Study of the Impedance Growth and Capacity Fade of High Power Lithium-Iron Phosphate, Valve Regulated Lead Acid, and Nickel Metal Hydride Batteries When Cycled in High Rated Pulsed Profiles

Electrochemical energy storage has become more power and energy dense in recent years, considerably opening up the application space for which they can be used. Among these applications are high power loads which operate in a pulsed manner. The most common chemistries considered for use in these applications include valve regulated lead acid (VRLA), nickel metal hydride (NiMH), and lithium-ion (LI). The University of Texas at Arlington’s (UTAs) Pulsed Power and Energy Laboratory (PPEL) has assembled roughly 60 V modules of each chemistry and experimentally characterized their capacity fade and impedance growth when they are used to drive a 40 kW constant power load in a 5 seconds on / 5 seconds off pulsed profile. The VRLAs are discharged at four times their ten hour rating, the NiMH are discharged at eight times their one hour rating, and the LI are discharged at thirteen times their one hour rating. All of the batteries are recharged at elevated rates, in most cases the maximum allowed by the manufacturer. Using standard voltage, current, and thermal diagnostics as well as periodic electrochemical impedance spectroscopy (EIS) measurements, the capacity fade and impedance growth of each chemistry has been studied to understand how the high rate pulsed profile ages the cells. Empirical and lumped element models of the impedance rise have been developed to better understand what is aging internally and how it impacts the usable life of the batteries compared with manufacturers rated life predictions. The experimental and modeling efforts will be summarized and conclusions will be drawn to provide the audience with a better understanding of how each of these technologies ages when used in high rate, pulsed applications.