Voltmeter measuring electromotive force of a battery
In your model, the battery is a voltage source with a resistance in series.  When the battery is delivering no current, there is no current thru this internal resistance, and therefore the voltage it drops is 0.  The open circuit voltage of a battery is therefore the same as its internal voltage.  When you put a load on the battery, current flows, the internal resistance drops some voltage, and the external voltage is then less than the internal voltage.

Voltmeters are designed to have high impedance.  They present such a tiny load to the battery that we can ignore it for practical purposes.  The internal and external voltages of the battery are therefore the same for practical purposes when measured with a voltmeter.  Put another way, measuring a battery only with a voltmeter is measuring it under open circuit conditions.

I am referring to the internal voltage as being that produced by the cell directly, and the external voltage as that minus whatever is dropped across the inevitable and unavoidable internal resistance of the battery.