title: On the dynamic effects upon unsteady cooling of Lithium-ion batteries
Creators: name: Saeed, Ali
Publisher: name: University of Glasgow
Year: 2023

description: The consumption of fossil fuels has been recognised as a major contributor to climate change, air pollution, and environmental degradation. In response to the growing public awareness of these issues, regulatory bodies and the public are emphasising the need to reduce harmful gas emissions and our dependence on fossil fuels. The transport sector, in particular the automobile market, has been a significant source of pollutants, leading to a shift towards electric vehicles (EVs) to combat emissions.

The broader adoption of EVs faces many challenges related to their initial costs, driving range, and overall performance – all of which are linked to the battery cell. Battery themal management systems (BTMS) play a vital role in overcoming these barriers due to the direct influence on the battery cells’ operational temperature to their performance.

While extensive research has focused on steady-state scenarios, the unsteady nature of EVs necessitates a deeper understanding. In particular, battery cooling is influenced by unsteady heat generation, extreme driving conditions, and mechanical vibrations. Therefore, robust BTMS technologies require dynamic tools capable of predicting the thermal evolution of battery cells.

This thesis utilises a battery module based on real cell dimensions and employs high-fidelity simulations to investigate various temporal scenarios. The research explores the impact of disturbances on the battery cells and evaluates the system linearity using Fast Fourier Transforms (FFT) and Phase portraits (Lissajous patterns).

Firstly, a battery module subjected to unsteady surface heat flux and the corresponding numerical data is analysed to examine the extent of nonlinearity of the thermal system. Water and nonfluids are found to be far better at attaining linearity. However, regardless of fluid type, as long as the disturbances are of low amplitudes and short, the system can be approximated as linear. Increasing the forcing frequency causes the nonlinearity of the system to increase.

Furthermore, a battery module is subjected to realistic transient scenarios from standard driving cycles. The resultant surface-averaged temperature of each battery cell is analysed to determine the maximum overshoot, settling, heating, and cooling times. The results show that water-cooled battery cells consistently remain within their safe operating range and exhibit quicker response times to changes in the internal generation compared to air. regardless of coolant type, short period ramps result in higher values of settling time.

Lastly, batteries can experience mechanical vibrations due to several reasons, such as road roughness, the effects of which have largely been unexplored. A series of high-fidelity numerical simulations of a vibration battery cell are conducted to attain further understanding of the heat transfer processes involved and to identify conditions under which the thermal dynamics can be predicted. The resultant data is analysed and shows that only the water-cooled battery cells under low modulation amplitudes can be characterised as linear. Using air always leads to a strongly nonlinear thermal response.