Carl Hutzler's Blog

Recently I ranted about how Apple has changed their dock connector numerous times to render old accessories useless. And while some of it seems to be necessary as I explained, other changes seem like greed.

Anyway, in my continuing saga to understand how to do simple things like get “line out” to work, I spent 6 hours over two weekends figuring out how to simply charge my iPhone 3G in the car. And this issue also affects the 3GS and most of the newest iPod Touch 2G and likely the latest Nano.

Basically, you can not simply use a standard Apple USB cable (30 pin dock connector on one end, USB on the other) to charge one of these things anymore unless it is connected to a computer. So take a standard car cigarette lighter adapter with a USB connector on it. These are commonly available for a couple of bucks and very convenient as they can charge more and more things that simply take 5V on a USB connector – blackberry’s, GPS, Jawbone headsets and more. But it won’t work with an iPhone.

Why? Well it seems a bit complicated to me, but I think I understand it. Basically the iPhone needs to know if it is connected to a computer’s USB port (limit of 500mA) or a regular wall charger (some are limited to an amp or even more). This way it “knows” how much current it can draw from the connector.

Now there are a lot of discussions in the forums about using a voltage divider network (two resistors between V+ and GND) to create +2.8V and another two resistors to create +2.0V for the USB Data- and Data+ pins, respectively. If these voltages are set on the DATA- and DATA+ pins, the iPhone will charge. And I found this to be true. In fact, the Apple wall charger has exactly those voltages on those pins which is likely where everyone got the idea.

But from playing around with it, I found that you could set both of these pins to the same voltage as long as it was above maybe 2.0V which makes it a little easier for the voltage divider network….now you just need one set of resistors – maybe 33K and 33K to give you +2.5V on Data- and Data+.

But I later found through more experimenting that simply shorting Data+ and Data- together (no voltage…just a solder joint), would cause the iPhone 3G, 3GS, last gen Nano, iPhone 2G, iPod 5G and likely many more to charge. I tested them all. They all work.

So again, all you seem to need is a standard USB port which supplies 5V on pin 1, GND on pin 4 and pins 2 and 3 (Data+ and Data-) soldered together.

I took a cheap USB charger like this one which runs $3 on sale. I opened it up, soldered the middle two connectors on the USB jack together, and viola!

So if you have an old charger laying around and your iPhone says “not compatible” you may be able to make it work. Keep in mind though that some old chargers output 12VDC (not 5VDC). These will not work with the newer iPhones/iPods because they no longer have the ability to charge from a “firewire” 12VDC input anymore. But if you are sure you have 5VDC and it still won’t charge, try soldering the middle two USB pins together and you should be happy.