Battery management device, battery system, and battery management method for estimating transition of AC resistance value of secondary battery

A battery management device comprising:

• a state management unit that estimates first values of AC resistance of a secondary battery at a plurality of points of time, estimates transition of a value of the AC resistance of the secondary battery by referring to the estimated first values, and estimates a remaining period of use elapsed until the value of the AC resistance of the secondary battery reaches a resistance threshold value corresponding to a point of time of end of use of the secondary battery;
• and
• an operation management unit that changes a method of use of the secondary battery to a method of use that imposes a smaller load or stops using the secondary battery, when the remaining period of use of the secondary battery is shorter than a predetermined period of time,
• wherein when estimating each first value of the AC resistance of the secondary battery at a respective point of time, the state management unit changes, after the secondary battery is started to be charged or discharged, a charging current or a discharging current of the secondary battery in stages until the charging current or the discharging current of the secondary battery reaches a current value defined by a load, measures values of voltage changes of the secondary battery in the stages, and estimates second values of the AC resistance in the stages.