Several years ago I bought a Dension Ice Link to charge my iPod and play it through my Toyota stereo. This system has served me well though a 3rd generation iPod, a 5.5th generation iPod, and a 1st generation iPhone. A friend of mine went out and got himself a sweet 3G iPhone only to find out that many of the older accessories do not work with it. To date, there are no car cradle systems that both play the iPhone through the stereo and also charge the unit.

It appears that the newest generation of iPods (4th gen nano, 2nd gen touch, 3G iPhone) do not charge through the firewire pins of the dock connector anymore. They are USB only, and not just any USB either. So many USB chargers are blissfully ignored by the iPhone.

My friend’s desire to get a car integration system for his 3G iPhone and m desire to understand what had changed with the dock connector has led me to try to understand the changes in the newest generation charging circuit.

Information is pretty scant out on the interwebs, but I did find this: http://pinouts.ru/Devices/ipod_pinout.shtml
which talks about what appears to have changed in the charging circuit, and why any old USB charger won’t charge a 3G iPhone. Cutting to the chase, the issue is that the USB dock pins are in fact used for charging, but having 5 volts present on the 5 volt pin of the USB plug of the sync isn’t enough (and ground obviously). The Data+ and Data- lines need to be held at a certain voltage for the phone to recognize that it should charge.  The Data+ pin needs to be held at 2.8 volts, the Data- pin needs to be held at 2.0 volts.

I have a craptastic generic USB car charger that would not charge my first generation iPhone.  This charger like every generic USB charger I’ve measured leaves the data pins floating.  This strikes me as a very wise decision, but makes it a non-starter to charge the iPhones.  I cracked my generic charger open and added a pair of voltage dividers form the 5volt pin to the ground pin.  I chose resistor values I had laying around to produce the 2.0 and 2.8 volts.  The pinouts.ru link suggested different values for the divider resistors, the absolute choice seems to not be critical.  Shown blow is some pictures of the process.  

This is the first step in modifying my Dension Ice Link for charging 3G iPhones.  My friend bought an Ice Link from ebay for $30 and we are going to mod the cradle for charging 3G iPhones.  Stay tuned for that later.