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Parallel-Cell Current Sharing and its Effect on the Expected Lifetime of a Battery Pack for Directed Energy Systems

By B.M. Huhman, J.M. Heinzel, L. Mili, D.A. Wetz

As the Navy begins to integrate directed energy and electromagnetic weapons into the fleet, the power requirements for these new mission loads requires careful investigation for optimum performance, maximum energy efficiency, and lifetime characteristics. An intermediate storage system, such as a high-power battery, is necessary to both protect the prime mover on the ship and provide instantaneous response to a “call for fire” from a mission load. High-power battery pack construction typically places series cells (strings) in parallel to limit the discharge current per string while maintaining a high potential on the output of the battery. At high discharge currents (>10C), the sharing of current between each string is a concern, as uneven sharing can accelerate the aging of the battery pack. NRL has designed an experiment focused around a single series string (1S) with four-parallel (4P) cell arrangement, for simplicity. Four electrochemically identical cells were inserted into a baseline array and cycled to end of life. Three duplicate 4P1S arrays with randomly selected cells were used to repeat the experiment until end of life for the replicant arrays was reached. Electrochemical impedance spectroscopy was performed on a specified schedule to assist in performance analysis. Data and conclusions from these experiments will be presented.