Parallel-connected Battery Cells Research Articles

Current Imbalance in Dissimilar Parallel-Connected Batteries and the Fate of Degradation Convergence

Abstract This paper proposes an analytical framework describing how initial capacity and resistance variability in parallel-connected battery cells may inflict additional variability or reduce variability while the cells age. We derive closed-form equations for current and SOC imbalance dynamics within a charge or discharge cycle. These dynamics are represented by a first-order equivalent circuit model and validated against experimental data. To demonstrate how current and state of charge (SOC) imbalance leads to cell degradation, we developed a successive update scheme in which the intercycle imbalance dynamics update the intracycle degradation dynamics, and vice versa. Using this framework, we demonstrate that current imbalance can cause convergent degradation trajectories, consistent with previous reports. However, we also demonstrate that different degradation assumptions, such as those associated with SOC imbalance, may cause divergent degradation. We finally highlight the role of different cell chemistries, including different OCV function nonlinearities, on system behavior, and derive analytical bounds on the SOC imbalance using Lyapunov analysis.

Internal Short Circuit Detection for Parallel-Connected Battery Cells Using Convolutional Neural Network

Reliable and timely detection of an internal short circuit (ISC) in lithium-ion batteries is important to ensure safe and efficient operation. This paper investigates ISC detection of parallel-connected battery cells by considering cell non-uniformity and sensor limitation (i.e., no independent current sensors for individual cells in a parallel string). To characterize ISC-related signatures in battery string responses, an electro-thermal model of parallel-connected battery cells is first established that explicitly captures ISC. By analyzing the data generated from the electro-thermal model, the distribution of surface temperature among individual cells within the battery string is identified as an indicator for ISC detection under the constraints of sensor limitations. A convolutional neural network (CNN) is then designed to estimate the ISC resistance by using the cell surface temperature and the total capacity of the string as inputs. Based on the estimated ISC resistance from CNN, the strings are classified as faulty or non-faulty to guide the examination or replacement of the battery. The algorithm is evaluated in the presence of signal noises in terms of accuracy, false alarm rate, and missed detection rate, verifying the effectiveness and robustness of the proposed approach.

Progression of cell-to-cell variation within battery modules under different cooling structures

Cell-to-cell variation generally exists within battery packs, due to factors attributed to manufacturing and operating. Non-uniform temperature distribution, caused by the uneven cooling condition, contributes significantly to cell-to-cell variation over time, particularly to capacity variation, as temperature significantly influences the battery degradation rate. Especially for parallel-connected cells, the lack of individual current sensing and actuation makes it challenging to detect and control the capacity variation. In order to understand how cell-to-cell variation evolves, we investigate the effect of cooling structures on the progression of variation within parallel-connected battery cells using an electro-thermal-aging model for battery cells and a thermal model of the cooling system. The simulation result shows that the cell-to-cell variations increase initially because of uneven cooling conditions, but then the variation decreases over time, thanks to the self-balancing mechanism among parallel-connected cells. Moreover, when comparing the sequential and round cooling structures, it is found that the round cooling structure, which provides a more uniform cooling condition to all cells, has significant advantage in terms of suppressing the cell-to-cell variation, especially for large battery strings. The even cooling structure is preferred in practical applications; however, the trade-off between cooling system complexity and performance needs to be carefully considered.

Active cell balancing control strategy for parallel connected LiFePO4 batteries

While several recent studies have focused on eliminating the imbalance of energy stored in series-connected battery cells, very little attention has been given to balancing the energy stored in parallel-connected battery cells. As such, this paper aims at presenting a new balancing approach for parallel LiFePO 4 battery cells. In this regard, a Backpropagation Neural Network (BPNN) based technique is employed to develop a Battery Management System (BMS) that can assess the charging status of all cells and control its operations through a DC/DC Buck-Boost converter. Simulation results demonstrate the effectiveness of the proposed approach in balancing the energy stored in parallel-connected battery cells in which the state of charge (SoC) estimation error is found to be only 1.15%.

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

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.

Current distribution within parallel-connected battery cells

Parallel connections can be found in many battery applications. Therefore, it is of high interest to understand how the current distributes within parallel battery cells. However, the number of publications on this topic is comparably low. Furthermore, the measurement set-ups are often not clearly defined in existing publications and it is likely that additional impedances distorted the measured current distributions. In this work, the principles of current distributions within parallel-connected battery cells are investigated theoretically, with an equivalent electric circuit model, and by measurements. A measurement set-up is developed that does not significantly influence the measurements, as proven by impedance spectroscopy. On this basis, two parameter scenarios are analyzed: the ΔR scenario stands for battery cells with differing impedances but similar capacities and the ΔC scenario for differing capacities and similar impedances. Out of 172 brand-new lithium-ion battery cells, pairs are built to practically represent the ΔR and ΔC scenarios. If a charging pulse is applied to the ΔR scenario, currents initially divide according to the current divider but equalize in constant current phases. The current divider has no effect on ΔC pairs but, as a rule of thumb for long-term loads, currents divide according to the battery cell capacities.

Detachable electrical connection of battery cells by press contacts

In battery applications, every electrical connection of battery cells is important because it influences functionality, efficiency, and safety. Increased contact resistances generate more heat at the affected terminals and if contact resistances of parallel-connected battery cells differ the current divides unequally.Amongst different techniques, detachable connections are easy to handle and have advantages for service, repair and recycling. Usually, detachable electrical connections are based on the functional principle of press contacts. So, press contacts are frequently used for consumer batteries in electronic devices, for pouch battery cells in scientific tests, and for large battery cells with screw connections.This paper focusses on the electrical contact resistance of press contacts for battery cells. A novel measuring method to assess the contact resistance itself is presented. Common terminal and connector metals for battery cells and the dependence of their contact resistances on the contact pressure, surface roughness, and contact area are investigated and compared with welding techniques. Only if these dependencies are considered, contact resistances as low as for welded connections are achieved. Furthermore, due to its hard passivating Al203 surface film, aluminum contacts need a certain surface roughness. The obtained results can serve for modeling and development of battery assemblies.

Thermal runaway due to symmetry breaking in parallel-connected battery cells

SUMMARY A battery energy system consists of networks of cells. This paper considers the coupling among the cells that are connected in parallel. Cells are designed to react uniformly in response to charging and discharging. We study the possible occurrence of symmetry breaking when small perturbation leads to further deviation from uniform responses. By including coupling among the cells to the existing battery models, we formulate a dynamical system governing the rate of change of thermal and electrical quantities of the cells. Limited numerical solution using fourth-order Runge–Kutta method was carried out, and stability criteria are obtained from algebraic analysis. The results show that a much more stringent effort in thermal management is required to prevent thermal runaway initiated by cells interacting with each other due to asymmetric perturbations. The increased heat transfer between the two cells serves to offset the undesirable effect of the parallel connection, but the stabilizing effect could be greatly compromised if the cell incorporates three parallel units as in some commercial Li-ion battery cells. Copyright © 2013 John Wiley & Sons, Ltd.