Current Distribution Measurements in Parallel-Connected Lithium-Ion Cylindrical Cells under Non-Uniform Temperature Conditions

Abstract

Understanding internal state non-uniformity that occurs across the electrodes in large-format Lithium-ion batteries, and among parallel-connected cells, is a critical part of the cell and battery module design process. Two separate groups of parallel-connected 18650 cells were tested using LiFePO 4 /C 6 (LFP), and LiNiMnCoO 2 /C 6 (NMC) chemistries. Pulse and full-capacity discharges were performed at various States of Charge (SOC), C-rates, average temperatures, and levels of temperature non-uniformity. Current nonuniformity for the pulse testing was always lower for the LFP group compared to the NMC group. The hottest cell in the LFP group produced up to 40% more current than average, while this was up to 80% for NMC. Conversely, under charge depleting conditions the NMC group experienced less current non-uniformity, and in certain cases provided a nearly uniform current distribution in the presence of non-uniform temperature. The results indicate that higher temperature sensitivity in the impedance of a cell will cause larger current non-uniformity under pulse conditions. However, due to the presence of non-uniform SOC for charge depleting, the Open Circuit Voltage (OCV) versus SOC gradient plays a significant role in dictating the current distribution behavior, where steeper OCVs provide a corrective action that minimizes the effect of the non-uniform impedance. Understanding the evolution of non-uniform current and temperature that occurs either across the electrodes in Lithium-ion cells and/or among sets of interconnected cells is a critical part of the cell and module design process for Lithium-ion battery packs.1-3 The issues of non-uniform current and temperature cause a two-way coupled problem. For example, non-uniform current density can cause non-uniform heating, and therefore non-uniform temperature. 4 Additionally, nonuniform temperature causes non-uniform electrochemical impedance across the electrodes of individual cells and/or across cells in a module which will generate non-uniform current density and therefore non-uniform heating. [5][6][7] Several studies, particularly at high charge/discharge rates (i.e., above 1C), show single cell temperature differences greater than 20• C. 3,4,8,9 As a result battery thermal management systems (BTMS) must be incorporated into large scale battery packs, particularly in automotive applications where high C-rates are common. Traditionally, the BTMS is designed in such a way that thermal non-uniformity is kept below 5• C throughout the cell, module, and pack. 4 In order to optimize the BTMS to ensure efficient packaging, thermal performance, and proper electrochemical performance, a thermally-coupled spatially-resolved electrochemical model must be used to understand the coupled effects between the BTMS and the electrochemical performance.In recent years much progress has been accomplished to model the inhomogeneous behavior of large-format cells. Generally, these can be classified as either True Multidimensional Models (TMM) or Distributed Models (DM). The d...

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Power Flow Distribution Strategy for Improved Power Electronics Energy Efficiency in Battery Storage Systems: Development and Implementation in a Utility-Scale System

“…In contrast, topologies that connect battery packs of multiple units in parallel may lead to heterogeneous current flows between parallel-connected battery packs due to variances for the battery impedance and capacity, which cannot which cannot be controlled [5,6]. This is of particular importance for systems which feature battery packs of varying State of Health or different battery chemistries, such as the battery packs in the Second-Life battery systems of this work.…”

Measurement of the Temperature Influence on the Current Distribution in Lithium‐Ion Batteries

“…Regarding temperature, there are both simulative [ 26–28 ] and experimental [ 27–29 ] investigations on its influence on the current distribution in parallel connected cells. The studies differ in terms of cell chemistry, number of cells, temperature differences, and temperature level, however, they achieve similar conclusions.…”