How to detect the State of Charge of an iPhone's battery?

[arzan]

I am building a battery pack charger (Li-Po) for the iPhone, and need to know a way to detect the state of charge of the iPhone's internal battery, so that I could stop charging it when it's completely charged.

The battery pack charger will connect to the USB. While connected to the USB, there are two batteries that need to get charged. The iPhone's internal battery, and the battery pack inside the charger.
When plugged in, I want the current from the USB to first charge the iPhone, and then, once the iPhone is completely charged, direct the current to the battery pack.

Essentially I need a circuit or component that can help me detect whether the iPhone is fully charged or not, and always direct current to the iPhone if it is not a 100% charged