A power supply unit includes a rechargeable battery for supplying power to a body circuit of an electrical apparatus, a current measurement circuit for measuring a currently-discharged current value, a voltage measurement circuit for measuring a currently-discharged voltage value, and a controller for detecting whether a memory effect occurs by determining whether a currently-discharged voltage value measured at a currently-discharged current value is lower than a predetermined voltage value when a detected battery capacity is equal to or less than a predetermined capacity.