PURPOSE: To obtain the battery performance detection device in which a drop in the battery performance of even at least one battery element out of a plurality of series connected battery elements can be detected accurately and automatically by a comparatively simple circuit constitution.

CONSTITUTION: A plurality of series connected battery elements are divided according to their connecting direction, and battery blocks 1, 2 are formed. Then, voltage detectors 4, 5 which detect individual block terminal voltages are installed with reference to the battery blocks 1, 2. A comparator 6 which detects a level difference between both detected voltages and a timer 7 are installed. When the level difference at a definite value or higher is detected only for a prescribed time, it is judged that an irregularity is generated in the battery performance between the battery elements inside the blocks.