Patent ID: 8194367

Claim:
An electricity storage device storing power generated from a generator, comprising: an electricity storage unit; a DC/DC converter having an electricity storage unit terminal connected to the electricity storage unit and an input/output terminal configured to be connected to the generator; an electricity storage unit voltage detection circuit configured to detect a voltage value of the electricity storage unit from the electricity storage unit terminal; an input/output voltage detection circuit configured to detect a voltage value of power input to and output from the DC/DC converter from the input/output terminal; an input/output current detection circuit configured to detect a current value of power input to and output from the DC/DC converter; and a control circuit, wherein the control circuit calculates an efficiency of the DC/DC converter when the DC/DC converter carries current, by use of an input and output power amount for the DC/DC converter calculated based on the voltage value detected by the input/output voltage detection circuit and the current value detected by the input/output current detection circuit, and a power amount for the electricity storage unit calculated based on the voltage value detected by the electricity storage unit voltage detection circuit and a capacitance value of the electricity storage unit; determines the electricity storage unit and the DC/DC converter to be normal when the efficiency lies in a predetermined range; and determines at least one of the electricity storage unit and the DC/DC converter to be abnormal when the efficiency lies out of the predetermined range.