If this is a summative project, you'll want to talk about a few things;
How does a battery work?
What's in a battery?
How each component of a battery is measured (mass, concentration, density maybe?)
How the change in concentration works to increase (or decrease) voltage
There's different kinds of batteries too...I don't know if you have a particular one in mind...but this post will turn out to be an interesting one.
Good luck!