Novel battery thermal management systems for electric vehicles

The evolution and optimisation of battery thermal management systems (BTMS) are critical for enhancing the performance and lifespan of electric vehicles (EVs), which rely heavily on the efficiency of lithium-ion cells. The durability and longevity of these cells are significantly influenced by operating temperatures, necessitating advanced thermal management to mitigate adverse effects on battery health and function. This thesis explores various aspects of thermal management in lithium-ion batteries, including electrochemical modelling, BTMS technologies, and electrical connection topologies.

Before conducting the thermal analysis of novel BTMSs, it is important to quantify the heat generation of the battery module. The lumped single particle model (LSPM) is an ideal electrochemical model for module-level simulation due to its balance of computational cost and accuracy, particularly at higher C-rates (i.e. > 1C). However, temperature sensitive electrochemical parameters, such as ohmic overpotential, exchange current, and diffusion time constant, require calibration based on Arrhenius dependencies, which need activation energy values. The conventional approach to derive activation energy values is repetitive and time-consuming. To streamline this process, a Fast Activation Energy Derivation (FAED) approach is proposed, enabling rapid fitting of temperature-sensitive parameters through a single discharge process. After calibration based on experimental data, cell heat generation can be accurately predicted. Experimental validation on a Tesla battery module shows good agreement between simulation and experimental results up to 1.5C, with battery temperatures ranging from 16 °C to 46 °C.

Based on the calibrated heat generation from cells using FAED approach, this thesis investigates two novel BTMS technologies for future EVs: 1) a 3D printed polymer-based liquid BTMS is evaluated for its cooling efficiency, parasitic weight, and power consumption compared to the conventional ribbon BTMS used in the Tesla Model S. The proposed polymer-based BTMS shows better cooling performance and a 40.43% reduction in weight. (2) A flat plate Loop Heat Pipe (LHP) BTMS is numerically analysed for its potential to offer superior thermal management through long-distance heat transfer and efficient cooling in cylindrical cell formats. Comparative analysis against passive air convection systems illustrates LHP’s advantages in promoting uniform temperature and current distribution, significantly enhancing battery lifespan.

However, the temperature gradient is inevitable in BTMSs, especially in the conventional BTMS (i.e. the ribbon BTMS used in the Tesla Model S), leading to inhomogeneous ageing, and current/voltage maldistribution. This thesis further investigates the impact of electrical connection topologies on battery module level inhomogeneous ageing, as electrical connection topologies have emerged as a solution to further regulate the current or voltage distribution. By numerically investigating two typical topologies on a representative 6S4P battery module: the straight design, where the sub-modules consisting of parallel-connected cells are serial connected in a linear configuration, and the parallelogram design, where the sub-modules are serial connected in a parallelogram configuration, results indicated that the straight design exhibited a more uniform voltage distribution among serial sub-modules but experienced uneven current distribution among parallel cells. Conversely, the parallelogram design demonstrated a more balanced current distribution but a higher voltage gradient. This leads to sub-modules reaching their max/min voltage prematurely, resulting in a 0.8% reduction in effective capacity at a 1C discharge rate due to limiting module voltage. For the ageing rate, when the parallelogram design reaches its end-of-life (80% state of health (SOH)), the SOH for the straight design is slightly higher at 80.15%. This insight informs the straight design is the preferred electrical connection topology.