Heat Generation Measurements of Prismatic Lithium Ion Batteries
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Electric and hybrid electric vehicles are gaining momentum as a sustainable alternative to conventional combustion based transportation. The operating temperature of the vehicle will vary significantly over the vehicle lifetime and this variance in operating temperature will strongly impact the performance, driving range, and durability of batteries used in the vehicles. In the first part of this thesis, an experimental facility is developed to accurately quantify the effects of battery operating temperature on discharge characteristics through precise control of the battery operating temperatures, utilizing a water-ethylene glycol solution in a constant temperature thermal bath. A prismatic 20Ah LiFEPO4 battery from A123 is tested using the developed method, and temperature measurements on the battery throughout discharge show a maximum variation of 0.3°C temporally and 0.4°C spatially at a 3C discharge rate, in contrast to 13.1°C temperature change temporally and 4.3°C spatially when using the conventional air convection temperature control method under the same test conditions. A comparison of battery discharge curves using the two methods show that the reduction in spatial and temporal temperature change in the battery has a large effect on the battery discharge characteristics. The developed method of battery temperature control yields more accurate battery discharge characterization due to both the elimination of state-of-charge drift caused by spatial variations in battery temperature, and inaccurate discharge characteristics due to battery heat up at various discharge and ambient conditions. Battery discharge characterization performed using the developed method of temperature control exhibits a reduction in battery capacity of 95% when the operating temperature is decreased from 20°C to -10°C at 3C discharge rate. A reduction of 35% in battery capacity is observed when for the same temperature decrease at a 0.2C discharge rate. The observed effect of operating temperature on the capacity of the tested battery highlights the importance of an effective thermal management system, the design of which requires accurate knowledge of the heat generation characteristics of the battery under various discharge rates and operating temperatures. In the second part of this thesis, a calorimeter capable of measuring the heat generation rates of a prismatic battery is developed and verified by using a controllable electric heater. The heat generation rate of a prismatic A123 LiFePO4 battery is measured for discharge rates ranging from 0.25C to 3C and operating temperature ranging from -10°C to 40°C. Results show that the heat generation rates of Lithium ion batteries are greatly affected by both battery operating temperature and discharge rate. At low rates of discharge the heat generation is not significant, even becoming endothermic at the battery operating temperatures of 30°C and 40°C. Heat of mixing is observed to be a non-negligible component of total heat generation at discharge rates as low as 0.25C for all tested battery operating temperatures. A double plateau in battery discharge curve is observed for operating temperatures of 30°C and 40°C. The developed experimental facility can be used for the measurement of heat generation for any prismatic battery, regardless of chemistries. The characterization of heat generated by the battery under various discharge rates and operating temperatures can be used to verify the accuracy of battery heat generation models currently used, and for the design of an effective thermal management system for electric and hybrid electric vehicles in the automotive industry.