Parallel-connected Lithium-ion Cells Research Articles

A Novel Quick Screening Method for the Second Usage of Parallel-connected Lithium-ion Cells Based on the Current Distribution

Battery screening is the key segment of secondary applications. The benchmark for conventional methods is mainly based on the series connection and makes parameter difference as the screening index a gold standard. However, because of self-balancing current in parallel connection, the existence of a certain degree of parameter difference is allowed and parameter difference may not be the best option, which leads to lower screening efficiency due to the higher uniform of parameters. This work firstly identifies the boundary of parameter difference and provides the ideal working point (IWP), which is related to maximum capacity utilization efficiency, as a novel screening index for parallel connection derived from the current distribution. A modified shepherd model is employed to calculate the IWPs and is verified that the maximum dynamic error is below 1.1%. Therefore, it is quick to achieve screening by judging whether the IWP falls within the normal working range. The results show that the maximum capacity utilization efficiency always occurs at the load current close to the IWP, which verifies the validity of IWP. Compared with the conventional method, the proposed method is validated based on a case study to improve screening efficiency and provides different ideas for flexible grouping.

A Novel Measurement Technique for Parallel-Connected Lithium-Ion Cells with Controllable Interconnection Resistance

Due to the broad use of parallel-connected cells across a wide variety of applications, it is essential to understand the current distribution between them. Variations in resistance, temperature and capacity can lead to an inhomogeneous current distribution and have a deleterious influence on ageing and safety. It is therefore crucial to investigate the current distribution within such systems. Many researchers have designed distinct test-benches to measure the current distribution. In any test-bench, known and unknown resistances caused by wiring, interconnections and measurement equipment can adversely affect the current distribution. The aim of this work is to design a low-complexity test apparatus that does not itself affect the current measurement.This work describes a novel measurement technique [1] to analyse the current distribution within parallel-connected cells without the use of complex custom-made test benches. While the cells are physically connected in conventional test benches, the proposed technique utilizes the software of a commercially available battery cycler to connect the cells in parallel. Because the software replaces the parallel junction within equations, this approach is called “virtual parallel connection“. Each cell is individually connected to the battery cycler and each cell voltage or current can be controlled separately. While a current pulse is used to determine the voltage across the reference cell, the control unit imposes the same voltage across each cell. Therefore, each cell benefits from the 4-wire measurement of the battery cycler, which generates no additional interconnection resistances. For this reason, only the Open Circuit Voltage (OCV), the capacity and the resistance of the cell itself, as well as its relationship to other cells can influence the current distribution. Additionally, any desired connection resistance can be chosen by adding a voltage drop in the equation of the battery cycler. Connecting cells virtually has many advantages, such as the ability to decouple the cells for check-ups and recouple them for cycling without ever touching them physically, independent location of each cell, scaling to n-parallel constellations, low assembly effort on the test-bench and defined contact resistances. In addition, different cell formats can be investigated with no additional effort.In this work, the virtual parallel connection will first be demonstrated, validated and discussed for constant current-, constant voltage- and rest phases using of a conventional test bench [2] within two NMC cells. Next, to understand the influence of additional resistances in two parallel-connected cells, two measurement studies were carried out in which interconnection resistances were varied between 0 and 5 mΩ. The first study investigates the influence of an inhomogeneous resistance increase within one of the parallel paths. Subsequently, the second study examines a homogeneous resistance increase within both parallel-connected paths.The conclusion of both studies is that the height of the local minima and maxima of the current distribution are mainly dependent on the resistance ratio of the parallel legs. The minima and maxima divide relative to the ratio of the combined cell and interconnection resistance in each pathway. In contrast, neither the local minima and maxima measured at various cumulative charge throughputs, nor the intersection points of the current distribution are affected by varied resistances. Meanwhile, OCV interactions between both cells determine the shape of the current distribution.Additionally, the shape of the Differential Voltage Analysis (DVA) showed correlations with current distribution. Some local minima and maxima within the current appear in almost the same region as the local minima and maxima of the DVA. By assigning different peaks of both half-cell profiles of the DVA to the anode and the cathode, the current distribution of both currents is expected to be affected by these characteristic peaks.

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

In this article the influence of Cell to Cell Variances (CtCV) and the thermal design of the battery on electrical and thermal cell imbalances are experimentally investigated for different cell configurations. Measurements with six cell groups are conducted, four cell groups with two parallel cells holding different CtCV and one cell group each with three and four parallel cells. In order to emulate different thermal conditions within a battery, the cells are separated locally and clamped between temperature-controllable aluminum plates. Due to this setup different conditions and battery system components can be emulated, such as insulation material, an adjacent cell or the battery housing. Asides from full discharge/charge cycles with constant current, the relaxation behavior and isolated selective current pulses are considered. It is found that the current inhomogeneity is superimposed by two effects: the voltage source difference ΔUsc and the reciprocal resistance ratio R2/(R1+R2), with the resistance of cell 1 R1 and cell 2 R2. The full discharge/charge cycles with constant current leading to a build up of ΔUsc acting against the resistance ratio. In case of subsequent relaxation, only ΔUsc is effective, since the resistance term is multiplied by the battery current IBatt. By applying an inversely directed pulse, ΔUsc and R2/(R1+R2) act in the same direction.

Electrochemical and Post-Mortem Degradation Analysis of Parallel-Connected Lithium-Ion Cells with Non-Uniform Temperature Distribution

Prior work on Li-ion cells which were parallel-connected in a stack and subjected to long term cycling showed that non-uniform temperature distribution caused non-uniform and accelerated degradation. To elucidate the degradation mechanisms, electrochemical and post-mortem degradation analysis were performed. Electrochemical impedance spectroscopy analysis suggested that the main degradation mechanism for the middle cell was a solid electrolyte interface (SEI) layer growth. Nevertheless, post-mortem analysis using X-ray diffraction, optical microscope, and scanning electron microscopy paired with energy dispersive X-ray spectroscopy shows the presence of Li2CO3 in both baseline and middle cell anodes. This points towards a combined degradation mechanism of SEI layer growth and lithium plating. A combination of microstructural particle cracking and lithium plating is considered the main mechanism for blocking the anode’s porosity network, which hindered further lithium diffusion, resulting in the abrupt failure for the middle cell. The observation and quantitative analysis provides insight into the performance and reliability impacts of non-uniform conditions within lithium-ion batteries.

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

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 novel measurement technique for parallel-connected lithium-ion cells with controllable interconnection resistance

Due to the broad use of parallel-connected cells across multiple applications, it is essential to understand the current distribution between them. Variations in resistance, temperature and capacity can lead to an inhomogeneous current distribution and have a deleterious influence on ageing and safety. It is therefore crucial to investigate the current distribution within such systems. However, the task of designing a low-complexity test apparatus, that does not itself affect the current measurement, remains incomplete.This work investigates a novel measurement method to connect cells in parallel with controllable interconnection resistances. Instead of a physical connection, the presented method couples the cell using Kirchhoff’s laws via a commercial battery cycler. This connection allows investigation of parallel-connected cells, without influencing factors such as contact resistances or an additional measurement environment.Further, two studies demonstrate the influence of the additional interconnection resistance caused by the parallel connection of two cells. The results of measurements including a differential voltage analysis show, that the cell current divides according the ratio of the combined cell and interconnection resistance, whilst the open-circuit-voltage influences the shape of the current distribution.

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

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.

On the Impact of Internal Cross-Linking and Connection Properties on the Current Distribution in Lithium-Ion Battery Modules

The often-observed current distribution between parallel-connected lithium-ion cells within battery modules is probably evoked by the properties of the connection, inhomogeneous contact and power line resistances, the impedance behavior of single cells and of the DoD. The extent to which each of the contributors and the interaction between them affects the current distribution within the battery module is crucial to improve the system’s efficiency, which is investigated here via various electrically cross-linked, physicochemical-thermal simulations with variable system terminal (ST). Consequently, cross-connectors balance the system and reduce DoD shifts between cells. Furthermore, if one compares ST-side and ST-cross, the position of the ST is negligible for topologies that incorporate at least two cells in serial and parallel. Depending on the ST configuration, point and axis symmetry patterns appear for the current distribution. Compared to welding seam and cross-connector resistances, the string connector resistance dominates the current distribution. Like the behavior of a single cell, the system’s rate capability shows a non-linear decrease with increasing C-rate under constant current discharge. As a recommendation for the assembly of battery modules using multiple lithium-ion cells, the position of the ST is of minor importance compared to the presence of cross-connectors and low-resistance string connectors.

Correlations of cell-to-cell parameter variations on current and state-of-charge distributions within parallel-connected lithium-ion cells

Parallel-connected lithium-ion cells are extensively applied in electric vehicles to satisfy the capacity and power capability. However, unavoidable cell-to-cell parameter variations lead to inhomogeneous current and state-of-charge distributions, causing early degradation of the cells. Therefore, knowledge of correlations of the parameter variations on current and state-of-charge distributions is crucial for improving these inhomogeneities. In order to clearly analyze the correlations, this paper provides a simplified analytical circuit model consisting of a resistance and a capacitance in series, and the capacitance is a function of capacity and open circuit voltage slope. Using the two parallel-connected cells analytical model, the current and state-of-charge distributions are theoretically investigated by varying cell parameters. For a large number of parallel-connected cells with Gaussian distributed parameters, a Laplace transform-based approximate analytical model is proposed based on the simplified analytical circuit model and validated by the simulation. Then the model is utilized to deduce mathematical correlations of the standard deviation and mean value of the cell parameters on maximum differences of current rate and state-of-charge. Results verify that decreasing the deviation of the product of cell resistance and capacity can effectively reduce the inhomogeneities, which is helpful for optimal screening approach for parallel-connected new and retired batteries.

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

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.

Influence of circuit design on load distribution and performance of parallel-connected Lithium ion cells for photovoltaic home storage systems

In parallel cell circuits within Lithium ion home storage systems, both the components as well as the topological structure of the circuit have an enormous influence on the current distribution among the individual cells and as consequence on the battery lifetime. For large battery assemblies welding techniques are often used for which a weldable current collector is needed. Due to the high specific resistance of the commonly used materials, this resistance measurably influences the current distribution.In this paper the current distribution in battery modules with Li ion cells connected in parallel has been measured and modelled. The cell internal resistance, the current collector resistance and the topology of the circuitry have been varied in order to determine the influence of each of these parameters on the current distribution. Model calculations for the cycle lifetime of battery modules have been performed assuming realistic charge and discharge profiles matching the conditions to be found in commercially available home storage systems.From the experimental data and the modeling results the most important parameters determining the homogeneity of the current distribution and the cycle lifetime of the battery modules have been extracted. Finally, design recommendations for Li ion batteries with cells connected in parallel are derived.

Modelling and experimental evaluation of parallel connected lithium ion cells for an electric vehicle battery system

Variations in cell properties are unavoidable and can be caused by manufacturing tolerances and usage conditions. As a result of this, cells connected in series may have different voltages and states of charge that limit the energy and power capability of the complete battery pack. Methods of removing this energy imbalance have been extensively reported within literature. However, there has been little discussion around the effect that such variation has when cells are connected electrically in parallel. This work aims to explore the impact of connecting cells, with varied properties, in parallel and the issues regarding energy imbalance and battery management that may arise. This has been achieved through analysing experimental data and a validated model. The main results from this study highlight that significant differences in current flow can occur between cells within a parallel stack that will affect how the cells age and the temperature distribution within the battery assembly.

Internal resistance matching for parallel-connected lithium-ion cells and impacts on battery pack cycle life

When assembling lithium-ion cells into functional battery packs, it is common to connect multiple cells in parallel. Here we present experimental and modeling results demonstrating that, when lithium ion cells are connected in parallel and cycled at high rate, matching of internal resistance is important in ensuring long cycle life of the battery pack. Specifically, a 20% difference in cell internal resistance between two cells cycled in parallel can lead to approximately 40% reduction in cycle life when compared to two cells parallel-connected with very similar internal resistance. We show that an internal resistance mismatch leads to high current into each cell during part of the charging cycle. Since capacity fading is strongly dependent on temperature, and hence on charging rate when this rate is sufficiently high, the high current leads to substantially accelerated capacity fade in both cells. A model, based on the formation of a solid-electrolyte interphase, is able to explain the dependence of lifetime on resistance mismatch, and also identifies the importance of random sudden capacity losses.