Jon has written an article detailing his portable battery harness project:
This year, I’m building a new version of the battery harness, one that is portable. The idea is, you attach it to your phone, and spend the day wandering around, doing whatever it is that you do, and all the while the battery harness is logging power usage data to a micro SD card.
I assume the people reading this are more interested in how it all works, so I’ll dive down into the details now.The battery harness basically pulls the battery out of the phone, and provides a secure mechanical adapter to power the phone from the battery, while allowing the positive line to be intercepted, between the battery and the phone. This shunt goes to the ammeter (a custom board that I designed and built), and the ammeter measures voltage drop across a resistor to get instantaneous current.
Project info at Jon’s blog.