Terminal voltage of a battery refers to the potential difference measured across the terminals of a cell or battery. When the battery is connected in a circuit, the terminal voltage depends on the total resistance of the circuit (also called the "load").

For a given cell or battery, as the load decreases, the current in the circuit increases and the terminal voltage decreases.
V = emf - I*r
emf is the electromotive force of the cell and it is a constant. r is the internal resistance of the cell.
As I increases, the product I*r increases and V decreases.

An appropriate graphical representation of the equation V = (-r)*I + emf allow s us to find r and emf.

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