Terminal voltage is the actual voltage between the terminals of a battery or voltage source. When current is drawn from a battery, the potential difference between the electrodes of the battery is called its terminal voltage. The terminal voltage is given by the equation V = emf - Ir, where emf is the electromotive force, I is the current flowing at the time of the measurement, and r is the internal resistance of the battery. The internal resistance of a battery is the resistance offered by the electrolyte inside the battery to the flow of electric current through it. The magnitude of the terminal voltage represents the amount of electric potential energy (voltage) that is available to the circuit outside of (external to) the battery.