Investigation of Open Circuit Potential of Lithium-Ion Battery by The Taguchi Design

Lithium-ion batteries are one of the most preferred energy storage materials with their high energy density for numerous applications, including electric vehicles, hybrid electric vehicles, mobile phones, notebooks, battery-powered vehicles [1-4]. Performance of the lithium-ion batteries in the vehicle systems determine the safety, durability, reliability, and efficiency [5]. Equivalent circuit models are used to estimate the performance of lithium-ion cells due to concerning battery dynamic parameters. To improve the reliability of model-based battery management systems, equivalent circuit models have been extensively investigated, as reported in [6-10]. There are generally two terms that define the state of lithium-ion batteries, namely state-ofcharge and state-of-health [11]. The state-of-charge can be defined as the quantified value of the usable energy of battery, while the remaining performance is indicated by the state-of-health [12]. In the literature, the studies have also been performed to determine the state-ofcharge and state-of-health [8, 13]. Battery management systems use the term state-of-charge to observe the state of the battery and avoid over-charging or over-discharging [14]. The state-of-charge as a substantial parameter for a battery management system can be estimated if the battery’s open circuit potential is known. The open circuit potetial thermodynamically reflects the battery’s equilibrium potential under zero-current loading. Therefore, the open circuit potential is crucial to design a robust battery management system in real world operating conditions. Charging operation is also important for battery-powered vehicles [15]. The constant current constant potential charging procedure consists of two steps, namely constant current mode, and constant potential mode [16]. In the constant current step, a constant charging current is applied to the lithium-ion battery until its potential reaches a cut-off voltage. The charging current gradually decreases when the battery potential is reached the cut-off voltage. The battery potential is kept constant during the constant potential mode while the charging current is decreased to a cut-off current that still charges the battery [17]. There are various charging factors influencing the battery characteristics during their life cycle. Charging current is one of the most important controllable factor. High charge currents provide a short charging time, while low charge currents are usually suggested by the battery manufacturers considering safety restrictions [18]. Another controllable charging factor is the relaxation time in which the battery potential is allowed to relax to its equilibrium potential. The effects of these charging factors on battery dynamic parameters have been studied in the literature [18, 19]. Schmidt et al. have charged the lithium-ion battery with a pulse current of 1 C to different battery potentials [20]. The charging current was applied during 10 s. They did not observe selfdischarge between 10-60% state-of-charge. The self-discharge was clearly observed after approximately 60% state-of-charge. They proposed a model to predict the full-cell potential and half-cell potential. However, they did not statistically determine the influence of charging factors on the open circuit potential. In a recent work, Shkrob et al. Research Article