The current distribution of lithium-ion batteries connected in parallel is asymmetric. This influences the performance of battery modules and packs. The ratio of asymmetry depends on the differences between the battery cell parameters and the dynamics of the load profile. This detailed simulative study varies both of these factors and shows the influences on current and charge throughput. The cell parameters are based on real-world effects caus. The current distribution of lithium-ion batteries connected in parallel is asymmetric. This influences the performance of battery modules and packs. The ratio of asymmetry depends on the differences between the battery cell parameters and the dynamics of the load profile. This detailed simulative study varies both of these factors and shows the influences on current and charge throughput. The cell parameters are based on real-world effects caused by production and operation. Differences in impedance generate higher current deltas and charge throughput differences compared to capacity differences due to manufacturing fluctuations.The simulation model in this study uses mainly a linear open circuit voltage (OCV) so that the results are not influenced by nonlinearities. A subsequent analysis uses a defined nonlinearity in the OCV to show its impact on the current distribution. The results show that the temporary difference in current caused by the nonlinearity of the OCV exceeds the effect of the chosen parameter differences.Finally, a comparison of the different cell dimensioning shows that high-energy (HE) cells display an inert behaviour with respect to current asymmetry. High-power (HP) cells are more dynamic. This means that impedance differences h. ••Modular, matrix-based state-space model to calculate the current distribution.••Capacity variation has less impact on current difference than impedance variation.••Changes of the OCV slope influence the current distribution significantly.••High-power systems are more likely to be influenced by capacity differences.••High-energy systems are more likely to be influenced by impedance differences.Current distributionParallel connectionBattery cell variationElectrical equivalent circuit modelBattery systemLithium-ion battery cellApplications for battery cells and systems cover a wide field. Smartphones use only one battery cell. Power tools, mobile electronic systems and starter batteries have several cells in series and sometimes in parallel. Traction batteries for electric vehicles (EVs), as well as home or grid storage batteries, have an output voltage of several hundred volts, with series connections being needed to achieve these high voltages. The costs of semiconductors and the volume of electrical insolation limit the maximum voltage of these large battery systems. To increase the energy content, either the cells need to have a higher capacity or small cells must be connected in parallel.Both approaches and hybrid forms can be found in commercial applications. The 2017 BMW i3 model uses no parallel connections at all. Its battery system consists of 96 cells connected in series, each with 96Ah. Nissan's Leaf features two parallel cells. In the automotive field, Tesla uses the largest number of cells connected in parallel; its Model S uses up to 86 parallel cells. In the field of stationary storage, almost all manufacturers build systems with a large number of small cells connected in parallel.Parallel connections are very flexible. Different requirements of different applications can be fulfilled with the same type of cell but a different number o. 2.1. State-space modelThe electrical voltage Un of a lithium-ion cell n is composed of the sum of OCVn, the resistive URs,n, and the dynamic voltage drops Up,n (2). Fig. 2 shows the structure of the EEC model for N parallel connected cells. The dynamic voltage drop for a cell can be made up of the sum of K single voltages of each RC-member k. Each dynamic voltage drop Up,k,n can be described in terms of the parameters Rp,k,n and Cp,k,n, as well as the current in of cell n via a common differential equation of the first order (3). If N cells are connected in parallel, a total current itot results from the sum of all phase currents in (4).(2)Un=OCVn+URs,n+Up,nwithUp,n=∑k=1KUp,k,n(3)dUp,k,ntdt=-Up,k,ntRp,k,n·Cp,k,n+inCp,k,n(4)∑n=1Nin=itot2.2. Model verificationAn EEC model with three parameter sets is used and varied in this paper to show different influences. To validate the EEC, an HP cell from LG (Type ICR18650HB2), and an HE cell from Panasonic (Type NCR18650PF) are connected in parallel. This constellation can represent an impedance difference on the one hand and a capacity difference on the other. Measurements at 25°.
