Challenges of a Fast Diagnostic to Inform Screening of Retired Batteries

With the increased pervasiveness of Lithium-ion batteries, there is growing concern for the amount of retired batteries that will be entering the waste or recycling stream before they are fully utilized. Although aged batteries no longer meet the demands of their first application, many still have a significant portion of their initial capacity remaining for use in secondary applications, but evaluating this capacity is difficult and time intensive. In this paper, we investigate the use of cell (or parallel sets of cells) internal resistance as a surrogate of the capacity of parallel cells. We also highlight the challenges of testing batteries as a full pack despite the cell-to-cell variability from lack of balancing and differences in resistance and capacity. First, we verify that the charge-interrupt resistance from parallel cell pairs from twelve retired battery packs can eliminate the need for the hybrid pulse power characterization (HPPC) test as long as the charge-interrupt tests were not applied at low cell pair terminal voltages. Then, the relation between cell internal resistance and capacity across the various packs is investigated. Initial experimental results from this study show a correlation between internal resistance and remaining capacity which can be approximated with a linear ft, suggesting internal resistance measurements taken above a threshold cell pair terminal voltage may be a suitable initial screening metric for aged batteries.