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. In battery types where EMF remains approximately constant during discharge, but resistance increases, voltage across battery terminals is not a good indicator of capacity.
When considering how a battery discharges, its electromotive force (EMF) may decrease or internal resistance rises; in most cases, the EMF stays relatively stable throughout the discharge process. The actual voltage supplied is determined by the drop across the internal resistance. In some situations, it's not possible to accurately determine the remaining charge using a voltmeter when the battery isn't connected to a system monitoring usage. In batteries where the EMF remains steady during discharge but internal resistance increases, the voltage at the terminals does not provide an accurate indication of capacity.