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…


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  9. Octopus says:
    Where do you get your numbers from? Official Apple chargers claim to draw 2.1A not 1A as your article says. See for example this product: but there are others.
    • Tomi Engdahl says:
      The links to information sources are in the article.

      You are right that Official Apple chargers afe now 2.1A, but this was not always the case.
      My posting was written several years ago, and at that time USB chargers were less powerful.

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  11. Dominic says:
    I think Apple don’t want people to figure out how they regulate for charging battery from 0% to 100%. First ,D- and D+ must have internal pull up or pull down resistor with specific value. Second, a MCU chip battery sensor value., when battery reading value reach a specific value MCU will control D- or D+ line by pulling up or pulling down, this will cause voltage level changing in D+ and D- pin if we use right value of resistor(cus final voltage reading depend on voltage divider resistor AND internal iphone pull up and pull down resistor in data line). Finally, from new value of D- and D+ the MCU will set a new charging rate for battery. This is general concept for charging battery with protection. If Apple use this concept for design, YOU HAVE TO USE RIGHT VALUE OF RESISTOR.
  12. Tomi Engdahl says:
    Micro-USB failure leads to a USB power adapter education

    The most innocuous of glitches sometimes blossom into much bigger engineering exercises and educational opportunities.

    My aspirations were thwarted, however, by the e-book reader’s completely drained battery.

    Its companion USB-to-micro-USB charge cable was buried deeply in my attaché case, so instead I grabbed a Samsung-branded 5′ cable I’d recently bought, which was sitting on my desk. I plugged it into an Apple 10W USB power adapter I also had handy, and … nothing.

    None of the ports on either of the chargers did the trick, leading me to suspect that I had a bad cable on my hands. However, on a hunch, I connected the Samsung cable to my two Android smartphones, a HTC One M7 and a first-generation Motorola Moto X … and they both happily reported that they were charging just fine,

    Had my Kindle Keyboard died? No, it charged fine using the cable it originally came with,

    What I learned hasn’t led to any definitive conclusions, but it’s certainly bolstered my respect for designers of USB power adapters, along with those of the devices whose batteries get recharged.

    First off, as a reminder, the USB hardware standard is nominally a four-shielded-wire configuration: a twisted pair for half-duplex differential data (USB 3.0 adds two more twisted pairs, for full-duplex and higher-speed support), 5V power, and ground. Micro-USB connectors add a fifth “ID” pin for OTG support, grounded at the host and left floating at the device.

    I was also already conceptually aware that chargers and devices conducted an initial negotiation process upon first connection-via-cable to determine the device’s maximum-accepted current draw needs versus the charger’s maximum-possible current output. I’d thought that this handshake was analog (and 5V wire-based) in nature

    The data wire-based reality is more complicated and explains why I’ve also experienced a few other charger-plus-cable-plus-device combination non-starter situations in the past.

    Device makers being a competitive lot, there’s no one standard negotiation – it’s a bit like having to speak six languages.

    While the USB forum released a generic ‘signature’ standard, other manufacturers came up with their own signatures and in the end, there are at least six D+/D- signatures in the wild:

    2.0V/2.0V – low power (500mA)
    2.0V/2.7V – Apple iPhone (1000mA/5-watt)
    2.7V/2.0V – Apple iPad (2100mA/10-watt)
    2.7V/2.7V – 12-watt (2400mA, possibly used by Blackberry)
    D+/D- shorted together – USB-IF BC 1.2 standard
    1.2V/1.2V – Samsung devices

    These days, resistance-based voltage sensing options like the first four are described as ‘legacy’ modes and all new devices we believe use chip-based detection.

  13. Tomi Engdahl says:
    USB Charging Past, Present, and Future – Type-C

    Eventually the USB Implementers’ Forum (USB-IF) designed a standard to resolve this issue, Battery Charging 1.2, and slowly it has been adopted into most modern devices, including later generations of the iPhone and iPad.

    Today several chipset manufacturers make smart chipsets that try to intelligently detect what device you are using and emulate the best charging signal for that device. Most major phones and tablets are supported from Apple iOS, Android, and Windows Mobile based devices from many different manufacturers. For 2015 we have introduced a whole new line of dedicated smart chargers and an update to our bestselling USB 3.0 hub to be BC 1.2 compliant ensuring almost any USB device will charge at the fastest rate possible.

    The future for USB charging appears bright with the introduction of USB Type-C, a new standardized universal connector that will hopefully become commonplace on future devices from cell phones to laptops. Currently there aren’t many USB Type-C devices on the market but the two that we’ve been testing have interchangeable power adapters thanks to cross-compatibility of the USB-IF Power Delivery standard.

  14. Tomi Engdahl says:
    Multi-port USB charge stations’ performance falls short–performance-falls-short?_mc=NL_EDN_EDT_EDN_weekly_20151119&cid=NL_EDN_EDT_EDN_weekly_20151119&elq=5fcbb209ce23439586d6007163c23f3d&elqCampaignId=25813&elqaid=29411&elqat=1&elqTrackId=317af95435f24e9fa2f2d7148a87f6d2

    In a recent article discussing a faulty Micro-USB cable, I mentioned two multi-port USB chargers, “an Orico DCT-5U, which offers both 1A and 2.4A output options, along with an OTG (on-the-go)-compatible port pair”

    “and a Sabrent AX-U5PB, which is supposedly able to deliver 2A to each of its five USB ports”

    If you’ve followed any technology retailer (brick-and-mortar, Internet, or both) for any amount of time, you likely already realize that this particular product class is one of the hottest in consumer electronics. A diversity of charge port options exists in the marketplace, which the combination of USB Type C and wireless charging may eventually simplify, although a diversity of the latter also currently exists. However, the connector-and-voltage combination on the other end of the charge cable is largely standardized nowadays; 5V at 1A (or more), sourced over USB.

    This commonality is fundamentally what’s fueling the multi-port USB charger’s popularity. However, as abundant user feedback on Amazon and elsewhere indicates, consumers’ experiences with these products are less than stellar.

    I found the printed user manual, which included the following broken-English statement:

    OTG Ports are not Dedicated Charger. They may can’t Charge Some Devices. Charge Devices via Dedicated Charger is Recommended.

    Calling the Orico DCT-5U a five-port charger under these circumstances seems disingenuous to me. Do you agree?

    Next, let’s look more closely at the Sabrent AX-U5PB. It’s marketed as a “50W” charger, which, if you do the math on its five-port, 5V-output specifications, translates to ~2A maximum per-port output current capability. There’s nothing in the product markings or documentation to suggest otherwise. However, a look at the Amazon user feedback suggests that something’s amiss. Some units don’t deliver more than ~1A per port, even when only one device to be charged is plugged into them. Other folks find that the AX-U5PB works as claimed only if a particular power-on sequence is employed. And still others find that the device prematurely dies. My particular unit hasn’t yet experienced any of these problems, but a 20% 1-star rating percentage isn’t exactly confidence building …

    But given that consumers naturally act in response to a “bigger numbers is better” mentality and don’t have a clue what maximum currents their various devices accept, inevitably something that should be hooked up to 2.4A (like a tablet) is instead connected to 1A because that’s the only available port left, with molasses-slow charging as the result. Other manufacturers instead label their ports with specific appropriate-product guidance, such as “smartphone” and “tablet.” But what if you have a smartphone that’s 2A-compatible?

    Still, given the inevitable not-insignificant number of customer returns from folks who obtained either a flat-out failing unit or one that doesn’t charge as rapidly as expected, I can’t help but wonder if it’s more profitable at the end of the day to bite the bill-of-materials costs bullet, sell a charger that’s inherently 2.4A-capable on all ports, and benefit from the reduced product-return costs that result. And more generally, I’m disappointed to have to wrap up this particular writeup with the same words I used in another recent post: “This isn’t the first time I’ve encountered a product that’s incapable of meeting the specifications touted in its marketing materials. But it’s nonetheless disappointing.”


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