nexus 7 battery repurposing
It may be a little easier than suggested above. I agree 100% with them - do not just add power or you will likely be sorry!

From your picture above, it is a little fuzzy, but it looks like the battery says it is a 3.7 nominal 4.2 volt max battery which means it is ONE cell.  So all of the worries about balancing etc don't play here as there are no series cells to balance.

Why the extra wires?  Usually they are to monitor for temprature, discharge rate, overdisharge protection etc.

There are a lot of $5-$10.00 CC/CV power sources on e-bay that you could hook up to the positive and negative poles to charge that battery.  Set the CV to 4.2 volts and CC to something small to start to be safe and increase it if it takes too long to charge.

Using a simple multimeter you can determine for certain which are the positive and negative nodes of the battery.

OF Course, ignore all of this if your aove picture is wrong and the battery contains multiple cells in series!!

Be careful!