I understand that a device only draws as much current as it needs. So assuming the power supply and the device have the same voltage (5V), using a power supply able to provide more amperage (= more watts) will not change anything.

However if the power supply can't provide enough current (nominal current of the device), that will either overheat the supply and/or the voltage in the circuit will drop.

Also regarding the voltage it is important that it be the same.

  • If that of the power supply is higher than the device, the current in the circuit will be too high (assuming the power supply can supply this new level of amperage) with a risk of frying the device.

  • if that of the power supply is lower, then the device might not be working properly as not enough current circulates inside.

My question is regarding the statement that new iphones can be charged with both the 5W and 12W charger, the charging time being faster with the latter.

How comes? To me either the 5W matches the device's inputs and the 12W shouldn't change anything, that is, it works but not faster. Or the 12W is the correct one but then it would mean that with the 5W the power provided is too low and we are in the case explained at the top where the power supply rating is below the device requirements, which can create issues.

The only explanation I can think of is that the circuit in iphone is more complex and somewhat intelligent with variable internal resistances to regulate the amperage and adapt to the power supply specs. Would that be correct?

  • Moderators note - this is a little more technical than most charging questions, but it’s also a bit less technical than the site from which it came. Please let us know in Ask Different Meta is this didn’t land well here.
    – bmike
    Commented Jul 31, 2017 at 4:23
  • 3
    @bmike it was only migrated because the mod involved disliked that his wrong answer was downvoted. This is 100% on topic on EE.
    – cde
    Commented Jul 31, 2017 at 4:27

4 Answers 4


The only explanation I can think of is that the circuit in iphone is more complex and somewhat intelligent with variable internal resistances to regulate the amperage and adapt to the power supply specs. Would that be correct?

Yes. Modern phones have intelligent power management. The iPhone can charge at various rates depending on what they believe they are connected to. They will only charge at <500 mA when connected to a USB host they have negotiated with. When the charger uses Apple's 1 Amp charger resistor signaling, they charge at that rate, and when it uses the 2.1 Amp charger resistor signaling, they charge at a faster rate. They can also charge faster when connected to a Mac's usb port due to special kernel modules that Apple has written for themselves.

They do not charge at a uncontrolled rate. They also specifically refuse to charge from non-apple standard chargers unless they have usb enumeration. So you cannot simply connect 5V and Gnd and charge them.


Apple chargers have distinctive "charger signatures", in a form of different voltage combination on D+ and D- lines, see for example this answer, and many-many similar answers. That's how an Apple device "knows" what is the power capability of a particular charger, and takes more current (by changing input parameters of their internal battery charger) and therefore charges faster from a 12W charger than from a 5W charger.


5W charger is 5V 1A. 12W charger is 5V 2.4A.

If the device max input current is 1A, then using both charger will give you the same time to charge from 0 to 100%. However if the device max input is 2.4A, then using the 1A charger will be way slower than a 2.4A charger.


The long answer to the question is an app note: Introduction to USB Power Delivery Applications.

In short, mobile devices that use USB for battery charging "negotiate" current with the DC power supply 1 which provides +5V on USB. The mobile doesn't simply draw the maximum current that it can to draw. At the beginning of negotiation, the mobile will draw no more than 100mA (any USB port is required to provide at least 100mA). If the mobile draws more than a USB port can provide, then a breaker inside the port would trip, and the USB the port would disconnect, which is a inconvenience.

1 Among lay consumers2, the wall adapter which supplies +5V through the USB jack is called "charger". But that's just a DC power supply. The actual charge controller for the battery is inside of the mobile device.

2 How's that for gender neutrality?

  • Nick, wall adapters-chargers are not just DC voltage supplies. They are a special class of devices, chargers. They are sort-of "soft" voltage supplies. For distinctive features of a charger, see this answer, electronics.stackexchange.com/a/239945/117785
    – Ali Chen
    Commented Jul 31, 2017 at 2:37
  • Apple uses none of that. They have their own standard.
    – cde
    Commented Jul 31, 2017 at 2:47

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