A relative capacity estimation method for lithium batteries used in electric vehicle applications

Abstract

Depending on the consciousness about environmental harms of fossil fuel usage and depletion in their reserves, the interest on clean transportation is rising today. Electric vehicles (EV) are important alternatives on clean transportation. In EV applications, lithium based batteries are commonly preferred due to their relatively high energy and power densities, higher open circuit voltages and lower self-discharge rates, when compared to other secondary battery types. Performance of a battery decreases with age. Therefore battery health and life information is important for reliable operation in EV applications. In this study battery health is represented in terms of relative capacity (RC) which is the comparison between actual and nominal capacity values of a battery and a simple RC estimation method is proposed. In the method, RC is estimated by using relative cycle number by using reference cycle number (RCN). For this purpose a RCN model, which is based on the change of terminal voltage under a significant load signal, is developed. A battery model, which was proposed in an earlier study is improved in order to reflect aging effects. Behaviors of batteries in different reference cycles are simulated. Different responses of batteries to the same load signal, by means of differences in terminal voltages are investigated. These differences are transformed to numerical quantities to develop RCN model and thereafter RC is estimated by using the relationship between RCN and RC. The method is validated with experiments.