DIY External Battery

Guys, I've looked around some, but can't find the information I need for this project. I've come into posession of a friend's Nexus 7 8GB that's cracked, as I have two projects in mind for it. The first is making my own external battery pack with the battery from the tablet.

I know the battery is rated for 3.7V at 4325mAh. A standard USB input is powered with 4.5-5.5V. I'm somewhat electrically knowledgeable, but I'm stumped on how to make sure I don't fry my phone or my Nexus 7 by plugging this battery into the USB port. My other dilemma is charging the battery back up, I'm assuming that this will take a small circuit to do.

Any ideas?

Thanks,
Austin