 A real battery can be modeled as an ideal battery with a specified EMF, in series with an internal resistance.As a battery discharges, the EMF may drop or the internal resistance increase; in many cases the EMF remains more or less constant during most of the discharge, with the voltage drop across the internal resistance determining the voltage supplied.Determining the charge remaining in many battery types not connected to a system that monitors battery use is not reliably possible with a voltmeter.