Small consumer batteries (AA, etc.) are often rated in terms of mA-hrs. For the sake of a small school simulation project, I would aim to make a convenient idealized model of the battery. Be sure to describe your model and the assumptions you make!
Specifically, look up battery discharge curves which show voltage vs time at a particular constant current discharge. Take one and transform it into voltage vs. (mA-hrs of depletion). Now you have a useful relationship: if you can keep track of how many mA-hrs have been pulled out of the battery, you can determine how much voltage it will have.
So you'll code up a way to keep track of cumulative mA-hrs. You'll allow the user to choose different constant resistances as the load. Then you'll simulate time moving forward. Current = Voltage/Resistance. Determine mA-hrs of depletion since previous time step and accumulate the total. Use the total to determine voltage for the next time step from your discharge curve data. Advance to the next time step and repeat.
Something like that. That should be enough for a small school project.
-Kevin P
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