The electrochemical cell operating irreversibly or reversibly

A battery is an electrochemical cell operating irreversibly, and is capable of generating a current and a potential difference. The potential difference is set up by virtue of the chemical reactions occurring at the electrodes of the battery. This potential difference can be measured by connecting a voltmeter across the terminals of the battery. When current is taken from the battery, e.g. in electrolysis, it is operating irreversibly. 

however, the battery is connected to another cell and the potential difference is balanced by an equal and opposite potential difference the cell can operate reversibly (see Sections 9.3.4 and 9.3.5 below). 

The chemical reactions occurring externally by virtue of the battery generating a current are studied in electrolysis. In contrast, the chemical processes occurring at the electrodes of the cell itself are the subject of emf studies. 

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