POWER SUPPLY DEVICE

A power supply device includes a secondary cell, a detection device, a storage device, and a control device. The secondary cell includes an electrolyte solution. The detection device detects the voltage and temperature of the secondary cell. The storage device stores first information and second information. The first information indicates the relationship among the voltage, the temperature, and the amount of decrease in the electrolyte solution during energization of the secondary cell. The second information indicates the relationship among the voltage, the temperature, and the amount of decrease in the electrolyte solution during storage of the secondary cell. The control device calculates a first index indicating the amount of decrease in the electrolyte solution.