Current distribution within parallel-connected battery cells

Brand, Martin J.; Hofmann, Markus H.; Steinhardt, Marco; Schuster, Simon F.; Jossen, Andreas

doi:10.1016/j.jpowsour.2016.10.010

Cited by 136 publications

(67 citation statements)

References 20 publications

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“…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.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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Utility-scale battery storage systems typically consist of multiple smaller units contributing to the overall power dispatch of the system. Herein, the power distribution among these units is analyzed and optimized to operate the system with increased energy efficiency. To improve the real-life storage operation, a holistic system model for battery storage systems has been developed that enables a calculation of the energy efficiency. A utility-scale Second-Life battery storage system with a capacity of 3.3 MWh/3 MW is operated and evaluated in this work. The system is in operation for the provision of primary control reserve in combination with intraday trading for controlling the battery state of charge. The simulation model is parameterized with the system data. Results show that losses in power electronics dominate. An operational strategy improving the energy efficiency through an optimized power flow distribution within the storage system is developed. The power flow distribution strategy is based on the reduction of the power electronics losses at no-load/partial-load by minimizing their in-operation time. The simulation derived power flow distribution strategy is implemented in the real-life storage system. Field-test measurements and analysis prove the functionality of the power flow distribution strategy and reveal the reduction of the energy throughput of the units by 7%, as well as a significant reduction of energy losses in the units by 24%. The cost savings for electricity over the system's lifetime are approximated to 4.4% of its investment cost.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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“…Range anxiety being still a major concern of customers deciding to acquire an EV [2,3] drives manufacturers to build batteries with higher and higher capacity. This can be done in two ways, either by using a small number of large capacity cells (e.g., BMW i3, Mitsubishi iMiEV) or a greater number of lower capacity cells connected in parallel (e.g., Tesla Model S, VW e-Golf and Nissan Leaf) [4,5]. Both ways have their specific challenges.…”

Section: Introductionmentioning

confidence: 99%

“…Both ways have their specific challenges. Parallel-connected cells need special attention during normal operation in relation to their current distribution due to cell resistance and capacity mismatch high transient balancing currents possibly occurring [5][6][7] and severely threatening the safe operation of the cells. These cell mismatches can also increase through degradation or cell connection errors and need to be properly handled [8][9][10].…”

Section: Introductionmentioning

confidence: 99%

Discharge by Short Circuit Currents of Parallel-Connected Lithium-Ion Cells in Thermal Propagation

et al. 2019

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The increasing need for high capacity batteries in plug-in hybrids and all-electric vehicles gives rise to the question of whether these batteries should be equipped with a few large capacity cells or rather many low capacity cells in parallel. This article demonstrates the possible benefits of smaller cells connected in parallel because of discharge effects. Measurements have been conducted proving the beneficial influence of a lower SoC on the thermal runaway behaviour of lithium-ion cells.A second test series examines the short circuit currents during an ongoing thermal propagation in parallel-connected cells. With the help of a developed equivalent circuit model and the results of the test series two major system parameters, the ohmic resistance of a cell during thermal runaway R tr and the resistance post thermal runaway R ptr are extracted for the test set-up. A further developed equivalent circuit model and its analytical description are presented and illustrate the great impact of R ptr on the overall discharged capacity. According to the model, cells with a capacity of no more than C cell = 10-15 Ah and a parallel-connection of 24 cells show the most potential to discharge a significant amount.

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“…Non-uniform impedance is also be caused by potential gradients along the current collectors, and this effect has been studied recently. 28,30,[46][47][48][49] U eq, j = U eq, j − U eq,avg [1] A simplified case where U eq is zero can occur, and in this case the impedance block is the only contributor to non-uniform current. This models the behavior observed in our pulse results, and also the behavior observed in portions of our charge depleting discharge where the slope of U eq versus SOC is effectively zero, and thus non-uniform SOC cannot cause non-uniform U eq .…”

Section: Introductionmentioning

confidence: 99%

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

Klein

Park

2017

J. Electrochem. Soc.

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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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Current distribution within parallel-connected battery cells

Cited by 136 publications

References 20 publications

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

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

Discharge by Short Circuit Currents of Parallel-Connected Lithium-Ion Cells in Thermal Propagation

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

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