at Linuxslate.com, 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
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:
- Why does it sometimes
seem to take a phone hours to charge, and other times it seems to
charge up rather quickly.
- What's the difference between a slow
and a fast charger? Will a "Fast Charger" charge all phones more
- Will I burn out my phone or it's battery if I charge it
with the charger that came with my tablet?
- Can I use my friend's charger for his HTC phone on my Samsung phone (or vise-versa)?
- Can someone steal the
pictures and other personal data from my phone if I use an unknown charger?
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
(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
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
The problem is that not all USB power supplies, or all devices, try each
form of communication.
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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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
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
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.
Look at all those
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)":
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, linuxslate.com 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 (Li
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
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.
Let the Craziness Begin:
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:
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
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.
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do we Deal with all the InSaNiTy?:
Here's a few tips to make sure you get Watt
you pay for:
- Avoid "cutesy" colorful AC and 12 V (car) adapters -- even if
they match the bottle of hair coloring you bought at the same store.
- Addendum to above - Don't buy any electronics at stores that also
sell "Safety Green" or "High-Vis Pink" hair coloring.
- Read the packaging. Look for "2.1A" or "10 Watts".
- 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.
- 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.
- 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?:
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
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:
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.
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.