Apple and other USB charger secrets

Everybody seems to be saying that you can’t charge Apple devices with normal USB power supplies. You need a special power supply from Apple or approved by Apple. I saw this kind of discussion at slashdot some time ago.

Usually, device makers need to sign a confidentially agreement with Apple if they want to say their charger ‘works with iPhone / iPod,’ and they’re not allowed to talk about how the insides work. I hate when manufacturers do crap like this to keep peripherals locked into a more profitable licensing agreement. Apples tendency toward total control is one of the things i don’t like about them. And many other manufacturers are just as bad. I wish companies would back off and be more open and/or use standard micro USB chargers.

The mysteries of Apple device charging article includes a 7-minute video we explore the mysteries of Apple device charging. The secret of Apple chargers is simple: just few resistors. If you don’t put these secret resistors on the data lines too, you get the dreaded Charging is not supported with this accessory. Those resistors like a way to signal to the iPhone that it can go ahead and “fast charge” by pulling 1A, or “slow charge” by pulling 0.5A. The iPhone needs to do a power negotiation to determine if the port is capable of providing 1000ma of power, because the upper-limit of a standard USB port is 500 mA. They just didn’t tell anyone about how to do that. I get why the resistors were initially added but I’m not understanding why it needs to be a trade secret.


There is nothing to stop them just drawing the 500mA if the right sort of charger is not detected. Refusing to charge at all unless the licensed parts are present is pure market control, nothing else. Here is the resistor configuration for 500 mA charging:


Resistance is Futile. The The mysteries of Apple device charging article demonstrates how anyone can make their own chargers that work with iPhone 4, 3Gs, etc. The pictures on this blog posting are from that article.

Apple devices are not the only one USB charged devices that can have some problems with USB chargers. So here are some resources on USB charging in general.

USB As A Power Source article gives an introduction USB Power Form.

European Commission has reached a voluntary agreement with some of the biggest names in the electronics industry to introduce a common charger for cell phones that fits all models. Information on this USB charging connector is available at USB Approved Class Specification Documents document directory. Read also Battery Charging v1.1 Spec and Adopters Agreement document.

Dealextreme USB charger discussion posting says that USB standard has 4 lines (+5V, ground and +/- data lines). Most USB chargers let the data lines float. Technically, the USB standard says that a USB charger should set the two data lines to specific voltages (~ 2V) to indicate how much power it can provide (I have not verified that from standards yet). The recent iPhones will not charge if the data lines are set incorrectly (i.e. not according to the USB standard).

USB Charging Guide comment: I believe having the data pins connected to each other is in the latest USB specification for charging. I had to interconnect the D+ and D- pins inside my USB AC charger to get it working with my Zune. Perfectly according specs but frustrating enough.

Wikipedia USB article: The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines. For USB 2.0 the voltage supplied by low-powered hub ports is 4.4 V to 5.25 V.

A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0.
All devices default as low-power but the device’s software may request high-power as long as the power is available on the providing bus.

In Battery Charging Specification, new powering modes are added to the USB specification. A host or hub Charging Downstream Port can supply a maximum of 1.5 A when communicating at low-bandwidth or full-bandwidth, a maximum of 900 mA when communicating at high-bandwidth, and as much current as the connector will safely handle when no communication is taking place (USB 2.0 standard-A connectors are rated at 1500 mA by default).

A Dedicated Charging Port can supply a maximum of 1.8 A of current at 5.25 V. A portable device can draw up to 1.8 A from a Dedicated Charging Port. The Dedicated Charging Port shorts the D+ and D- pins with a resistance of at most 200Ω. The short disables data transfer, but allows devices to detect the Dedicated Charging Port and allows very simple, high current chargers to be manufactured. The increased current (faster, 9 W charging) will occur once both the host/hub and devices support the new charging specification.

Without negotiation, the powered USB device is unable to inquire if it is allowed to draw 100 mA, 500 mA, or 1 A. Some non-standard USB devices use the 5 V power supply without participating in a proper USB network which negotiates power draws with the host interface

In most cases, these items contain no digital circuitry, and thus are not Standard compliant USB devices at all. This can theoretically cause problems with some computers; prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host’s permission, into high-power mode.

USB Charging Guide tells some more details on mini-USB plug: the mini-USB plug actually has 5 pins in it. This can be important as the extra pin (Pin 4) USB_ID is usually either connected to ground or left floating. Sometimes a pull up resistor needs to be added to from the USB_ID to Pin 1 (VDD) to select “Device Mode” rather than “Host Mode”. This resistor is in the device side plug as the USB_ID pin is not wired through to the PC side connector. The good news is that quite a few USB cables have this. So sometimes you can get round the not charging problem simply by trying out different leads and one may work rather than buying the manufacturers “special” cable. On some Creative players you can also solve this by pulling down both data lines (with 2x15k resistors) at the source to emulate what the host (PC) does when setting line speed. This is not so common.

So the current state of USB charging is a little bit of mess…


  1. Mitchell says:

    How can the USB charger detects the apple products (such as Iphone)?

  2. reza says:

    I’m interested in figuring out how to negociate higher charge currents (1A) from USB ports – know where i could find that information (i.e. which documents)? Thnx!

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    Typically a laptop can’t charge an iPad, but with this magic dongle, it can. And it’s $25 less than a competing product.

    Digital Innovations’ ChargeDr, a small USB dongle that promised to do the seemingly impossible: boost the power output of a typical USB port, thus allowing it to charge an iPad (or other tablet).

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    My only real gripe with the ChargeDr was the price: $29.99.

    Guess what? Meritline sells a USB Charging Adapter for iPad for $4.99 shipped. It looks virtually identical to the ChargeDr and promises to work the same magic. Could it really?

    It’s a smidgen smaller than the ChargeDr, but functionally they’re the same: Meritline’s generic dongle works. With it I can charge my iPad from my laptop. Without it, I can’t.

  21. Tomi Engdahl says:

    DIY USB Power Strip

    I considered simply replacing the power supply and carrying on, but having a few more bed-side USB ports would be handy. I therefore opted to build myself a USB power strip.

    On the right is a 2.1mm barrel jack to accept standard 12V wall warts (which I’ve made a standard of as many of my projects as possible), and on the left is 5 “dumb” USB ports for charging devices. Originally, USB wasn’t at all designed to be a charging port, and required quite a bit of intelligence on both ends of the connection to negotiate how much power the client device can consume. Manufacturers had no interest in spending the extra cost to make a phone charger that intelligent, so each manufacturer came up with its own bastardization of the USB spec (usually via random resistors between various pins).

    This was little-to-no-fun, so the USB spec eventually caught up with reality and defined “dedicated charging ports,” which are dumb USB ports where the two data pins are simply shorted together to indicate to the client that they do nothing except provide power. Manufacturers like this because they no longer need to add a 15 cent IC to their chargers, and us hobbyists like it because shorting the two data pins together is a much easier way to build USB chargers than trying to reverse-engineer every manufacturer’s creativity on the issue

    The 12V input is regulated down to 5V 3A with an LM2596 switching regulator module

    The one point of interest that I added to this was 600mA polyfuses to four of the ports. These are solid-state fuses that don’t permanently blow, but will reset themselves after the load is removed. I left the fifth port un-fused

    In hindsight, the fuses were a good idea that work out great, except that Apple seems to hate every spec they didn’t invent themselves. Dedicated charging ports are unable to digitally tell client devices how much power they can deliver (500mA? 1A? 2.5A?), and instead simply shut down the port when this limit is exceeded. It is then the client’s job to figure out where this limit is and to do the best it can to stay below it.

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  35. Jeroen Houben says:

    Hi Tomi… I read this a little and was hoping you could give your input on a problem I have regarding USB charging my galaxy S4 from my bike.

    Well, I have a bike that produces power to an USB hub. The harder you drive the more mA it delivers at 5V.
    (details are: 1. SON28 wide hub 2. Tout Terrain The Plug 2.0+ Pat cable)

    Details about how much mA the plug delivers can be seen here:

    It boils down to this. Depending on your speed the plug delivers let us say between 300 and 700mA, reaching 500mA pretty quick!

    However the Galaxy S4 don’t want to charge, apparently renegotiating the current, constantly asking more(?), and in the end not charging.
    That is what I think happens?!

    Now I have one mini USB cable with a cheap mini-to-micro converter. If I use that cable, my Galaxy S4 does charge (I don’t know how much).
    That is strange, only with that cable + converter.
    Something must be different.
    I have a feeling you could tell me more about this? :)

    If so your input or solution would be very welcome.
    You are also very welcome in helping us out at :

    Your help and time is much appreciated!
    gr, J

    • tomi says:

      Thank you for your feedback.

      I don’t have any specific tips how to solve your problem.

      Sometimes on charging the way the cables are wired (how that fifth “id” pin is wired) has difference how different equipment see the charger.

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  38. Bummer says:

    “There is nothing to stop them just drawing the 500mA if the right sort of charger is not detected. Refusing to charge at all unless the licensed parts are present is pure market control, nothing else.”

    No, it is not. You are wrong.

    The USB spec says that you are allowed to draw max 100 mA before you have negotiated! When you have your 100 mA, you can possibly negotiate it up to 500 mA, or more in later versions of the specs.

    All those things that draw 500 mA without negotiation are violating the spec!

    If you want to “not like” Apple because they are following the spec, it is purely because of your ignorance. Apple is one of the few on the market that is actually following the USB spec.

    In the beginning they even disconnected devices that draw to much without negotiating about it first. Since almost no PC did the same (they typically didn’t have either measurement or protection, they just burned if you draw way to much power out of a USB port), many device manufacturers didn’t correctly implement negotiation, so Apple had to change to allow 500 mA out through the port even if the device hadn’t negotiated. But that was only an adaption to bad devices, and that has now continued. I honor them for still following the spec!

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  44. Stevearino says:

    I just bought a car charger made by Saicoo. It has four USB ports with on/off switches for each port.
    They bill it as the “most powerful car charger. They say it has 6 amps 30W (6000 mA) of total charging power with a single port able to put out 3A 15W (3000 mA). 2 of the ports are labeled A (apple) and two are labeled as NA for none apple devices. After reading this article I am guessing that Saicoo has put the resisters in the A ports so any apple device with go ahead and fast charge at the 1000 mA rate. So if I put an apple device on the NA ports it will only charge at the 500 mA rate or not at all? Any non apple device put on either port should draw whatever it needs I am guessing, would everyone agree?

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  46. Jani says:

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  47. Tomi Engdahl says:

    I am not joking.

    There are lots of facts in the posting backed with reference to the information source, and very few opinions.

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      Haha. 99% of these comments are clearly spam created by a bot with no reference to the actual content. Try adding a captcha to your site and watch the number of comments like “your informative of websiting very will be interesting” drop to zero.

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        Spam is a problem and lately the this problem is better handled. I get 99.9… percent of spam filtered out already. Thousands of spam comments rejected automatically every day.

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  49. Tomi Engdahl says:

    Cheap USB chargers are ‘a real danger’
    Australian warning follows one death

    AN AUSTRALIAN Fair Trading Office has warned people of a “real and present danger” that they have invited into their homes, the cheap USB charger.

    Cheap knock-off device chargers are associated with several things, especially dangerous incidents.

    Many times a simple charging process has turned a piece of hardware into a smokey melted lump and on some occasions more damage has been done.

    Stowe, reacting to a report that a knock-off charger started a fire that killed a woman, said, “These devices pose a serious risk of electrocution or fire,” and recommended that no one buy or sell them.


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