journal articles

A. Fill, S. Koch, A. Pott and K. P. Birke –

„Current distribution of parallel-connected cells in dependence of cell resistance, capacity, energy density and number of parallel cells“

– J. Power Sources, vol. 407, 2018, pp. 147-152.

Abstract: Parallel-connection of lithium-ion cells is of increasing research interest, caused by the commercialization of large-scale applications and their needed amount of energy. However, correlations of cell parameters on the current distribution of parallel-connected cells are not given in present literature. This paper provides an insight into the influence of cell resistance and capacity on the current distribution, open circuit voltage (OCV) dif- ferences and the corresponding time constants for parallel-connected cells. Cell currents and OCV differences are evaluated via an analytical model for two parallel-connected cells. The system can be described by four characteristic values, what is mathematically shown. The analytical solution and characteristic values are extended to n parallel-connected (np) cells by a transformation of the np equivalent circuit model (ECM) to a corresponding 2p ECM. The models are used in order to investigate the effects of Gaussian distributed cell parameters on the current distribution by Monte Carlo simulations. Thereby, the influence of the number of parallel-connected cells and the product of cell resistance and capacity is examined. Simulations show increasing cell currents with increasing number of parallel-connected cells and an accelerated SoC balance within the logical cell by decreasing product of cell resistance and capacity.

A. Fill, S. Koch and K. P. Birke –

„Analytical model of the current distribution of parallel-connected battery cells and strings“

– J. Energy Storage, vol. 23, 2019, pp. 37-43.

Abstract: The parallel-connection of lithium-ion cells and strings is of increasing research interest, mainly due to the energy and power demands of large-scale applications, e.g. electric vehicles (EV). For battery systems an accurate estimation of the current distribution within these parallel configurations is crucial for optimal operation and system design. The present paper provides an analytical model for the current distribution of cells and strings connected in parallel. Eight characteristic values are determined, fully describing these systems. These characteristic values are used to investigate the impacts of cell resistance and capacity distributions on maximum currents, open circuit voltage (OCV) and state of charge (SoC) differences within the parallel-connected cells and strings. It is found, that maximum cell currents and SoC differences within parallel-connected cells increase almost logarithmically with the number of parallel- and serial-connected cells. A linear correlation of these values to the related standard deviation of cell’s resistance and capacity is shown by Monte Carlo simulations. Maximum string currents decrease by square root with the number of parallel- and serial-connected cells, due to statistical averaging, which cause a square root correlation of the related standard deviation to the number of parallel- and serial-connected cells.

A. Fill, S. Koch and K. P. Birke –

„Algorithm for the detection of a single cell contact loss within parallel-connected cells based on continuous resistance ratio estimation“

– J. Energy Storage, vol. 27, 2020, 101049.

Abstract: Large-scale battery applications like electric vehicles (EV), hybrid electric vehicles (HEV) and stationary energy storage systems have high energy and power demands. In order to provide these requirements these battery systems mostly consist of parallel-connected cells and/or strings. These parallel configurations enable closed electric circuits even if a single cell or string gets disconnected, which can be caused by, e.g mechanic stress and chemical degradation. This type of connection error has rarely been investigated in present articles and is therefore the subject of this article. The detachment of one or more cells has to be detected in order to prevent electrical or thermal overloading of the remaining former parallel-connected cells. Otherwise in the worst case non detection can lead to hazardous safety issues like thermal runaway and propagation. This article provides an algorithm detecting the detachment of a cell within a few seconds based on continuous resistance ratio estimation. The algorithm is validated by measurements on a module with three serial-connected cell groups, each consisting of two parallel-connected 50 Ah pouch cells. Under typical driving loads a cell detachment is detected in less than 14 s. Furthermore, the estimated resistance ratios are compared to the pulse response approach with a mean deviation of ξ = 0.69% for the reference, respectively ξ = 0.9% for the measurements with one detached cell.

A. Fill, T. Mader, T. Schmidt, R. Llorente and K. P. Birke –

„Measuring Test Bench with Adjustable Thermal Connection of Cells to Their Neighbors and a New Model Approach for Parallel-Connected Cells“

– Batteries, vol. 6, 2020, 6010002.

Abstract: This article presents a test bench with variable temperature control of the individual cells connected in parallel. This allows to reconstruct arising temperature gradients in a battery module and to investigate their effects on the current distribution. The influence of additional contact resistances induced by the test bench is determined and minimized. The contact resistances are reduced from RTab+ = 81.18 μΩ to RTab+ = 55.15 μΩ at the positive respectively from RTab− = 35.59 μΩ to RTab− = 28.2 μΩ at the negative tab by mechanical and chemical treating. An increase of the contact resistance at the positive tab is prevented by air seal of the contact. The resistance of the load cable must not be arbitrarily small, as the cable is used as a shunt for current measurement. In order to investigate their impacts, measurements with two parallel-connected cells and different load cables with a resistance of RCab+ = 0.3 mΩ, RCab+ = 1.6 mΩ and RCab+ = 4.35 mΩ are conducted. A shift to lower current differences with decreasing cable resistance but qualitatively the same dynamic of the current distribution is found. An extended dual polarization model is introduced, considering the current distribution within the cells as well as the additional resistances induced by the test bench. The model shows a high correspondence to measurements with two parallel-connected cells, with a Root Mean Square Deviation (RMSD) of ξRMSD = 0.083 A.

A. Fill, T. Schmidt, T. Mader, R. Llorente, A. Avdyli, B. Mulder and K. P. Birke –

„Influence of cell parameter differences and dynamic current stresses on the current distribution within parallel-connected lithium-ion cells“

– J. Energy Storage, vol. 23, 2020, 101929.

Abstract: To satisfy energy and power requirements of electric vehicles, traction batteries contain a high number of parallel- as well as serial-connected lithium-ion cells. Due to cell manufacturing and battery architecture tolerances, inhomogeneous electrical and thermal stresses on the individual cells arise during operation. To guarantee the service life of the battery, the power output of the battery has to be limited depending on the most loaded cells. This article investigates the influences of cell parameter variances and dynamic current stresses on the current distribution within parallel cells. A series of measurements considering two consecutive pulses with varied dynamic current conditions, continuous alternating charge and discharge pulses, as well as a typical drive cycle are conducted. Correlations of cell parameter variances and the current conditions to the current distribution are provided and validated using experimental data. This knowledge helps predicting the cell currents onboard and optimizing the operating window of the battery while guaranteeing the service life.

A. Fill, T. Schmidt, T. Mader, A. Avdyli, B. Mulder and K. P. Birke –

„New semi-analytical model approach of the current distribution within parallel-connected lithium-ion cells“

– J. Energy Storage, vol. 40, 2021, 102653.

Abstract: Automotive batteries contain a high number of parallel- and serial-connected cells to fulfill the energy and power requirements of electric vehicles. Due to current, State of Charge (SoC) and temperature differences among the lithium-ion cells, caused by cell production and battery design tolerances, the power and energy of the battery is limited to the most stressed cell. In order to provide the best battery performance, regarding fast charging, recuperation and acceleration of the electric vehicle, the current distribution within the parallel- connected cells has to be estimated onboard. This article provides a semi-analytical model of the current distribution, with low parametrization and computational effort sufficient for an implementation in the Battery Management System (BMS). The model is validated by measurements of two parallel-connected cells with constant and dynamic current stress. The model output fit well to the experimental data with a related Root Mean Square Deviation (RMSD) of 𝜎RMSD ≈ 0.5 % (constant stress) and of 𝜎RMSD ≈ 1 % (dynamic stress). Further, the model mathematically describes the affects of the Open Circuit Voltage (OCV) bending on the current distribution among parallel cells, which closes an important lack of knowledge. This correlation explains the characteristic current cross at 0.2 ≤ SoC ≤ 0.45 and the peak of the current difference at the end of a constant discharge stress.

A. Fill, T. Mader, T. Schmidt, A. Avdyli, M. Kopp and K. P. Birke –

„Experimental investigations on current and temperature imbalances among parallel-connected lithium-ion cells at different thermal conditions“

– J. Energy Storage, vol. 51, 2022, 104325.

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.