Getting Faster USB Charging from an In Car Charger

The other day I was driving to work with my brand new Google Nexus S Android phone on the dashboard, as usual. A new version of Google Maps had just come out, and coupled with the fact that I had just got a new phone, I decided to test the navigation.

Nexus S

Google Nexus S

When I got to work, I noticed the battery level on my phone was lower than I would have expected, so I viewed the battery discharge graph, and was surprised to see that the battery charge had been depleting all the while I was navigating with the screen on, even though the Nexus S was connected to the in car USB port, and charging all the while. My in car USB port is rated at 1A, but something was surely up.

After some Googling, I came to the conclusion that the phone was thinking it was connected to a standard USB port, not an AC adapter, and was therefore limiting itself to only draw 500mA of current, irrespective of what the charger can deliver, to remain compliant with USB standards (where a computer USB port can only deliver a maximum of 500mA).

To confirm what I had suspected, I used the Nexus’s diagnostic mode. To enter this mode, I simply tap the code *#*#INFO#*#* into the dialer. When the main diagnostic screen appeared, I clicked “Battery information” to see what source the Nexus thought it was connected to. Sure enough, when connected to the in car USB, this screen was reporting “USB”, meaning that the phone is not able to take advantage of the 1A output of the car charger.

So what I wanted to do was make the phone think it was connected to an AC adapter. To do this, the data lines on the USB connector (D+ and D-) must be shorted together. this signals to the phone that it is not connected to a computer and then the phone will go into fast charging mode.

I then took a cheap micro USB cable and used a multimeter to check the connections on the cables, as we don’t want to short out the power + and – lines, that would be really bad! Once the correct cables were determined, I simply cut them, and soldered them together, as shown in the image.

USB

Shorting the data lines

After putting the case back on the connector on the cable, I connected the phone back up to the car’s USB port, and sure enough, the phone reported itself as connected to AC as the charging mode on the INFO Screen.

Testing this the next day, the battery graph showed a slight increase in charge over the course of the journey rather than the slight discharge of before, so the hack was a success.

So now I have a cable specifically for using in the car. I do not recommend using this to connect to a PC or other “real” USB port, just in case!

About Tris

Tris is a software engineer with a PhD in image analysis. He likes real beer, photography, electric guitar based music and Unix operating systems.
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14 Responses to Getting Faster USB Charging from an In Car Charger

  1. Pingback: Anonymous

  2. Diekolade says:

    Thanks a lot for this, I really need that for my android . Thanks again.

  3. Shibu says:

    I just did this today before reading your article. I shorted out the green & white wires & left the black & red alone. In your pic it looks like you shorted out the white & black. Aren’t they all standard colors?

    • Tris says:

      Hi Shibu,

      It seems that the colours should be standard, Red and Black for +ve and GND, then Green and White for data, however, looking at my picture again, the colours seem to be Green, Red, Yellow and White.

      To determine which lines were which, I used a continuity tester connetced to the pins on the other end of the cable, to be sure that I had the correct wires.

      Hope this helps,
      Tris

  4. Tom says:

    Hi,
    I have the same problem and I am very glad to find solution on your website.
    Fortunately it is not so difficult to make the wire :-)
    However I am puzzled about typing info in your dialer. I can type *#*# but cannot type info… since there are only numbers showing… do you know the trick how to type in dialer letters not only numbers? I was trying to hold the key the same way when in old phone when typing text message using alpha numeric keypad but this faild….

  5. wildfiyah says:

    I saw a similar hack, but done differently. The person opened up the USB charger itself and soldered the two terminals to remove the limitation. On putting it back together he had a higher output as well. In his situation, the charger wasn’t putting out enough juice.
    However what you’re saying is that it was the cable in your scenario. Do you think you could have had similar results using the above mentioned method? that perhaps without cutting up the cable, it would not be receiving a signal on those two wires and therefore take in more current? Definitely would like to understand this better.

  6. Tris says:

    The two wires that I shorted do not carry any current that the device can use for charging. They are signal wires used to carry data (when syncing for example). Shorting them works with Nexus phones because that’s how they work. – they assume that if this is shorted then it is not connected to a USB port on a computer but to an AC adapter.

    I’m sure there are ways of modifying chargers to get more current as well, but I am not aware of any.

    On thing interesting to note is that Apple devices like iPhones, iPods etc look for voltages across the two data lines in order to decide how much current to draw, as can be seen on ladyada:
    http://www.ladyada.net/make/mintyboost/icharge.html

    Thanks,
    Tris

  7. Giorgos says:

    The D+ and D- must be shorted from the phone side or the charger side?
    If i have a whole charger without separate usb cable, can i use a separate usb cable (with
    shorted D+ and D-) which would conected to charger output?
    +5V and GND of usb cable connected to charger output and D+ D- shorted together in phone side

    • Tris says:

      You can connect the data lines on either side. Make sure you do not connect the + or ground to anything else, only connect the data lines together.

      • Giorgos says:

        ok, i’ll connect the data lines in phone side. it is safe for phone, right?

        • Tris says:

          All I can say is it never harmed my Nexus S as far as I can tell. Of course your experience may vary, and anything you do to your phone is ultimately your responsibility.

  8. Giorgos says:

    My car charger has already “AC charge” so, i dont have anything to do. right?
    But the charge still not working after few minutes. may this happens because
    the phone are heated from the sun? i check the battery and the temp reached 59 celcius.
    it is possibly the charge stop from high temp?

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