Cheap and Simple USB Car Fast-charger Mod

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Introduction: Cheap and Simple USB Car Fast-charger Mod

A few years ago I had to make a fairly long road trip. Around 8 hours each way. I typically use my phone to provide music through the aux jack on my stereo, and when I travel to parts unknown I also use my phone for GPS navigation assistance. On this particular journey I had also planned to play with a new bluetooth gadget that I had just purchased.

Obviously, a phone simultaneously running 3G, bluetooth, GPS, & playing music uses a lot more power than a sleeping phone running no services lying on the seat. I soon discovered that my off-the-shelf lighter-plug car charger would not provide sufficient current to keep up. Even when only using the GPS my phone's battery was draining. After several purchases of progressively more expensive chargers I finally broke down and did some R&D.

I've since built a multi-port linear supply from scratch into the dash of my truck, but my wife wanted a no-dash-cutting solution for her car.

...more on the bigger linear supply charger in a separate instructable later...

Step 1: Background

You may have noticed that your device charges faster when using a wall charger than when using a car charger. On some set-ups that charge through USB you may have also noticed that your device knows the difference. There may be a different icon displayed when charging with one than the other. The reason for this is that some devices can charge at different rates under different scenarios.

To simplify the jargon and only address what's necessary for this article: The USB standard provides for a non-data charging connection that will provide higher current. You can get the full rundown at wikipedia

I'm going to disclaim here for a second: I'm not responsible if you melt down, electrocute, or violently explode your very expensive device, your car, your home, or your dog. These are all very real possibilities if you misuse or make any alterations to electrical devices. Use sense and caution when voiding any warranty.

That being said, the method that devices use to determine when they can charge at a higher current is through the USB connection itself. A standard USB type-A connector uses 4 pins: The 2 outside pins are for supplying current and the 2 center pins are for data. If the center 2 data pins are shorted together then it is obvious that no data can flow. This is an indication to the device that it can charge at a faster rate.

Step 2: Mod

I've found a car charger that, once modified, happily provides me with enough current to run all of the services I want. It appears to be rebranded and sold by a number of different vendors and is readily-available for under $10 at just about every chain store and interstate truck stop I've visited since. Another advantage it has over some other chargers is that when it is installed it sits back flush enough to the receptacle to allow the ashtray door to close. When selecting a charger to perform this mod on be mindful of the output current. The higher the better.That in and of itself won't charge your device faster, however. You still have to alter the charger. Personally, I would not perform this mod on anything that is not capable of providing at least 1 amp.

Once you have the charger in hand the mod is simply this: open it up, solder the center 2 pins together, and put it back together.

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33 Comments

Question: Fast Charges Qualcomme 2 exist today. did you try changing your car usb to one that have this chip? Apparently i couldn't find one article that talks about that. The problem is It's either I have to use the my car charger to charge fast or use Android Auto to mirror the phone but loose the fast charging and in those new phones No fast charging is a pain

Chargers supplying 2 amps and more are pretty common and really inexpensive now so I've just been using a cheap 2.5A. I had an antique truck that I'd made a linear supply using an LM series chip and installed into the ash tray. Soon I'll likely either be making another of those or disassembling a cheap charger and installing it directly into my dash. The cigarette plugs aren't the most stable or secure connectors and mine come loose fairly frequently.

You do not specifically mention if the device you are referring to was modified.

Thanks I found your tips very useful. But we can do this in the USB cable and then use the cable anywhere (by shoring the Data+ and Data- pins on the output side). Also on some Android ROMs like SlimRom you can enable Fast Charging mode in software.

NOTE: I have used "Charger Doctor" to monitor USB output currents in various configs and have found the slow charging culprit: "A Bad USB Cable!" The cable I used was 60 cm long and dropped both the voltage to 4.5 volts and the current to 0.19 amps while charging my Samsung Android phone (AC charging mode)!

The stock USB cable that came with my phone allowed 0.49 A in the USB mode and 0.65 A in the AC USB charging mode.

-Bad USB Cable (~60cm long):

USB Current Measurement: 0.15 A (USB) - 0.19 A (AC-USB)

- Stock Samsung USB Cable (~30 cm long):

USB current measurement: 0.49 A (USB) - 0.65 A (AC-USB)

So the USB cable is very important. The shorted the cable, the better.

II don't like the idea of a shorted cable first because the cable can no longer be used for data. Outside the car tricking the device into fast charging from a port that is not designed to handle it can potentially damage that device. Inside the car the charger is the logical place to make the modification because the charger only has 1 purpose.

Thats been my concern with shorted cables. A computers usb port can only supply 500 miliamps at most, but a shorted cable would trick a device into trying to draw more than the computers port can handle and fry a component. Secondly, when I need a data cable, I dont want to mistake a shorted usb cable for a data cable.

Length can impact the charg rate. So can the diameter of the wires as well as the type of metal in the wire (copper or aluminum), of the cable.

Each parameter above effects a cable's internal resistance, thus effectively creates a voltage dividing network with your phone that ultimately draws less current and delivers even less current.

I bought a USB charger doctor for less than $3 on ebay. They are extremely handy.

Also, don't forget that your Samsung phone (Galaxy or Note I'm assuming) charges at a higher current with the screen off so you are missing out on this information keeping the screen on. My S5 pulls around 1.1a with the OEM charger with the screen on and 1.7a with the screen off.

There are reasons why it didn't come soldered. One is although it can supply with 1A the chip will run hot and will not last long, within hours it will just failed and gives out full 13volts of whatever fuse rating you have. If your device is smart enough it will shut itself off, if not you will have a fried device.

As you can easily see, the device is rated for 1.5A continuous. The reason it didn't come soldered, is because in the early days of USB charging, smartphones use different voltage divider networks on the data[ins, to tell the device how much current it could supply. This standard has since been abandoned, and we now use the standard as written by the USB-IF, that says a dedicated power device, shall have the two middle pins shorted to signal that it's a power supplying device.