A storage system includes control devices and a second processor. The second processor determines a number of abnormal batteries when an abnormality has occurred in a first battery. The second processor assigns a second cache currently assigned to a second control device associated with the first battery to a first control device when the number is smaller than a threshold. The second processor assigns a mirror cache currently assigned to the second control device to a third control device when the number is smaller than the threshold. The second processor instructs the first control device to control write to a first storage device associated with a first cache by using the first cache. Data of the first cache is mirrored to the mirror cache. The second processor instructs the first control device to control write to a second storage device associated with the second cache by using the second cache.