ENERGY STORAGE APPARATUS, ENERGY STORAGE SYSTEM, INTERNAL RESISTANCE ESTIMATION METHOD, AND COMPUTER PROGRAM

An energy storage apparatus includes an energy storage device and a management unit. The management unit: lowers an SOC of the energy storage device by discharge of the energy storage device; acquires current and voltage of the energy storage device while the SOC of the energy storage device is within a predetermined SOC range in a process of raising the SOC of the energy storage device by charge of the energy storage device; and calculates an internal resistance of the energy storage device based on the acquired current and voltage. The SOC range is a range in which a change in voltage of the energy storage device with respect to a change in SOC of the energy storage device is larger than a range in which a value of the SOC is larger than the SOC range.