A battery system includes a controller that performs a calculation to calculate an internal resistance of a battery, and executes a predetermined operation based on the result. The calculation includes obtaining a regression line through a regression analysis of a current-voltage plot obtained from voltage sensor and current sensor, and calculating the internal resistance from a slope of the regression line. The controller calculates a first internal resistance through the calculation based on a first group of detection values of the voltage and current detected a plurality of times, and calculate a second internal resistance through the calculation based on a second group of detection values. The controller executes the predetermined operation when a resistance difference between the first and second internal resistances is smaller than a reference value, and does not execute the operation when the resistance difference is larger than the reference value.