POWER SUPPLY DEVICE AND BATTERY DETERIORATION DEGREE DETERMINATION SYSTEM

To provide a power supply device and a battery deterioration degree determination system that enables continuous use of stable electric power even if used battery cells of various deterioration degrees are used. A power supply device for performing charging from a charging part and discharging from a discharging part includes plural battery units including plural battery cells connected in series or in parallel, a connection selection part configured to select at least one of the battery units to be connected to the charging part or the discharging part, an accommodation part configured to detachably accommodate each of the battery units. The battery cells are classified into plural deterioration ranks according to a deterioration degree, and the battery cells included in one of the battery units have the same deterioration rank.