An algorithm for dip point detection in lithium–sulfur battery cells

Abstract

This article examines the problem of developing a simple, model-free algorithm for detecting and identifying the time instant when a Lithium–Sulfur (Li–S) cell passes through its “dip point” during discharge. The dip point marks a sharp transition between two different sets of redox reactions involved in Li–S battery discharge, and is characterized by a significant change in the slope of battery potential with respect to charge processed. This makes it possible to detect the dip point accurately using a simple algorithm that uses a moving-horizon least-squares method to estimate the above slope, then detects the dip point by detecting changes in this slope. We validate this algorithm both using a physics-based battery simulation and experimentally, using custom-fabricated Li–S coin cells. One potential benefit of this algorithm is the degree to which it makes it possible to pinpoint battery cell arrival into the low plateau region accurately, which is important in light of the well-recognized difficulties associated with Li–S battery state estimation in this region. This opens the door to potential innovations in Li–S battery pack balancing that rely on dip point detection as a simpler and reliable alternative to full battery state estimation.