The self-discharge mechanism during storage in open-circuit states of a Ni–MH battery using a Mg2Ni-type hydrogen storage alloy anode was investigated by electrochemical impedance spectroscopy (EIS) and X-ray diffraction (XRD). The loss of discharge capacity for this battery can be ascribed to two causes: (i) desorption of hydrogen from the Mg1.95Y0.05Ni0.92Al0.08 hydride anode; and (ii) anode surface degradation resulting from oxidation of the magnesium alloy in the electrolyte. At the higher open-circuit voltages (OCV), the former was mainly responsible for a high self-discharge rate, while the latter might dominate the loss of capacity at the lower OCV. XRD results confirmed that Mg(OH)2 formed on the magnesium alloy anode after storage in an open-circuit condition for 20 days.