Evaluation of the Effects of Thermal Management on Battery Life in Plug-in Hybrid Electric Vehicles

We develop a simulation model that aims to evaluate the effect of thermal management on battery life. The model consists of two submodels: a thermal model and a battery degradation model. The temperature rise in the battery is calculated using the thermal model, and a temperature profile is obtained under pre-defined driving, charging and stand-by scenarios. The temperature profile and the energy requirement required to achieve a driving profile act as inputs to the degradation sub-model, which is used to predict the battery life. The degradation model is derived from models and test data available in literature, and the model is constructed for aircooled cylindrical LiFePO4 cells based on the Hymotion Prius-conversion configuration. Preliminary results suggest that peak temperatures have the greatest impact on degradation: Thermal management increases life substantially in climates with high peak temperatures (Pheonix) and for more aggressive driving cycles (US06), while thermal management has less influence in climates with lower peak temperatures (Miami) and with gentle driving cycles (UDDS). Use of cabin air vs. outside air for thermal management has minor impact on battery life for the control strategy used, but thermostat control settings are important for lowering peak temperatures and extending battery life. Introduction Plug-in hybrid electric vehicles (PHEVs) have the potential to reduce operating cost, greenhouse gas (GHG) emissions, and petroleum consumption in the transportation sector. One of the most important factors affecting the commercialization of PHEVs is the battery cost, which should be reduced for PHEVs to be cost competitive with other vehicles [2-5]. While reducing the cost, other requirements should also be satisfied such as power, energy, weight, size, and life. Often, improving one of these factors causes an adverse effect in others. If the battery reaches end of life (EOL) before vehicle life, there would be need for battery replacement, which raises the costs for the consumer substantially since the battery is the most expensive part of the vehicle for many electrified vehicles. Although different design choices can lead to different battery EOL criteria [6], EOL is typically defined as the time when 20% capacity loss or 30% internal resistance growth is reached. According to the goals set by US Advanced Battery Consortium (USABC), a PHEV battery is targeted to have 15 years of calendar life and 300,000 cycles of cycle life One of these stress factors that strongly affects degradation rate is temperature. The relationship between degradation and temperature can be formulated by an Arrhenius type behavior where degradation rate increases exponentially with temperature [7]. To achieve these goals, it is necessary to improve battery life by managing the stress factors that affect battery life.