NOVEL APPROACHES TO FAST OCV CHARACTERIZATION AND IMPROVED CAPACITY ESTIMATION IN LITHIUM ION BATTERIES

Abstract

This thesis considers the problem of open circuit voltage (OCV) to state of charge (SOC) characterization in li-ion batteries for battery reuse applications. The traditional approach to OCV-SOC characterization is done by collecting voltage and current data through a slow discharge and charge process; this process usually takes about 60 hours. Such OCV-SOC characterization is performed on a few sample batteries because the OCV-SOC characterization is considered to be the same for new batteries coming out of the same manufacturing process. However, the characteristics of a battery may change as it is used for years in different environmental and usage conditions. Hence, they may need to be re-characterized before secondary use. Unlike primary characterization, the secondary characterization may have to be done faster in order to save time and cost. This thesis presents a faster approach for OCV-SOC characterization. The proposed approach in this thesis consists of constant-current profiles that halves in magnitude after a specified time. Such reductions allows us to fully deplete the battery; similarly, the battery is charged back with a reducing current profile in order to make sure the battery is fully charged. The resulting current profile reduces the total characterization time by 1/5. Secondly, we explore the idea of discharge and charge capacity of batteries. A traditional low-rate-OCV test consists of constant-current charging which results in a voltage drop based on the internal resistance and charging/discharging current. This thesis presents an approach to counteract this voltage drop, by appropriately over-charging and over-discharging the battery to obtain the most accurate representation of the capacity of the battery