Detection of Degraded/Aged Cell in a Li-ion Battery Pack using Spread Spectrum Time Domain Reflectometry (SSTDR)

This paper presents a cell-level state of health (SOH) measurement technique based on spread spectrum time domain reflectometry (SSTDR). This arrangement can identify the location and amount of aging associated to the degraded cell in a large battery pack. Variations in SOH of the lithium-ion (Li-ion) cells in a series-parallel connected battery pack is unavoidable because of the manufacturing tolerances and non-uniform operating conditions. As a result, uneven SOH in series-parallel connected cells can lead to affect the performance of the entire battery pack. Therefore, cell level SOH along with the respective cell location is a crucial metric for the battery management system (BMS) to predict the remaining useful life of the entire battery pack. Today's BMS considers the SOH of the entire battery pack/cell string as a single SOH and therefore, cannot monitor the SOH at the cell level. A healthy battery string has a specific impedance between the two terminals, and any aged cell in that string will change the impedance value. Since SSTDR can characterize the impedance change in its propagation path along with its location, it can successfully locate the degraded cell in a large battery pack.