Effects of Reference Performance Testing during Aging Using Commercial Lithium-Ion Cells

Abstract

The Advanced Technology Development Program, under the oversight of the U.S. Department of Energy’s FreedomCAR and Vehicle Technologies Program, is investigating lithium-ion batteries for hybrid electric vehicle applications. Cells are aged under various test conditions, including temperatures and states-of-charge. Life testing is interrupted at regular intervals to conduct reference performance tests (RPTs) which are used to measure changes in the electrical performance of the cells and determine degradation as a function of test time. Although designed to be unobtrusive, data from the Advanced Technology Development Generation 2 (Gen 2) cells indicated that RPTs actually contributed to cell degradation and failure. A study was performed at the Idaho National Laboratory using commercially available lithium-ion cells to determine the impact of RPTs on life. A series of partial RPTs was performed at regular intervals during life testing and compared to a control group that was life tested without RPT interruption. It was determined that certain components of the RPT were detrimental to cell performance. Consequently, a new “mini” RPT was designed as a less obtrusive alternative that did not include those detrimental components. Initial testing with commercial cells indicates that the mini RPT has less negative impact than the Gen 2 cell RPT.