State-of-Health (SoH), a specified measure of stability, is a critical parameter for determining the safe operating area of a battery cell and battery packs to avoid abuse and prevent failure and accidents. A series of experiments were performed to evaluate the performance of a 4P1S battery array using electrochemical impedance spectroscopy to identify key frequencies that may describe battery state of health at any state of charge. Using a large sample number of cells, the state of health frequency, fSoH, for these LiFePO4 26650 cells is found to be 158 Hz. Four experiments were performed to evaluate the lifetime in different configurations: single-cell at 1C (2.6A), single-cell at 10C (26A), four cells in parallel at 10C (ideal match), and four cells in parallel (manufacturer match). The lifetime for each experiment set degraded substantially, with the final parallel series reaching end of life at 400 cycles, a 75.32% reduction in life compared to operating solo. Analysis of the fSoH data for these cells revealed a change in imaginary impedance at the critical frequency that corresponded to changes in the capacity and current data, supporting the development of a single-frequency diagnostic tool. An electrochemical model of the battery was generated, and it indicated the anode material was aging faster than the SEI layer, the opposite of normal cell degradation. A post-mortem analysis of cells from three configurations (baseline, single-cell, and parallel-cell) supported the modeling, as physical damage to the copper current collector in the anode was visible in the parallel-connected cell. / Ph. D. / Lithium-ion batteries are used in a large number of applications, from cellular phones to laptops and electric vehicles. In low power devices, such as a laptop, these batteries can be relatively stable if the associated circuitry is designed properly. However, as the amount of power required from the battery increases, the possibility of an internal battery fault will also increase. The ability to determine the stability of the battery for military applications such as laser weapon systems, electromagnetic railguns or commercial systems such as electric vehicles or industrial-scale micro-grids becomes critical to prevent catastrophic events such as fires. Additionally, the ability to determine the battery State-of-Health (SoH), a specified measure of stability, will enable advance warning of a failing battery to optimize the logistics chain in an operational system. A battery marked as “bad” can be scheduled for replacement before a failure actually occurs. This dissertation has designed a series of experiments that establishes the technology to detect these internals faults, and applies them to a scaled battery system that represents a much larger system. When batteries are placed in parallel and discharged at very high currents, typical of the military applications, the lifetime for the cells was reduced by 75% when compared to batteries discharged under the same conditions by themselves. A post-mortem analysis of cells from three different conditions (uncycled, single-cell, parallel-cell) reveals physical damage to the internal electrodes that indicates a high level of internal destruction occurs at high currents when in parallel arrays.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/80573 |
| Date | 29 November 2017 |
| Creators | Huhman, Brett Michael |
| Contributors | Electrical Engineering, Mili, Lamine M., Odendaal, Willem G., Zaghloul, Amir I., Wetz, David Alan, Centeno, Virgilio A., Brown, Alan J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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