Power storage system

Processing circuitry functionally includes: an internal resistance calculation unit; a reference value obtaining unit configured to obtain a reference value for internal resistance; a correction coefficient calculation unit configured to calculate a correction coefficient indicating how great difference between the internal resistance and the reference value is: a correction value calculation unit configured to calculate a correction value by correcting the reference value using the correction coefficient; and an estimation unit configured to calculate estimated internal resistance, based on the correction value and present voltage. The processing circuitry is configured to control charge and/or discharge of the battery, based on the internal resistance or the estimated internal resistance. The correction value calculation unit is configured to calculate the correction value by correcting the reference value using the correction coefficient calculated in the past when the internal resistance calculation unit is unable to calculate the internal resistance.