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Current and temperature imbalances in parallel-connected grid storage battery modules

Joseph Ross, Damien Frost, Stratos Chatzinikolaou, Stephen Duncan, David Howey

TL;DR

The paper tackles current and temperature imbalances in parallel-connected grid storage modules, focusing on large-format LiFePO4 cells and the sensing limitations of battery management systems. It presents a computable electrothermal model of a four-cell parallel module, calibrated and validated against experiments that include single-cell and interconnect failures. A Sobol sensitivity analysis reveals which cell parameters most influence thermal imbalances, and a constrained optimization yields safety limits for parameter variability under different C-rates and DoD. The work provides robustness metrics and design guidance to improve safety and performance in industrial BESS deployments with limited per-cell sensing.

Abstract

A key challenge with large battery systems is heterogeneous currents and temperatures in modules with parallel-connected cells. Although extreme currents and temperatures are detrimental to the performance and lifetime of battery cells, there is not a consensus on the scale of typical imbalances within grid storage modules. Here, we quantify these imbalances through simulations and experiments on an industrially representative grid storage battery module consisting of prismatic lithium iron phosphate cells, elucidating the evolution of current and temperature imbalances and their dependence on individual cell and module parameter variations. Using a sensitivity analysis, we find that varying contact resistances and cell resistances contribute strongly to temperature differences between cells, from which we define safety thresholds on cell-to-cell variability. Finally, we investigate how these thresholds change for different applications, to outline a set of robustness metrics that show how cycling at lower C-rates and narrower SOC ranges can mitigate failures.

Current and temperature imbalances in parallel-connected grid storage battery modules

TL;DR

The paper tackles current and temperature imbalances in parallel-connected grid storage modules, focusing on large-format LiFePO4 cells and the sensing limitations of battery management systems. It presents a computable electrothermal model of a four-cell parallel module, calibrated and validated against experiments that include single-cell and interconnect failures. A Sobol sensitivity analysis reveals which cell parameters most influence thermal imbalances, and a constrained optimization yields safety limits for parameter variability under different C-rates and DoD. The work provides robustness metrics and design guidance to improve safety and performance in industrial BESS deployments with limited per-cell sensing.

Abstract

A key challenge with large battery systems is heterogeneous currents and temperatures in modules with parallel-connected cells. Although extreme currents and temperatures are detrimental to the performance and lifetime of battery cells, there is not a consensus on the scale of typical imbalances within grid storage modules. Here, we quantify these imbalances through simulations and experiments on an industrially representative grid storage battery module consisting of prismatic lithium iron phosphate cells, elucidating the evolution of current and temperature imbalances and their dependence on individual cell and module parameter variations. Using a sensitivity analysis, we find that varying contact resistances and cell resistances contribute strongly to temperature differences between cells, from which we define safety thresholds on cell-to-cell variability. Finally, we investigate how these thresholds change for different applications, to outline a set of robustness metrics that show how cycling at lower C-rates and narrower SOC ranges can mitigate failures.
Paper Structure (14 sections, 18 equations, 13 figures, 4 tables)

This paper contains 14 sections, 18 equations, 13 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Model Formulation
  3. Model Implementation
  4. Experiments
  5. Methodology
  6. Results
  7. Model Parameterisation and Validation
  8. Parameter estimation
  9. Model validation
  10. System Analysis
  11. Temperature sensitivity analysis
  12. Derivation of safety limits
  13. Thresholds for safe operation
  14. Conclusion

Figures (13)

  • Figure 1: Development of BESS parameter variation from manufacturing to end of life. Adapted from Baumann in BAUMANN2018295 and Zwicker in ZWICKER2020100017.
  • Figure 2: Model electrical-thermal diagram
  • Figure 3: Module configuration with four parallel cells
  • Figure 4: Current and temperature results from constant current discharge experiments.
  • Figure 5: Simulated cell currents and temperatures for model validation.
  • ...and 8 more figures