Characterization of graphite lithium-ion cells

Abstract

This thesis explores the characterization of graphite lithium-ion cells. A control procedure was performed to ensure any capacity loss or gain seen in tests was not the result of cell cycling. Vibration testing of the cells, on all three axes to simulate the spacecraft launch environment, showed a slight increase in capacity after vibration. Cell capacity was measured at two current rates at a variety of temperatures to obtain a family of curves to allow for a prediction of cell capacity at a given temperature. Voltage drift was explored and determined to not be a factor when matching cells for a battery. Using data from hard carbon lithium-ion cells, data for capacity loss over time, while in storage, was examined. It was determined that for an 18-month time period, these cells lost less than 2% of their capacity while in storage. Next, cells were cycled in simulated Low Earth Orbit power cycling to determine capacity loss while on orbit. Using a 0.25 Amp charge rate, the graphite cells retained 93% of their initial starting capacity by the 2,000th cycle. Finally, cells underwent accelerated Low Earth Orbit testing to validate the accelerated testing theory. This thesis concludes that accelerated testing is not a good representation of how cells will perform under real-time conditions.