A battery diagnosis apparatus, comprising: a voltage measuring unit configured to measure a cell voltage of each of first to n th battery cells, wherein n is a natural number of 2 or greater; and a control unit operably coupled to the voltage measuring unit, wherein the control unit is configured to: determine first to n th average cell voltages for a first rest period associated with the first to n th battery cells in a one-to-one relationship, based on the cell voltage of each battery cell measured a first number of times for the first rest period, wherein the first rest period is from a first time point to a second time point, determine first to n th average cell voltages for a second rest period associated with the first to n th battery cells in a one-to-one relationship, based on the cell voltage of each battery cell measured a second number of times for the second rest period, wherein the second rest period is from a third time point to a fourth time point, and the third time point is later than the second time point, and is a time point at which a threshold time has passed since the first time point, and diagnose a fault of each of the first to n th battery cells based on the first to n th average cell voltages for the first rest period and the first to n th average cell voltages for the second rest period. a voltage measuring unit configured to measure a cell voltage of each of first to n th battery cells, wherein n is a natural number of 2 or greater; and a control unit operably coupled to the voltage measuring unit, wherein the control unit is configured to: determine first to n th average cell voltages for a first rest period associated with the first to n th battery cells in a one-to-one relationship, based on the cell voltage of each battery cell measured a first number of times for the first rest period, wherein the first rest period is from a first time point to a second time point, determine first to n th average cell voltages for a second rest period associated with the first to n th battery cells in a one-to-one relationship, based on the cell voltage of each battery cell measured a second number of times for the second rest period, wherein the second rest period is from a third time point to a fourth time point, and the third time point is later than the second time point, and is a time point at which a threshold time has passed since the first time point, and diagnose a fault of each of the first to n th battery cells based on the first to n th average cell voltages for the first rest period and the first to n th average cell voltages for the second rest period. determine first to n th average cell voltages for a first rest period associated with the first to n th battery cells in a one-to-one relationship, based on the cell voltage of each battery cell measured a first number of times for the first rest period, wherein the first rest period is from a first time point to a second time point, determine first to n th average cell voltages for a second rest period associated with the first to n th battery cells in a one-to-one relationship, based on the cell voltage of each battery cell measured a second number of times for the second rest period, wherein the second rest period is from a third time point to a fourth time point, and the third time point is later than the second time point, and is a time point at which a threshold time has passed since the first time point, and diagnose a fault of each of the first to n th battery cells based on the first to n th average cell voltages for the first rest period and the first to n th average cell voltages for the second rest period.