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An ideal battery or cell has no internal resistance, ie., r = 0.

So there is no voltage drop across the materials in side the battery, when it is connected in a circuit with some external elements.

When a battery is not connected in a circuit or it is open,  then there is no current passing through it.  So the emf is the potential difference observed by measurement.

When a battery has an internal resistance  r,  and it is connected through a circuit, there is a current I flowing in the circuit.  Hence, there is a voltage drop across this internal resistance.    So P.d. across battery =  emf - r I.

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