Abstract: An energy storage system includes a first energy storage, a second energy storage, a transformer, and circuitry. The first energy storage outputs a first output voltage. The second energy storage outputs a second output voltage. The transformer transforms at least one of the first output voltage and the second output voltage. The circuitry is configured to detect a sign of a fault in at least one of the first energy storage and the second energy storage. The circuitry is configured to control the transformer so as to transfer power from one energy storage among the first energy storage and the second energy storage to another energy storage among the first energy storage and the second energy storage in a case where the detector detects the sign of the fault in the one energy storage.