USB Charger Craziness
You Don't Always Get Watt You Pay For

05 August 2014


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USB Symbol

USB charging isn't actually Evil,
It's just misunderstood
Here at, we have lots of gadgets that charge using some sort of USB charger.  Standardizing on USB for charging is a great idea.  With a few exceptions, most notably Apple, the industry has also standardized on micro-USB for the device connector.  This eliminates waste by not forcing you to send chargers and cables to the landfill every time you upgrade or replace your mobile device.  It also lets you borrow a friends charger (and/or cable) when you need to.

Unfortunately, it also allows some not so reputable companies to make all sorts of poor quality USB adapters and cables.  Since we can't see what is happening inside the charger or the charger cable, how can we know what's really going on?  Let's learn a little about these so-called wall-warts, and we'll answer a few questions like:

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Let's get a few terms straight.  First of all, the actual battery charger for a phone or tablet is not that little square thing you plug into the wall or into your car's cigarette lighter.  The actual Battery Charger Circuit is inside the device.  A chip called a Power Management IC, and some other discrete components inside the phone regulate all aspects of the battery -- charging, power saving, sleep mode, etc.

So calling that thing a "Charger" is technically wrong.  More correct terminology is USB Power Adapter, Power Supply, or even "Wall Wart", but we'll go with the (incorrect) colloquial, and still call it a "charger" for this article.

The thing you plug into the wall simply supplies 5 Volts (V) to that internal Power Management circuitry.  That brings us to the second point -- USB is always 5 volts (or very close to 5 V).  "Fast chargers" do not put out more voltage than slow chargers.  Applying much more than about 5.25 V to a USB device can damage it, and if it is less than about 4.75 V, the device simply ignores it (goes out of charging mode).

Now let's deal with the exceptions before we continue.  There are a few devices -- Several models of Nook eBook readers, as well as some of the larger Acer tablets for example - use what look like USB connectors, but sneak additional wires and connections in them.  These additional connectors may carry more than 5 V, but they are not part of USB, and if a normal USB device or cable is connected, it will not touch the extra connections.

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Watt's the Current Current?:

So if all USB chargers produce exactly 5 Volts, how can some charge a device faster than others?  How can some advertise to be 1 Amp or 2.1 Amps, or 5 Watts or 10 Watts or whatever?

To ask the same question another way:  What ever happened to Ohm's law?

The answer is that we are not dealing with a set load here.  That battery management circuit I mentioned previously is actually "smart".  It is going to actively control the battery charge current, based on the available USB power you connected to the device (as well as several other factors, such as temperature.)  Modern batteries require a smart charger that is internal to the device.

Further more, the device actually communicates with the USB power source.  Using several different methods, the portable device determines how much power the USB port it is connected to can supply.  Please note that it is not my intention to reproduce the USB charging specifications here, so I am going to take some liberties, and provide a simplified explanation.

Assuming the device desires to charge the battery as fast as possible -- i.e. temperature is in a proper range, the battery is not already charged, etc. -- it determines the available power using some combination of the 3 following methods:

1.  If the port is "intelligent" - e.g. the port is on a desktop or laptop computer -- the mobile device can actually negotiate a charging current over the center 2 data wires in the USB connector (the actual power is carried on the outer 2 wires.)

2.  The device being charged can sense the capabilities of the charger using some simpler or "analog" code on the center 2 lines.  If the 2 center lines are not connected to anything the device may assume that it is a slow charger.  If the 2 data lines are shorted together inside the charger, it may indicate that the charger is capable of a higher rate.  Apple uses a non-standard system of applying certain predetermined voltages to the data lines.

3.  The last method (the dumb method) is simply to try to pull a certain current and see if the voltage gets pulled down or drops out all together.

The problem is that not all USB power supplies, or all devices, try each form of communication.

Protocol Selections Switches

The device pictured above, a Dual USB charger, Bluetooth Keyboard and Speaker system, has switches that allow you to select different charging protocols, so that more devices will recognize it as a high-capacity charger.  Other devices may have separate USB sockets for Apple devices, and other devices.

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Communications Break Down

If we connect our phone or tablet to a generic or 3rd party USB power source, we don't know which of the "languages" each device can speak.   But wait... It gets worse.  In the rush to market, and in an effort to sell a device at the lowest possible price, the portable device might not even negotiate properly with the charger that it came with.  The device manufactures buy the chargers from China, and just toss them in the box with the phone or tablet.

In either case, the device will default to a safe, slow charge.  While it is important that a device charges quickly, it is far more important that the charger does not melt and run down the wall, burst into flames or electrocute the customer.  Manufactures are very aware of doing business in the "Nation of Litigation", so they error waaayyyy over on the side of safety.

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Paying for Every Electron:
Modern power supply design has resulted in some very small, and very cheap circuits for converting household 120 Volt AC power down to the USB specified 5 Volts DC.  The circuitry has changed, but the laws of physics have not.  A circuit that can handle more power still must be made with "beefier" componets.  Better components mean more cost.

Similarly, the design effort and components needed to properly implement the protocols listed above cost money.  Chinese manufactures building USB chargers that wholesale for about a dollar each can't afford the research and development to correctly implement every protocol.

So we end up with some USB adapters that can't put out enough power to fast charge our phones or tablets just because they are built with components that could never handle the current, and others that could produce more current, but don't know they are being asked to, because they don't speak the language.

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Look at all those Electrons!:
So if we cannot see the electrons flowing through the wire, how can we tell if our phone is fast charging or slow charging?  How can we tell if it is charging at all?

Without special Apps, our phones don't tell us much about what is going on.  Some devices will report "Not Charging" if a USB connector is inserted, but the phone cannot negotiate a proper charging current.  It should be noted that in this case, the device may actually be charging, but at a very slow rate.

Here's a couple of screen shots from Android that show "Charging (USB)" or "Charging (AC)":
Android Screenshots

From this, we might assume that "Charging (USB)" means slow charging, and "Charging (AC)" means fast charging.   Well... Unfortunately it is not that simple.  Depending on the USB port that it is connected to, it could possibly say "Charging (AC)" when connected to a PC, especially if the PC has special "Dedicated Charging Port" (DCP) USB connections, and depending on whether the PC was awake or asleep.  In rare cases, it could actually being charged faster when it says "Charging (USB)" than when it says "Charging (AC)".  I've also seen it simply get it wrong.

We can also just wait a few hours, and see how much the battery has charged.  Now, HQ is in a pretty boring town, and we actually joke that one of the most exciting things to do is sit around and watch the LiPo's (Lithium Polymer Batteries) charge; but if you are in a bit more of a hurry, there are inexpensive devices you can use to see exactly what your charger is doing:

USB Doctor

The USB Doctor is a simple and cheap way to accurately tell what is going on between our phone or tablet and it's USB power supply.  The Red LED display alternates between Voltage and Current.  The center 2 USB connections (the data lines) pass through unaltered, so it does not effect the negotiation between the device under charge, and the USB power source.

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Let the Craziness Begin:
OK, Great.  So I've got a little 3 dollar gadget that displays Voltage and Current on a "Retro" LED display.  How does that help anyone?

Take the two Dual USB charger blocks shown below:

Dual USB Chargers

The smaller one (green) claims to be rated at 2.4 Amps (that would be 12 Watts, since 5V x 2.4A=12W), and the larger one (black) is rated at 2.1 Amps (Usually rounded to 10W on the package).  That should make you suspicious right there.  It's possible I could fit components rated at 12W in that smaller package, but go with your intuition on this one... If if looks wimpy, it usually is.  Unfourtuanately, price is not an indicator of performance, as we will see.

Sure enough, I could not get the smaller one (Walgreens, US$ 15.00) to produce more than 1A (as indicated on the Charger Doctor) no matter what I connected to it.

But when I connected 2 devices to the black one (GE branded, from Target, $US 9.99), it happily produced over 1.5 A without even getting more than slightly warm.

Now here's where the craziness sets in.  When it charged my Nexus Tablet by its self, it would not charge the Nexus 7 at more than about 0.6A.  When I simultaneously connected another device (In this case, an Chinese Android Gaming Phone), the Nexus started charging at almost 1A.  Calm down Mr. Kirchhoff, we're not breaking any of your laws.  By plugging in the Phone, it simply negotiated in such a manner that the GE power supply decided to produce more current.  Since the 2 USB connectors are really just wired together, the change also affected the Nexus tablet.

USB Charging Craziness

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How do we Deal with all the InSaNiTy?:
Here's a few tips to make sure you get Watt you pay for:
  1. Avoid "cutesy" colorful AC and 12 V (car) adapters -- even if they match the bottle of hair coloring you bought at the same store.
  2. Addendum to above - Don't buy any electronics at stores that also sell "Safety Green" or "High-Vis Pink" hair coloring.
  3. Read the packaging.  Look for "2.1A" or  "10 Watts".
  4. If you see an adapter sold one place, and it is rated at 1 A, and you see what appears to be exactly the same charger at another store marked "2.4 A", be suspicious -- be very suspicious.
  5. Stick with name brands.  These days, it's all made in unknown factories in China, but you have a little bit better chance of accurate specifications if the packaging bears a name brand.
  6. Avoid products with proprietary or non-standard cables or connectors.  It will make it very hard (or slow) for you to charge the device without a charger and cable specifically designed for for that device.

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Is Charging my Phone from an Unknown or Public USB Charge Port Safe?:
You're in the Airport, your flights have been delayed all day.  You've been emailing, calling and texting, and your about to board that last 4 hour flight to your destination.  Without some entertainment, or at least some music or an ebook, you're probably going to have to be sedated long before final approach.  Just then your normally trusty Android says "Please Connect your Charger".  Your charger is in your checked lugage, but there is this funky little table/kiosk thing with USB outlets all around it. Your tempted to plug in, but you've heard that every file on your phone, along with contacts, texts, and passwords can be grabbed through a USB connection.

Yup, it's true, an entire Android or iPhone hacking suite, and wireless internet router can be hidden inside a USB wall-wart (or a USB equiped hotel lamp, or clock radio, or...).

So are those intimate selfies safe?  (Or perhaps, depending on your physique, I should ask: Is the internet safe from your intimate selfies?)

There are some settings you can use to prevent some of these, but most people don't know what to look for.  Even worse, in an attempt to simplify things, most of the options have been removed from the latest versions of Android (and were never there in iOS).

Similarly, the phone may ask for permission, or present a warning, but again, not everybody knows what to look for.  By the time you notice the little Icon at the top of the screen, a dozen hard-working civil servants at the NSA may already have been privy to (or subject to) your immitation of a Victoria Secrets fashion show.

So the short answer is:  Unless you know what you are doing, avoid unknown chargers and charging ports.

Another thing you can do is carry a Charge-only USB cable with you.  Without those 2 center connections, it is impossible to establish any sort of USB data connection.  You get the 5V to charge your phone, but if you do happen across a hacked USB device, the hackers won't get your personal data.

With a little soldering skills, or even just a pair of tiny pliers, it's easy to butcher a USB cable, and make one.  You may also be able to buy one:

USB Charge-only cable

Here's a USB Charge-only cable that was included with a cheap USB car charger.  Notice that the intermediate connector (the round one) has only 2 connections -- the inside and the outside.  With only 2 connections, a USB data connection is impossible.  Especially when traveling to certain countries with a reputation for espionage, such a cable is essential.

Do note, however that without the data wires connected, Options 1 and 2 above cannot take place to negotiate a charging rate.  Only number 3 will apply, and that could result in very slow charging.

Note also that the above only stops hacking via the USB connection. It does not stop wireless eaves dropping, phishing, malicious apps that are already installed, etc.  It also will not stop someone who gains physical access to your phone.  Never leave your electronic devices unattended in a hotel room.

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With a little knowledge, and a few inexpensive tools, you can make USB charging work for you, not against you.

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