Lithium ion secondary battery system

The present invention, while utilizing the high energy density of the lithium ion secondary battery comprising a positive electrode containing a positive electrode active material having an operating potential of 4.5V or more with respect to lithium metal, the output characteristics and / or storage characteristics are improved and an object thereof is to provide a battery system. The present invention comprises a first cell comprising the 5V-class battery, a second battery consisting of 4V class battery, a battery system and a control system.