Battery thermal management for electric vehicles operating in cold climates

Licentiate thesis, 2022

Electromobility has gained significance over recent years in an attempt to reduce greenhouse gas emissions which contribute to climate change. The requirements on the performance and efficiency of electric vehicles are high to make them an attractive alternative to the conventional fossil-fuel driven vehicles. Lithium-ion batteries are the primary energy source for electric vehicles as they have high power and energy density, excellent storage capabilities and long cycling life when operated under conducive conditions, i.e a temperature range of 15°C to 35°C. However, their performance and cycling life are drastically affected when the operating temperature is outside this range. Therefore, battery packs must be heated to optimal temperatures under cold climates. This energy is often provided by the packs themselves, which results in reduced driving range.
This work investigates thermal encapsulation of battery packs as a means of passive battery thermal management to improve the battery performance and decrease heating demand during the initial phase of driving in cold climates. In order to predict the effects of battery pack encapsulation, a robust battery model that captures the dynamic behaviour of large battery packs is necessary in addition to other simplified vehicle and powertrain subsystems. The presented work proposes an integrated simulation methodology that enables numerical simulation of the relevant phenomenon at battery module, powertrain and vehicle levels.
The battery modeling strategy uses a one-dimensional module discretized electrical-thermal approach. An electrical circuit model with 2RC Thevenin branches was used to capture the electrical performance and the Bernardi's heat generation equation was used to estimate the heat generated from each module. The developed strategy was found to be in good agreement with measured test data. Vehicle simulations were performed under parking-driving scenarios to investigate the effectiveness of battery pack encapsulation at different ambient temperatures. It was found that the percentage of energy saved with battery pack encapsulation increased with decreasing ambient temperatures. The thermal resistance of the encapsulation material played a significant role in reducing heat loss to the environment. The simulations indicated that there is a potential of approximately 15% energy savings as a result of increased initial battery temperatures.