System and Method for Diagnosing Degradation State of Battery

A diagnosis system for diagnosing a state of a battery includes the battery and a diagnosis apparatus configured to diagnose the state of the battery, in which the diagnosis apparatus is further configured to predict a side reaction rate of an electrode, based on an open circuit voltage (OCV) model defined with a state of charge (SOC) of the battery and an accumulative side reaction amount of the electrode and predict a degradation state of the battery based on the side reaction rate.