Instructables
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...
 
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Step 1: Background

Picture of 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.
hmdz10513 days ago

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.

RedBinary (author)  hmdz10513 days ago
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.
fkayani6 months ago
I made a video about this for people that want to see it in motion.

http://www.youtube.com/watch?v=Ro9BBCLJZnc
Forgive what might turn out to be a silly question, but why would a cigarette lighter charger not have the 2 centre pins already soldered together? It is no more able to transfer data than a wall charger!
RedBinary (author)  jasoncattnz1 year ago
I wondered about that myself, & the only reasons I can think of are that either the device would not be capable of delivering the amount of current - though the one pictured can, or that they just didn't think of it.

My guess is the latter simply because the fast rate standard is relatively new on the scene. I think that, due to the quickly increasing saturation of higher power mobile devices, it's quickly becoming more common for the car chargers to be designed with the fast rate capability. A few months ago a picked up a charger the exact same size as the one pictured here, but it has 2 ports - one can provide up to 1.5A & the other up to 2.1A. It was $20 - I need to pic up another so I can ruin it by opening it up. ;-)
There's also the issue that Apple uses a different standard (I use the term loosely); if you were to plug an iDevice into a charger with the center pins shorted, it would get very confused and either charge at the base 500mA, or worse (and not entirely out of the realm of possibililty for Apple) not charge at all.
Sorry, just read some of your details about testing with Apple devices...

While the newer iDevices seem to be happy with the shorted pins, companies making "universal" chargers will likely default to the open-pin standard rather than risk some older devices not being able to charge at all on there (and thus drawing the ire of consumers who thought the universal charger they bought would work for all their toys).
cuth_bert1 year ago
Where is the fuse on this device. I looked at mine and I'm guessing it's one of the chips. Mine stopped working and I assume the fuse blew.
RedBinary (author)  cuth_bert1 year ago
I don't know that there's any fusing in any of the ones I've opened. Funny coincidence that you mention this however - I just today had to change the fuse in my car's lighter circuit!
Robot Lover2 years ago
Pretty good instructable. There is one thing though. If you do this mod, you will most likely not be able to charge apple products. This is because you need each of the solder points you soldered together to go to a resistor and then to ground.
RedBinary (author)  Robot Lover2 years ago
Thanks for the kind words! I had come across some info regarding the Apple charging spec back when I made my linear charger. Based on what I'm reading now, this *should* only apply to older (pre-2007) products that were made before the USB standard introduced dedicated charging ports. [http://en.wikipedia.org/wiki/USB#Charging_ports]. I would think that things manufactured after that point would have to conform to the standard. This makes me think that I need to have some of my iOS-carrying friends try the chargers.

It makes me wonder then if all of the USB wall-wart supplies that have proliferated in the last few years will only charge Apple devices at the slower rate?
I am pretty sure that even the newer apple products require resistors. I tried charging my ipod nano on a non-apple charger and it did not work. I opened up the device to find that there were no resistors on the data lines. Plus, my nano is post 2007. Anyway, good hack!
RedBinary (author)  Robot Lover2 years ago
Another coworker just brought his old iPhone 3G in for us to test. We hooked it up to a cheap generic USB wall charger. The app "Battery+" reported battery at 49% with 2:20 left to charge. Then taking it out to the modded charger in my truck the app reports 50% with 1:20 left to charge. Based on this I'd say that it is charging at the faster rate on the modded charger while not on the wall charger. YMMV
RedBinary (author)  Robot Lover2 years ago
Just a an update: a coworker & I tried hid iPhone 4 with the charger pictured in this instructable & it worked. At what rate it was charging I couldn't tell, however.