Battery system

A battery system comprising:

• a power supply device which supplies power;
• a battery device in which a plurality of substantially identical battery units are connected in parallel, wherein each battery unit includes a power conversion unit which converts the power supplied from the power supply device and a battery module which is charged with power converted by the power conversion unit;
• and
• a control device including a storage unit which stores information about a reference power value corresponding to predetermined power conversion efficiency of the power conversion unit, an acquisition unit which acquires information about a supply power amount of the power supplied by the power supply device, and a selection unit which determines the number of battery units capable of distributing the supply power amount at a power amount greater than or equal to the reference power value based on the information about the reference power value stored in the storage unit and the information about the supply power amount acquired by the acquisition unit and selects the battery units to which the power is supplied from the power supply device among the plurality of battery units, wherein the selected battery units are equal in number to the determined number.