The simple answer is that in a battery the chemical reactions that produce the electrical current are produced from materials that are already in the battery itself, whereas in a fuel cell, the reactants, almost always Hydrogen and Oxygen are fed to the cell like an external fuel.
. However, while a battery makes electricity from the energy it has stored inside the battery, a fuel cell makes its electricity from fuel in an external fuel tank This means that while a battery may run dead, a fuel cell will make electricity as long as fuel is supplied. For hydrogen fuel cells, hydrogen is the fuel and it's stored in a tank connected to the fuel cell. When hydrogen in the tank runs low, you refill it, or replace it with a full tank.
