Thermal effects and management of lithium ion batteries for automotive applications

Abstract

In recent years the use of lithium ion batteries in hybrid and electric vehicles has increased enormously. Due to the high cost of lithium ion batteries, maximising their performance and reducing the rate of degradation is vitally important to vehicle manufacturers. There are many ways in which this can be done, however this thesis focuses on the thermal behaviour of lithium ion batteries and how thermal management can affect performance and levels of degradation. The importance of good experimental design is discussed, with a specific focus on thermal boundary conditions. In addition, some experimental examples are shown highlighting the impact that thermal boundary conditions can have when testing batteries. The thermal properties of cells were measured, and an approach based on Searle’s bar technique and using cells of different sizes was used in order to measure the thermal conductivity of different components and interfaces in a cell. This approach was successful in measuring the thermal conductivity through the layers of a cell, and the results showed a 32% discrepancy with often-used literature values, showing how important the measurement of thermal conductivity is when designing battery thermal management systems or parameterising coupled electrochemical-thermal models. The effect of temperature gradients on the performance of batteries was investigated, showing that a cell under a temperature gradient exhibits poorer performance compared to a cell at a temperature equivalent to the mean of the gradient. This result was reproduced using a simple data-driven model, which showed significant inhomogeneous behaviour of the different layers of the cell. Finally, the effects of cell surface cooling and cell tab cooling were investigated, reproducing two typical cooling systems that are used in real-world battery packs. For new cells using slow-rate standardized testing, very little difference in capacity was seen. However at higher rates, surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling. After cycling the cells for 1,000 times, surface cooling resulted in a rate of loss of useable capacity under load three times higher than cell tab cooling. Understanding how thermal management systems interact with the operation of batteries is therefore critical in extending their performance. For automotive applications where 80% capacity is considered end-of-life, using tab cooling rather than surface cooling would therefore be equivalent to extending the lifetime of a pack by 3 times, or reducing the lifetime cost by 66%.