Internal Parameters Identification for Equivalent Electrical Circuit Model Design of an Al-Ion Cell
Authors: Lee Danbi , Tak Yongsug , Kim Jonghoon 
Published: February 2020

The rechargeable cells (lead-acid & Li-Ion) have been broadly considered as the main source in electric-powered applications such as electric vehicle (EV), energy storage system (ESS), unmanned aerial vehicle (UAV), marine, robot, and aerospace. However, because of the limitation of energy density in these cells, the interest of some new generation rechargeable cells including Li-Air, Li-S, vanadium redox flow battery (VRFB), Al-Ion, Al-Air cells, etc have abruptly increased in these days. After manufacturing these cells, it is required to analysis cell’s internal operating mechanism and to check discharging/charging performance for an efficient usage of electric-powered application. For this purpose, the electrochemical modeling describing cell’s performance has been significantly adapted. Unfortunately, in the light of electrical engineering, it is numerously difficult to implement this modeling using electrochemical equation. Besides, most battery management system (BMS) solutions consider little electrochemical modeling and its equations. Namely, a different modeling should be developed for overcoming above weaknesses and implementing efficient BMS that can satisfy high-accuracy state-of-charge (SOC), state-of-health (SOH), state-of-power (SOP) and state-of-energy (SOE). Based on discharging/charging performance, an equivalent electrical circuit modeling (ECM) can be played a key role in the BMS. Fortunately, various literatures on equivalent ECMs of lead-acid and Li-Ion/polymer cells and packs have been investigated. On the contrary, there are little researches on the equivalent ECMs for new generation rechargeable cells, especially Al-Ion cell. Thus, this study newly gives insight to ECM design and internal parameters identification for an Al-Ion cell. The most important task for developing an equivalent ECM is to determine the model structure (e.g. RC-network in series/parallel connection) and internal parameter values. The reflection of high accuracy experimental-based internal parameters values in the ECM can be guaranteed more reliable SOx (SOC/SOH/SOP/SOE) information to users. First of all, the equivalent ECM consisting of one series resistance, one/two RC-networks, and open-circuit voltage (OCV) based on discharge capacity is constructed in this study. After ECM determination, each value of above components in the ECM needs to be precisely measured and identified based on various experiments. Under various experimental conditions including C-rate, SOC range, and temperature, an elaborate internal parameters identification becomes available. In case of series resistance and RC-networks, two methods of the direct current internal resistance (DCIR) and the hybrid pulse power characterization (HPPC) are carried out at fully SOC range with SOC 10% interval. Ampere hour-counting is calculated to acquire the discharge capacity from fully-charged state to fully-discharged state. With an identical condition of SOC, each OCV are measured and used as the OCV-SOC relationship in the equivalent ECM. According to comparison between two discharging/charging voltages of the real and estimated, it is capable of validating the ECM’s accuracy. The use of corrected internal parameters in the ECM leads to decreased modeling error. For reference, two University participated in this study. The ‘Materials and Electro-Chemistry Laboratory’ in Inha University performed the Al-Ion cell’s manufacturing and its experiments. Based on various experimental results, the ECM was designed by the ‘Energy Storage and Conversion Laboratory’ in Chungnam National University.

Figure 1