Holistic simulation models running in parallel to a battery system as so-called digital twins have drawn increasing interest of researchers in recent years, especially for large-scale and investment-intensive automotive, marine or stationary systems. Regarding the thermal domain, both the average temperature as well as the distribution of the individual cell temperatures within such multi-cell structures are of high relevance since they influence the aging progress as well as the intensity of the occurring electrical cell-to-cell variations. In this context, numerical CFD approaches and thermal equivalent circuit models represent the current state of the art in calculating the temperature distribution. However, the existing approaches usually come with two major drawbacks: 1) The common parameterization approaches are very demanding both regrading time and required measurement equipment. 2) During computation of the obtained models, both approaches do not allow a real-time simulation of complex multi-cell structures consisting of thousands of cells as common in the named applications.
In this context, this work intends to present a novel performant and non-destructive characterization and modeling process which allows the calculation of the temperature distribution within a lithium-ion battery module with a comparably low computational effort. The investigations are conducted within the EU-funded research project “Nautilus”, which investigates hybrid battery and fuel cell energy systems for next-generation cruise ships.
For modeling the temperature distribution, a two-stage approach is applied. In the first stage, the module is represented by a thermal equivalent circuit containing each individual cell as a 0D heat capacity, allowing a cell-specific calculation of the temperature. In the second stage, the model is reduced to a mean-difference model allowing the calculation of the temperature extrema in the module with a significantly reduced computational effort. For the parameterization, a set of measurements and methods for data analysis, which can be conducted in a non-destructive manner is proposed. Figure 1 shows a central measurement during which the module is externally heated on one side while being isolated on the long sides resulting in a lengthwise thermal gradient allowing the calculation of the heat transfer within the cell stack. The resulting two-stage model is validated against common marine and stationary application profiles using a commercial lithium ion battery module with 64 Ah NMC pouch cells in 14s2p configuration. Figure 2a shows the comparison of the average temperatures while Figure 2b plots the resulting temperature spread within the module. It can be shown that the novel two-stage approach is able to determine the temperature distribution with sufficient accuracy while heavily reducing the calculation time and characterization effort compared to existing approaches.