A charging cable for Android smartphones

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Sat, 2011-06-25 12:13 by Hans · Forum/category:

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Background

Modern smartphones and other phones are charged through a micro USB socket and cable. However, the USB standard introduces a problem. USB provides only an insufficient maximum current of 0.5 A (at 5 V), and even that only after a negotiation with the computer. Without negotiation the limit is some 100 mA.

Even worse, when a device exceeds its limit, the computer is supposed to, and many do, switch off the USB port for good, until the computer is rebooted.

This means that ingenious algorithms, like trying to pull a higher current and reducing the demand if it is not delivered, would not work. A typical smartphone must not attempt to pull more than 500 mA from a USB port to stay out of trouble.

But 500 mA are not enough. Most contemporary smartphones have nominal charging currents of 1 A, 2.1 A, or even up to 3 A for quick charging. Indeed, 500 mA are not enough to run the phone with heavy-load applications running, particularly navigation, which also keeps the screen lit up continuously, consuming even more power. In such a workhorse mode the phone consumes more than 500 mA, so even with standard USB charging of 500 mA it runs down the battery.

But the mains charger that comes with the phone somehow achieves the feat of quick-charging the phone with nearly 1 A or 2.1 A, although Ohm's Law explains that there is no way to "push a higher current down an electricity consumer's throat" without raising the voltage, which would kill a smartphone. So how can it do that?

The answer is that different manufacturers use different tricks, all outside the USB specification, to signal to the phone that it is allowed to pull a higher current while it is connected to the charger. These are some of the tricks:

  • A shortcut or a less than 200 Ω resistor between the data lines 2 and 3 (HTC and many others), which is prescribed by the Common External Power Supply (EPS) standard, also known as EN 62684:2010 and IEC 62684:2011
  • A shortcut or low-Ω resistor between line 4, which is otherwise unused, and line 5 (ground)

    Note that only micro USB connectors have 5 lines. All others (short of USB 3) have only 4.

  • A digital signalling protocol on the data lines (Apple)
  • Other methods I do not know

Of course the mains charging adapter that comes with the phone provides the matching method, so you don't have to worry about that and can even use any normal USB data cable.

But you do have to worry particularly when you obtain a car charger, typically a small device that plugs into the cigarette lighter socket and provides one or several USB type A ports with 5 V power for charging.

Since the manufacturer of such a charger has no clue which device you connect it to and since no manufacturer is honest enough to explain the predicament, so as not to curtail sales to unsuspecting users, you cannot expect the charging current to be more than 500 mA.

The only exception is a car charger for a particular device, typically sold by the manufacturer of the device for a relatively high price. A few car chargers are rumored to have the data-shortcut method already built-in, but check carefully to make sure.

This article describes one particular method to make a maximal-current charging cable for HTC, Samsung, and other smartphones and for chargers that do not have the device-specific capability, particularly for car chargers.

Obviously, shorting the data lines in a USB cable renders it incable of any data transfer, so you have to sacrifice one cable and use another one for data.

Quick and easy solutions

The best and easiest could be a Micro-USB charging cable made by portapow and sold in different lengths from 30 cm to 2.5 m through Amazon and other dealers. Search your Amazon for portapow. They seem to be nearly the only ones who really understand the problems of USB fast-charging.

Not only do their cables contain the required shortcut, but they also contain much thicker power wires, required for charging with currents up to 3 A. Note that these cables have no data wires and can only be used for charging. Note also that almost all commonly used USB cables are too thin for high-current charging. Usually the devices switch down to a lower current when they encounter a thin (too-high-resistance) cable.

There is also a fast-charge adapter that contains the required shortcut. I found it on the German Amazon web site, ordered and tested it. It works perfectly well, but is too thick to permit its use on chargers with two parallel USB ports that are too close to each other.

USB charging adapter on the German Amazon web site

It is strangely named, "Adapter (USB) für Samsung Galaxy Tab", and its function is never explained—an utterly ineffective way to sell a product. Sadly, the concealment of technical data is very common.

Recently a similar adapter by portapow has appeared on the German Amazon web site. It fulfills the same purpose.

With the availability of these cables and adapters the need for the following description is greatly diminished. I may remove it soon.

Requirements

To make your own cable, you need:

  1. A car charger or other charger that so far charges only with 500 mA, but can provide a higher current, like 1 A or more. In fact, you do not yet need the charger to make the cable, but of course you need it later to test and use it.
  2. A micro USB cable, i.e. a cable with one plug that fits your charger and a micro USB plug for the smartphone on the other end. For a typical car the cable should be at least 1 m (3 ft) long, so you can use the phone with the cable connected to the charger in the cigarette lighter socket.

    USB data cable with one micro USB plug
    USB data cable with one micro USB plug

  3. A soldering iron with a small tip
  4. A tiny bit of solder
  5. A little bit of soldering flux
  6. A very sharp knife
  7. Some sticky insulating tape

Cut

Take the sharp knife and cut the USB cable lengthwise as shown below.

Be careful. Depending on the quality of the cable it can have a good outer screen that you will have to unwrap to get at the actual 4 power and data lines, or it may have no screen at all, presenting the danger that you cut into the wires. Cut a little bit first and try to peep inside.

An unscreened cable is normally of unacceptably low quality, but it works well enough as a charging cable.

You can apply the cut wherever you like. I usually cut in the middle, half way down the cable.

Pull out the wires. You may have to open up the screening first. Try not to tear the screen apart. It should still be electrically connected along the cable.

Cable cut with wires pulled out
Cable cut with wires pulled out

You can click on the pictures to enlarge them.
Then use the back button of your browser to get back to this article.

Now you have to strip the insulation from the two data lines.

In all cases I have seen, the data wires were white and green. If you can measure, try to make sure yours are not differently colored, by measuring the resistance from each wire to the two inner contacts of the USB A plug, the bigger one. You can do that after stripping the insulation (see below) or by pricking the wire with a needle.

Do not try to work on both wires at once. Do one first, then the other. Push something with a hard surface in between the wire you want to work on and the rest, as shown here.

Preparing to strip the insulation off one wire
Preparing to strip the insulation off one wire

Take the sharp knife and very carefully carve off a bit of its insulation. It is better to do it with several attempts than to cut into the copper wires once. You don't want to cut through the cable.

Actually, you could cut through, because the requirement is only that the two data lines are connected on the micro USB plug side. The wires from the USB A plug, the bigger one, could be left dangling. But I never did that because the thorough connection on both sides cannot hurt and is mechanically and electrically stable. You can still go this way, however, if you accidentally cut through a data line.

Insulation carved off on one side of one wire
Insulation carved off on one side of one wire

Keep carving off the insulation on all sides, then do the same to the other data wire.

Insulation removed from both data lines
Insulation removed from both data lines

Solder

Heat up the soldering iron and use the soldering equipment to place a tiny drop of solder over the two data line contacts.

Soldered
Soldered

If you have no soldering equipment, you could achieve a reasonably good connection by stripping the insulation over a longer distance, like 5 cm (2 inches) and drilling the two lines. Push a shortened match between the two lines and rotate them at least for 10 revolutions. Then pull out the match.

Put it back together

Cut off a piece of insulating tape that is longer than the stripped distance of the two data lines, pull it through between the two data lines and the rest, and arrange it lengthwise. Then wrap and close it around the uninsulated part of the wires.

Insulated with tape
Insulated with tape

Stuff all wires back into the screen and the outer cable mantle. You can leave the insulating tape sticking out.

Wires stuffed back into the cable
Wires stuffed back into the cable

Finally wrap it all up with more insulating tape.

Wrapped up
Wrapped up

Of course now it doesn't look like new any more, but, as a redeeming fact, at least you cannot mistake this cable for a normal data cable.

Test

After you have done all this, you can try to measure your success, if you have an ohmmeter. If not, you have to have trust in yourself.

However, it cannot hurt to know where in your car the circuit breakers are, and it also cannot hurt to have spare circuit breakers on hand.

  1. Go into the car. Do not yet plug the USB type A plug into the car charger.
  2. Connect your mobile phone to your newly modified USB cable. Problems are unlikely to show here already.
  3. Plug the car charger into the cigarette lighter socket.
  4. Switch on the electricity in the car, if necessary. Most car chargers show a light to indicate available power.
  5. Slowly plug the USB type A plug into the car charger while watching and listening for sparks, smoke, or the lights going out. Good luck!

If everything worked according to plan, you now have a maximum current charging cable and can use your smartphone to navigate non-stop, while its battery charge goes up to and stays at 100%.

On an Android phone, while charging, you can immediately check whether it all works, by entering the following command:

  1. Menu, if the phone still has a menu button
  2. Settings
  3. About phone (at the very end of the list, scroll down)
  4. Status
  5. If Battery status shows "Charging (AC)", you know that it works. "Charging (USB)" would indicate a failure.
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