Secondary battery system and method for diagnosing abnormality in battery pack

A secondary battery system mountable on a vehicle, comprising:

• a battery pack to be charged by external charging control with electric power supplied from outside of the vehicle, the battery pack including a plurality of modules connected in series to one another, each of the plurality of modules including a plurality of cells connected in parallel to one another;
• a plurality of voltage sensors each provided for a corresponding one of the plurality of modules and each configured to detect a voltage of a corresponding one of the plurality of modules;
• and
• a controller configured to execute the external charging control,
• the controller being configured to, if a first condition and a second condition are satisfied, diagnose an abnormality in which a current path of a cell breaks, the cell being included in a module among the plurality of modules,
• the first condition being a condition that a voltage difference between a maximum voltage value and a minimum voltage value among a plurality of voltage values is less than a reference value before execution of the external charging control, each of the plurality of voltage values being detected by a corresponding one of the plurality of voltage sensors,
• the second condition being a condition that a voltage difference between the maximum voltage value and a voltage value other than the maximum voltage value and the minimum voltage value among the plurality of voltage values is more than the reference value after execution of the external charging control.