Improved Path Resistance Adjustment As a Valuable Tool to Homogenize the Current Distribution within Interconnected Lithium-Ion-Battery Modules
Authors: Schindler Markus , Durdel Axel , Sturm Johannes , Jocher Philipp , Jossen Andreas 
Published: December 2020

Range anxiety is one of the leading reasons why people buy conventional cars instead of battery electric vehicles (BEVs). At the same system voltage, an increase in range can be achieved by both higher cell capacities and parallel connection of cells. As cells with sufficient capacity still lack market maturity, parallel connection is currently the remaining alternative for car manufacturers. In parallel-connected lithium-ion battery packs, varying internal cell impedance, different contact resistances or an uneven state of health (SOH) of the cells cause inhomogeneous current distributions within interconnected lithium-ion-batteries (LIBs). However, varying system terminals, different connecting strategies, as well as the number of serial and parallel connections have a major impact on the inhomogeneity of the current distribution. The resulting path resistance of the parallel-connected cells combines all of these influences and is the key to reduce such inhomogeneous current distributions. The extent to which each of the contributors and the interaction between them affects the path resistance within the battery module is crucial to improve both the manufacturing process and the system’s efficiency.

In this study, the influence of the stated reasons on the path resistance of parallel-connected LIBs within a battery pack is analyzed, using physicochemical, electrical, and thermal coupled models. Experimental data of the commercially available cylindrical lithium-ion cell LG INR18650MJ1 was used for parameterization and validation of the physicochemical and thermal model. The cell incorporates NMC-811 as cathode active material as well as silicon doped graphite (SiC) on the anode side, which is a promising high energy material combination for future BEVs. To ensure a high practical applicability, the investigated system topologies were restricted from 1s2p to 4s4p. Due to limited installation space, the system terminals often cannot be freely chosen, which is why the positon of the system terminals were varied between side, middle and cross connections. Additionally, the influence of different linking strategies between the serial- and parallel-connected strings was examined.

An example of a resulting current distribution for a battery pack with 4s4p topology, using cross- connectors between serial- and parallel-connected LIBs as well as cross-connection of the system terminals is shown in Fig. 1. Part a) illustrates the schematic diagram, while the current distribution is shown in part b). For side connection the system terminal next to cell c4,4 would switch to cell c4,1.

As a result, axis and point (see Fig. 1) symmetry patterns within the battery modules could be recognized, depending on the number of serial- and parallel-connected cells and the choice of the system terminals. Furthermore, the presence of cross-connectors leads to equivalent current distributions within the analyzed systems, regardless of whether the system terminals were contacted on the side or crosswise. For car manufacturers, this implicates another level of freedom, since the presence of cross-connectors allows choosing the positon of the system terminals without negative impacts on the current distribution. Additionally, the analyses of different contact resistances showed that particular attention has to be paid to string connectors during the assembling process of battery modules. On the other hand, the impacts of welding spot resistances and cross-connector resistances on the path resistance cannot be neglected by the car manufacturers, but are of minor importance compared to the influence of string connector resistances and the presence of cross-connectors.

Current studies therefore focus on verifying these results using measurement data. In particular, the voltage response of parallel-connected cells is considered as an indicator for uneven path resistance, since measurement data of inhomogeneous current distributions are usually missing within battery modules. Furthermore, the influence of varying path resistances on the aging behavior of interconnected LIBs could be of great interest to car manufacturers and has therefore to be investigated in further studies.

The results presented were achieved in association with an INI.TUM project, funded by AUDI AG.

Figure 1