The deterioration mechanism of the storage characteristics of nickel-metal hydride batteries for hybrid electric vehicles (HEVs) has been investigated. The deterioration in the storage characteristics during charge–discharge cycling is caused by the microscopic short circuit in the separator which is influenced by the deposits from the positive and negative electrode elements such as zinc, cobalt and manganese. Based on the elucidated deterioration mechanism, the deterioration in the storage characteristics of nickel-metal hydride batteries was suppressed by employing a hydrogen-absorbing alloy with higher oxidation resistance for the negative electrode.