A little while ago then I stumbled on this very good article: http://www.ladyada.net/make/mintyboost/icharge.html
In which they decribe how they produced a battery powered USB charger. After reading that article I took my cheap USB charger and decided to modify it.
This will be a really, really easy modification and I think that anybody with a soldering iron could do it.
Step 1: A little Theory
Nowadays the iPhone expects a certain voltage on those two pins to decide how much current to absorb from the charger. Putting a 2.0 V voltage on both the pins the iPhone will absorb about 500 mA, while with 2.8 V on D- and 2.0 V on D+ it will absorb about 1000 mA.
The same behaviour I expected to be observed on my iPod.
On the images there are the schemes for the two configurations. As you can see, using an opportune couple of resistors it is possible to get the voltage required. Obviously the 1000 mA configuration is better if you want your phone charged quicker, but it is possible that your power supply can not support that much current.
Step 2: Opening and checking
The cases were glued together so I had to break open them. The Car USB chargher, though, had the top that was screwed on.
I was happy to find fuses inside the two: my iPod should be safe!
As you can see on the pictures the data pins are floating and that is why my iPod did not want to charge.
The specifications of the stickers on the items say that the wall plug can provide no more that 500 mA while the car charger no more than 1000 mA. I decided to stick to 500 mA for both of them.
Step 3: Resistors
Both the pins should be at the same voltage thus only 2 pairs of resistors are needed.
Step 4: Drilling holes
The car charger has pleny of space so I did not need to drill holes.
Step 5: Soldering
Step 6: Checking
Step 7: Testing
While charging the Samsung Galaxy, though, the charger got really hot. I am not sure if it is good or not, since I have never used it without my "fix". I should measure the current that the Galaxy drains, to see if it is in the parameters of the charger.
My conclusion is that: It Works!