Partial cycling aging of Li-ion batteries in frequency regulation applications

This research analyzes the performance and provides an initial estimation of the health of Lithium-ion (Li-ion) batteries in frequency regulation applications. Frequency regulation applications appear to produce a great strain on batteries as they are constantly cycled as a fast-ramping resource to regulate the frequency in the grid. The frequency regulation battery profile was extracted from data available only from the Pennsylvania-New Jersey-Maryland Interconnection (PJM). Once the battery profile was established, a Li-ion battery was cycled at 25 degrees C and 40 degrees C for a duration of 1500 and 4271 partial cycles. The aim was to analyze and compare the performance at 25 degrees C and 40 degrees C, and potentially identify the trends that show battery aging and capacity fade. To that end, different methods including capacity tests, throughput, and equivalent full-cycle analysis were performed. Electrochemical Impedance Spectroscopy (EIS) tests were done every 100 or 200 partial cycles to analyze the health and aging of the battery. An Equivalent Circuit Model (ECM) was also developed from EIS test results. One of the objectives was to study how the model parameters changed with the number of cycles and start to identify the main degradation mechanisms associated with the battery operation under frequency regulation. The results of cycling testing and a discussion of the analysis are presented here.