Incremental Capacity Analysis of a Lithium-Ion Battery Pack for Different Charging Rates

Introduction Incremental Capacity Analysis (ICA) is a method used to investigate the state-of-health (SOH) of batteries and to identify their possible aging mechanisms, by tracking the electrochemical properties of the cell 1 . The method is based on the differentiation of the battery capacity over the battery voltage (i.e., dQ/dV), over a full or a partial cycle, regardless the experimental conditions. Several ICA research studies have been performed on various Li-ion chemistries, and several mathematical approaches have been employed to acquire the differential curves 2 , 3 . However, all these studies are focused on cell level analysis while the influence on the whole battery pack has not been yet studied, to the best of our knowledge. Thus in the present work, we have performed an in-depth investigation of a battery pack composed of 14 Lithium-ion -60Ah cells, for the purpose of evaluating the applicability of the ICA on a battery pack level by means of different charging current rates. Method Experiments were conducted with C/5 and C/10 current rates at 28 o C and for a full charge (constant current – constant voltage) of the pack. In real-life, the data logged by the Battery Management System (BMS), about each individual cell from the battery pack, are not available to the user. Thus, the ICA will be applied to the battery pack voltage, which accounts also information about the impedance of the cables, the switch box, and other equipment, as well as the influence of temperature variation, while obtaining the data. For this reason, we decided to divide this work in two main parts. Firstly, the ICA is established at a pack level by directly averaging the BMS cells measured voltages, while the voltage drop due to connector, relays, fuses, etc are not included. This will be noted as ‘‘direct ICA’’. Secondly, ICA is obtained when considering the terminal voltage of the pack, i.e. at the charger terminals and will be noted as ‘‘terminal ICA’’. Results A comparison between the terminal Incremental Capacity (IC) curves, obtained for two different C-rates are presented in Fig. 1. Similarly to cell level analysis, the ability of this method to detect the changes of the pack’s electrochemical properties is illustrated. The voltage plateaus which are related to the first order phase transformation are undeniably associated to the ICA peaks for both current rates. Both can yield information on the cell behavior, however a reverse proportionality of the ICA peak