An aging model of Ni-MH batteries for hybrid electric vehicles

The extensive use of batteries in hybrid electric vehicles (HEVs) today requires establishing an accurate model of battery aging and life. During a battery´s lifetime, its performance slowly deteriorates because of the degradation of its electrochemical constituents. Battery manufacturers usually provide aging data that will show this degradation. However the data they provide result from standard aging tests, in which the battery is discharged and charged thousands of times with identical current profiles (or cycles). Using these data many aging models have been developed that relate the maximum number of battery cycles to the depth of discharge (DOD) of the current profile used. In this work, we focus on the development of an aging model suitable for applications in which the battery is used with no pre-defined cycles, as in the case of hybrid-electric vehicles. Laboratory experiments and concepts borrowed from fatigue analysis are applied to the relationship between battery aging and the most important operational conditions that affect its life, i.e. its operating temperature and current history.