Today I discovered that all smartphone chargers are not created equal. Let's start back with the basics of how USB connections work. In a USB 2.0 cable there are 4 wires and 4 contacts on the connector. The two outside contacts supply power to the device that you're plugging in, 5V at 500mA. The two inside contacts provide the data connection. Don't believe me? Take a USB cable with a Standard A male connector and place a piece of scotch tape so it covers the two center contacts. Now plug the cable into the device and into your computer. The computer will act like the device isn't even there. Wikipedia
can provide loads more information about this.
Now on to the interesting stuff. Android smartphones can be charged through a standard AC wall charger or through the USB port on a computer. The phone will charge at a different rate depending on what it is plugged into. The phone discovers what it's plugged into by checking to see if the two data pins in the USB connection are shorted together. (iPhones use a different technology to determine how fast they can charge.) If the phone detects a short between the two data pins, it thinks it's plugged into an AC charger and will start charging pretty quickly. If no short between the two pins is detected, the phone thinks it's plugged into a USB port on a computer and will only draw 500mA. Even if you are in fact plugged in an AC wall charger and not the USB port on your computer, if the phone doesn't detect that short, it's only going to draw 500mA. Even if you have a 2.1A charger, if the phone doesn't detect that short, 500mA is all you're going to get.
The screenshot below on the left shows that my phone detects a USB connection where the one on the right detects an AC charger connection.
Most of the time this isn't going to be that much of a problem. Your phone is just going to take longer to become fully charged. Where I was running into problems is if I tried to use Navigation while in my car. My car charger couldn't keep up with the amount of power that my phone was using. After a lot of frustration I discovered that the charger couldn't keep up with the draw from the phone because my phone thought it was being charged through a USB connection and was only drawing 500mA.
This is the innards of the charger just after I got it open.
What I did to take care of the issue was to open up my car charger and solder together the two center pins from the USB connector. It was about a 5 minute fix that seems to have solved the problem. The only thing left to do is to get some super glue and glue everything back together. After the charger is glued back together, no one should be able to tell I ever did anything to it. The problem could have been avoided completely if I would have been paying a little more attention when I bought my charger. Oh well though. You live and you learn.