Saurabh Bidari, MPAUT

Anisotropic temperature distribution within Li-ion cells in EV batteries


Conducted in co-operation with Volvo Cars Corporation, supervised by Zeyang Geng, Torbjörn Thiringer and Keerthana Arjun

Examiner: Torbjörn Thiringer, Dept of Electrical Engineering
Opponent: Pradeep Simpi

Abstract


One of the most pressing issues in the electric vehicle ecosystem is the service life of the Li-ion battery pack. As the cell's energy and power density improve, there has also been a reduction of thermal losses due to the internal resistance which depends on the electrical, chemical, thermal and mechanical properties of cell materials and their mutual interactions. Temperature plays a vital role in cell performance, safety and the nature of regular operation defines the variability of its service life. For a cell with a volume of 225 cm3, as in the current study, heat distribution through the volume becomes important, especially when the constituent materials are anisotropic, temperature-sensitive and possess poor thermal properties which inhibit effective cooling. Present work deals with understanding the interdependence of multiple physics contributing to the cell operation and creating an electro-thermal model of a Li-ion pouch cell determining temperature distribution within the cell volume. The finite element method is employed in multi-physics simulations. In addition, prior experimental test results are also utilized to calibrate the model to improve its accuracy and reliability. Pulse tests and standardized drive cycle current profiles are fed as input to correspond real vehicles on-road operations, hence obtaining respective temperature distributions.

It is observed that the temperature is the highest at the positive tab than at the negative and a substantial difference in temperatures at the cell surface and cell core is detected. The higher temperatures at specific sections of the cell resemble the experimental results. A difference of 3.3 °C is detected between extremes, i.e., positive tab and lower cell surface while the difference between the cell surface and cell core peaks at 0.5 °C for boundary conditions of an ambient room at 20 °C and 2C electric current.

A reference vehicle model with minor modifications is used to compare the powertrain thermal management system (TMS) performance. The TMS which used cell core temperature as a cooling system actuation parameter demonstrated a marginally higher drop in the state of charge as opposed to the system using cell surface temperature. This is due to the longer cooling requirement in the former case. However, the improvement in battery pack service life is not quantifiable because of single-cycle simulations and the complex chemical degradation phenomenon occurring over the cell’s lifetime. This reflection and its derivations provide an interesting scope for future studies.

Category Student project presentation
Location: Web seminar
Starts: 17 September, 2020, 08:00
Ends: 17 September, 2020, 09:00

Published: Thu 10 Sep 2020.