1.5 volts

Curiously, I was doing some reading on Batteries. Of course all you old timers and electronics guys know that a "Battery" has
to be more than 1.5 volts. A battery by definition is composed of 2 or more "cells" (1.5 volts each). So any combination of
Nickel, Zinc, Copper, etc. and whatever electolyte will give the same potential difference. With a few exceptions (Like Lithium is
1.3V or something like that)
What I want to know is WHY a chemical battery always produces approximately 1.5 volts? A quick and dirty chemistry lesson is
all I want, so I can impress my co-workers.