Battery system and method for controlling lithium ion battery

A battery system, comprising:

• a lithium ion battery including an electrode assembly;
• a voltage converter configured to be capable of changing a voltage of the lithium ion battery;
• and
• a control unit that controls the voltage converter,
• wherein the control unit calculates an index value corresponding to a degree of progress of deterioration occurring in the lithium ion battery due to bias in a lithium ion concentration distribution within the electrode assembly, and controls the voltage converter to cause the voltage of the lithium ion battery to fall within a predetermined voltage range including a specific voltage when the index value exceeds a threshold value,
• the specific voltage is a peak voltage on a dQ/dV voltage characteristic curve, the peak voltage being derived from structural change of a positive electrode active material contained in the electrode assembly, and
• the dQ/dV voltage characteristic curve is a curve indicating a relationship between dQ/dV that is a ratio of a change dQ of a stored electricity amount of the lithium ion battery to a change dV of the voltage of the lithium ion battery, and the voltage of the lithium ion battery.