conference papers

A. Fill and K. P. Birke –

„Impacts of cell topology, parameter distributions and current profile on the usable power and energy of lithium-ion batteries“

– IEEE 2nd International Conference on Smart Energy Systems and Technologies, 2019, pp. 1-6.

Abstract: In order to meet the energy and power requirements of large-scale battery applications, cells have to be connected in serial and parallel configuration. For the purpose of understanding the assembly of parallel-connected cells is referred to as logical cell. Caused by current and State of Charge (SoC) inhomogeneities between the serial and parallel cells, the usable power and energy will differ from the installed values. These differences are affected by the cell topology, cell parameter distributions and the current profile. The influences of these parameters are investigated by Monte Carlo simulations with Gaussian distributed cell parameters. The 5σ values of the usable power decrease almost logarithmically with increasing number of serial and parallel cells. The power is primarily limited by the logical cell with the highest current distribution, which in turn depends principally on the differences in cell parameters. In a battery the maximum difference of cell parameters statistically increase with the number of connected cells. Two further effects influence the usable energy. The Open Circuit Voltage (OCV) bending leads to a SoC balancing at the end of discharge. And the standard deviation of the logical cell capacity distribution decreases by square root with increasing number of parallel cells. These effetcs are leading to a higher usable energy by connecting the cells in parallel compared to a corresponding serial-connection, especially for discharging. Furthermore with increasing standard deviation of the cells resistance and capacity distributions a linear decrease of the usable power and energy is found.

A. Fill, A. Avdyli and K. P. Birke –

„Online Data-based Cell State Estimation of a Lithium-Ion Battery“

– IEEE 2nd International Conference on Industrial Electronics for Sustainable Energy Systems, 2020, pp. 351-356.

Abstract: To fulfill the battery requirements of an electric vehicle in terms of power, driving range and safety an accurate online estimation of the cell states is essential. In this paper two data-based approaches, which continuously estimate the resistance of each cell with low computational effort and memory capacity needed, are presented. The long- and short-term changes of the estimated resistance distribution are used to give insights into temperature and SoH gradients between the cells of a battery. The algorithms are validated by measurements of a battery, consisting of two modules each containing 50 automotive lithium-ion pouch cells. Decreasing cell temperatures close to the module edges are detected by the algorithms, which is validated by the use of temperature sensors.

A. Fill and K. Peter Birke –

„Influences of Cell to Cell Variances and the Battery Design on thermal and electrical imbalances among parallel Lithium-Ion Cells“

– IEEE 22nd International Conference on Industrial Technology, 2021, pp. 391-396.

Abstract: Batteries consist of a high number of single cells in order to fulfill the power and energy requirements of large-scale applications. Electrical and thermal imbalances among the cells of the battery arise during operation caused by cell to cell variations and battery design tolerances. This heterogeneous stress can result in a accelerated degradation of individual cells, which reduces the lifetime of the whole battery. To prevent this effect insights of the impacts on cell imbalances, regarding the State of Charge (SoC), temperature and current, are essential to improve the battery operation strategy. In this paper the influence of cell to cell variances and the thermal design of the battery on the mentioned cell imbalances are experimentally investigated for two parallel-connected cells. Experiments with three cell groups, having different cell parameter variances, are conducted. In order to investigate the thermal influence of the cell packaging and battery design, different cooling scenarios are considerd, like an insulation and a heterogeneous cooling of the cells.

A. Fill, D. Müller and K. P. Birke –

„Influence of the Cell Topology, Thermal Conditions and initial Cell to Cell Variances on Aging Gradients among Lithium-Ion Cells of a Battery“

– IEEE 15th International Conference on Compatibility, Power Electronics and Power Engineering, 2021, pp. 1-8.

Abstract: Due to the power and energy demand of large-scale applications, like electric vehicles, batteries consists of a high number of single cells connected in serial and parallel. Caused by cell manufacturing induced Cell to Cell Variances (CtCV) as well as system design tolerances in terms of contact resistances and cooling, heterogeneous electrical and thermal loads arise among the cells of a battery. This can lead to aging gradients, which can further increase the initial CtCV, resulting in an accelerated battery degradation. To evaluate the influence of initial CtCV, different thermal conditions of the cells, battery assembly tolerances as well as the cell topology on aging gradients an Equivalent Circuit Model (ECM) is coupled with a thermal and an aging model in this work. The models are implemented in Matlab® and Monte Carlo simulations are conducted to investigate the influences of the mentioned parameters.

A. Fill, D. Müller, T. Mader, A. Avdyli, T. Schmidt and K. P. Birke –

„Impact of Cell to Cell Variances on the Current Distribution among two parallel Lithium-Ion Cells at constant Current Conditions“

– IEEE 15th International Conference on Compatibility, Power Electronics and Power Engineering, 2021, pp. 1-7.

Abstract: Batteries of electric vehicles contain a high number of serial- and parallel-connected cells in order to fulfill the power and energy requirements. Due to Cell to Cell Variances (CtCV), induced by manufacturing tolerances, and the impact of the cell packaging, a heterogeneous electrical and thermal stress arise, among the single cells of a battery. This inhomogeneity can lead to an accelerated degradation of individual cells, reducing the lifetime of the whole battery. One essential factor of this effect is the current distribution within parallel-connected cells. This paper experimentally investigates the influence of the C-rate, Depth of Discharge (DoD) and CtCV on the current distribution within two parallel cells at constant current stress. Therefore measurements with three cell groups, which consist of two parallel cells, having different CtCV, are conducted. The measurements show a correlation of the current distribution to the Differential Voltage Analysis (DVA) and increasing current differences with increasing CtCV. The findings help to understand the dynamic of the current distribution and contributes to an optimization of the battery design and operation strategy.

A. Fill, M. Kopp, J. Hemmerling and K. P. Birke –

„Experimental Investigations on thermal and electrical Inhomogeneities within a Lithium-Ion Cell“

– IEEE Vehicle Power and Propulsion Conference, 2021, pp. 1-6.

Abstract: An important issue for a successful commercialization of battery-electric vehicles is the minimization of the cell degradation rate to guarantee the longest possible service life of the battery. One key factor is to ensure a homogeneous electrical and thermal stress among the cells of the battery and especially within each cell. There are two levers for this: on the one hand, the design of the cell and the battery, and on the other hand, the optimization of the battery operation. Otherwise, an accelerated aging of individual cells or certain cell areas, such as the core of the cell or the area close to the tabs, would lead to a faster aging of the entire cell or battery. In this article, the heterogeneous stress within the cell is studied experimentally. For this purpose, four cells are connected in parallel and the individual cells are thermally controlled that they represent different thermal conditions within a cell. The influence of the C-rate, cell temperature and current profile on the current, temperature and charge throughput gradients within the cell is investigated. Therefore, a cell pack, consisting of four parallel cells, is stressed with continuously alternating discharge/charge pulses. The experimental data show a predominant electrical and thermal load of the cells, which represent the layers in the center of the cell. In addition, the current and temperature gradients among the cells increase significantly with increasing C-rate and decreasing ambient temperature.