, the hardware still remains sensitive to the introduction of charging power to the 30 pin dock connector. Not only does this help in cases where the OS has frozen and run down the battery past the point where available voltage can even run the system (including the charging circuitry) but it also helps in the more common case of "my phone is dead" and I'd like it back without having to press buttons. Depending on how long the battery self-discharged after crossing the "empty" line, it may need tens of seconds or even minutes of charging to get back to the minimum viable voltage. At that point, the sleep/wake button can become effective again for starting the OS.

You can of course plug in the power and then shut off the phone to bypass this behavior. The charging circuitry runs whether the OS is running or not, and that circuitry is what kicks the OS to start when it senses a connection event on the 30 pin connector and the OS is not currently running.

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The short answer is: there's a better chance you want your phone ON than OFF.

That's why Apple chose to make it turn on whenever there's a power source available.

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True, but there is a specific case where this "feature" really messes you up: a device that is at 0% charge. It turns out that the voltage required to power the screen and display the apple logo exceeds the voltage input from the charger, and you are never able to recharge the device - the screen coming on uses up the voltage that the AC charger is delivering so you never get above the threshold and are stuck in an endless loop of: off - charge - apple icon - off... – Tom Auger Oct 25 '14 at 4:30

protected by bmike Oct 28 '13 at 1:36

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