A battery apparatus comprising: a plurality of assembled battery units, each of the assembled battery units including a cell module circuit in which a plurality of cell modules are connected, each of the cell modules including a plurality of cells which are connected in series and a cell monitoring unit configured to monitor voltages and temperatures of the plurality of cells; a current sensor for detecting a current flowing in the cell module circuit; and a battery management device configured to receive monitor data from each of the cell monitoring units, and detection data of the current sensor; a gateway control device configured to execute mutual communication with the battery management devices in the plurality of assembled battery units; and a measuring computer connected to the gateway control device and the plurality of battery management devices, and configured to acquire detection data from the battery management device at first time intervals, to calculate a predetermined characteristic value of each cell or cell module, based on the acquired detection data, and to send the acquired detection data or the calculated characteristic value of the cell or cell module to a control device at second time intervals which are longer than the first time intervals; characterised in that the plurality of cell modules are connected in series in each cell module circuit, and wherein the measuring computer is configured to preferentially select, from among the cell modules, a cell module having a highest voltage at a time of charge or a lowest voltage at a time of discharge, as a cell module which is a target of degradation diagnosis, and to execute the degradation diagnosis of the selected cell module. a plurality of assembled battery units, each of the assembled battery units including a cell module circuit in which a plurality of cell modules are connected, each of the cell modules including a plurality of cells which are connected in series and a cell monitoring unit configured to monitor voltages and temperatures of the plurality of cells; a current sensor for detecting a current flowing in the cell module circuit; and a battery management device configured to receive monitor data from each of the cell monitoring units, and detection data of the current sensor; a gateway control device configured to execute mutual communication with the battery management devices in the plurality of assembled battery units; and a measuring computer connected to the gateway control device and the plurality of battery management devices, and configured to acquire detection data from the battery management device at first time intervals, to calculate a predetermined characteristic value of each cell or cell module, based on the acquired detection data, and to send the acquired detection data or the calculated characteristic value of the cell or cell module to a control device at second time intervals which are longer than the first time intervals; characterised in that the plurality of cell modules are connected in series in each cell module circuit, and wherein the measuring computer is configured to preferentially select, from among the cell modules, a cell module having a highest voltage at a time of charge or a lowest voltage at a time of discharge, as a cell module which is a target of degradation diagnosis, and to execute the degradation diagnosis of the selected cell module.