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Using the external accessory kit could one iPhone charge another iPhone with a double headed dock connector?

Or would the power supplied be so small that it wouldn't worth charging it that way?

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migrated from stackoverflow.com Jun 9 '11 at 3:22

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3 Answers 3

I wouldn't recommend using this cable at all. The iPhone dock connector uses the pin 23 to provide 5V of electric flow. Since you're using a double headed dock connector, the most reasonable technique the manufacturer used is just to split the same cable that comes from the USB head into the four pins each dock connector should at least have to provide power and sync capabilities (or two pins if it only charges the iPhones).

Since the current must at least drop by half on each phone because of the splitting, neither would be properly charged. If both iPhones seem to charge, the least dangerous consequence would be that the charge time will be at least doubled.

Using this technique can (worst case scenario) disrupt the device's PMU. My advice: don't.

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Chargers usually deliver a voltage a bit higher than the battery voltage - otherwise there would be no reason for net flow toward the battery. The delivering battery voltage may not be enough to charge the receiving battery fully - but it would do something if the receiving battery is low (beyond where voltage dips) and the sending battery is charged well.

The other problem is that manufacturers (Apple especially) use special codes delivered by the charger when first plugged in that let the phone know it's ok to start receiving charge. This is a form of proprietary handshaking that lets the manufacturer only (and others that have sniffed the code) make chargers. If you can embed a microprocessor to handle that (or better program the phone to do that) then you're all set.

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As per this question:

How much current/power can be drawn from iPhone 30-pin connector?

The current one iPhone can supply to any device connected via the docking connector is so small that it wouldn't charge the other iPhone's battery - it would simply help power the other iPhone. If you turned the target iPhone off, then I suspect you might get a 50-100mA charging current, which is 1/20 or 1/10 of the normal charging current, but due to the inefficiency of conversion if your source phone started out at 100%, and your target phones started out at 0%, and you turned off all the wireless features of the source phone, you still wouldn't end up with even a 50% charge on the target phone once the source phone hit 0%.

And it would take 8 or more hours to do it, as well.

However, for an emergency charge that might give you enough power to make a call, it might work if you plug them together for an hour or so. I don't see any particular advantage to doing this, if you have enough foresight to buy the right cables and accessories, then you might as well buy a AA battery based iPhone charger instead and carry that where you would otherwise keep your special iphone to iphone charging cable.

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