iPhone Car Charger
Overview
The iPhone (and also some other devices) does not charge when connected to a standard car USB power adapter. These adapters provide 5V at VCC and GND of the USB port. The data lines (D+ and D-), however, are not connected. For the iPhone to start charging, it is sufficient to provide about 2.5V at the data lines, which can easily be achieved with a voltage divider consisting of 2 100k Ohm resistors. This page documents how you can make your car USB power adapter iPhone compatible.
Update
ladyada provides a slightly different schematic taken from a reverse engineered Apple charger and uses in their awesome Minty Boost kit
Instructions
- Open the car adapter and find the USB connector. The outer pins are VCC and GND. Verify with a voltmeter. The middle two are the data lines.
- Solder both data lines together.
- Connect an 100k Ohm resistor to both GND and VCC and connect them to the data lines, effectively building a voltage divider.
- Done. Put the adapter back together and give it a try.
Images
Car USB power adapter without cover
PCB from below, USB connector at the right.
D+ and D- connected together and to GND and VCC.