After being uber excited to get a brand new Nexus 7 for Christmas, I quickly realized that there was a problem with my new gadget! I found that no matter what I did the battery showed 0%, even after spending several hours on the factory charger. After a couple hours on el Goog finding some similar issues it was starting to look like my new friend was doomed to experience the exchange Charlie Fox of a process.
Finally I found the iFixIt writeup for replacing the battery and I decided I would try re-plugging the battery. Once I opened the back cover I found the problem right away, the battery plug was not inserted into the socket all the way! A quick and gentle push on the plug, then snapped the back cover on and upon a restart the issue was gone!
So, if your N7 is telling you that the battery is dead no matter how much you charge or clear the battery ini, try (at your own risk) checking the battery plug.
Let it be said that I am in no way a tech repair guy or an electronics guru, just a few years of experience working with my hands.