A battery charger as an embodiment of the present invention includes a charger/discharger, a measurer, an estimator, a determinator, and an outputter. The charger/discharger can charge and discharge a connected battery. The measurer measures a voltage and current of the battery during charging and generates measurement data. The estimator estimates a deterioration state of the battery on the basis of the measurement data. The determinator determines whether or not the battery is usable, on the basis of the estimated deterioration state of the battery. The outputter outputs information about the determination as to whether or not the battery is usable.