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The operating temperature is critical in achieving the right balance between performance, cost, and life for both Li-ion batteries and ultracapacitors. The chemistries of advanced energy-storage devices - such as lithium-based batteries - are very sensitive to operating temperature. High temperatures degrade batteries faster while low temperatures decrease their power and capacity, affecting vehicle range, performance, and cost. Understanding heat generation in battery systems - from the individual cells within a module, to the inter-connects between the cells, and across the entire battery system - is imperative for designing effective thermal-management systems and battery packs. At NREL, we have developed unique capabilities to measure the thermal properties of cells and evaluate thermal performance of battery packs (air or liquid cooled). We also use our electro-thermal finite element models to analyze the thermal performance of battery systems in order to aid battery developers with improved thermal designs. NREL's tools are used to meet the weight, life, cost, and volume goals set by the U.S. Department of Energy for electric drive vehicles.
Lithium-ion batteries have revolutionized our everyday lives by laying the foundation for a wireless, interconnected and fossil-fuel-free society. Additionally, the demand for Li-ion batteries has seen a dramatic increase, as the automotive industry shifts up a gear in its transition to electric vehicles. To optimize the power and energy that can be delivered by a battery, it is necessary to predict the behavior of the cell under different loading conditions. However, electrochemical cells are complicated energy storage systems with nonlinear voltage dynamics. There is a need for accurate dynamic modeling of the battery system to predict behavior over time when discharging. The study conducted in this work develops an intuitive model for electrochemical cells based on a mechanical analogy. The mechanical analogy is based on a three degree of freedom spring-mass-damper system which is decomposed into modal coordinates that represent the overall discharge as well as the mass transport and the double layer effect of the electrochemical cell. The dynamic system is used to estimate the cells terminal voltage, open-circuit voltage and the mass transfer and boundary layer effects. The modal parameters are determined by minimizing the error between the experimental and simulated time responses. Also, these estimated parameters are coupled with a thermal model to predict the temperature profiles of the lithium-ion batteries. To capture the dynamic voltage and temperature responses, hybrid pulse power characterization (HPPC) tests are conducted with added thermocouples to measure temperature. The coupled model estimated the voltage and temperature responses at various discharge rates within 2.15% and 0.40% standard deviation of the error. Additionally, to validate the functionality of the developed dynamic battery model in a real system, a battery pack is constructed and integrated with a brushless DC motor (BLDC) and a load. Moreover, because of the unique pole orientation that a BLDC motor possesses, it puts a pulsing dynamic load on the battery pack of the system. HPPC testing was conducted on the cell that is used in the battery pack to calibrate the model parameters. After the battery model is calibrated, the rotation experiment is conducted at which a battery pack is used to drive a benchtop BLDC motor with a magnetorheological brake as a programable load at varying running speeds. The voltage and current of the battery and the BLDC motor driver are recorded. Meanwhile, the speed and the torque of the motor are recorded. These data are compared to the predicted voltage of the battery pack using the mechanical analogy model. The model estimated the voltage response of a battery pack within 0.0385% standard deviation of the error.
This presentation discusses the characterization of battery thermal characteristics.
A major challenge in the development of next generation electric and hybrid vehicle technology is the control and management of heat generation and operating temperatures. Vehicle performance, reliability and ultimately consumer market adoption are integrally dependent on successful battery thermal management designs. It will be shown that in the absence of active cooling, surface temperatures of operating lithium-ion batteries can reach as high as 50 °C, within 5 °C of the maximum safe operating temperature. Even in the presence of active cooling, surface temperatures greater than 45 °C are attainable. It is thus of paramount importance to electric vehicle and battery thermal management designers to quantify the effect of temperature and discharge rate on heat generation, energy output, and temperature response of operating lithium-ion batteries. This work presents a purely experimental thermal characterization of thermo-physical properties and operating behavior of a lithium-ion battery utilizing a promising electrode material, LiFePO4, in a prismatic pouch configuration. Crucial to thermal modeling is accurate thermo-physical property input. Thermal resistance measurements were made using specially constructed battery samples. The thru-plane thermal conductivity of LiFePO4 positive electrode and negative electrode materials was found to be 1.79 ± 0.18 W/m°C and 1.17 ± 0.12 W/m°C respectively. The emissivity of the outer pouch was evaluated to enable accurate IR temperature detection and found to be 0.86. Charge-discharge testing was performed to enable thermal management design solutions. Heat generated by the battery along with surface temperature and heat flux at distributed locations was measured using a purpose built apparatus containing cold plates supplied by a controlled cooling system. Heat flux measurements were consistently recorded at values approximately 400% higher at locations near the external tabs compared to measurements taken a relatively short distance down the battery surface. The highest heat flux recorded was 3112 W/m2 near the negative electrode during a 4C discharge at 5 °C operating temperature. Total heat generated during a 4C discharge nearly doubled when operating temperature was decreased from 35 °C to 5 °C, illustrating a strong dependence of heat generation mechanisms on temperature. Peak heat generation rates followed the same trend and the maximum rate of 90.7 W occurred near the end of 5 °C, 4C discharge rate operation. As a result, the maximum value of total heat generated was 41.34 kJ during the same discharge conditions. The effect of increasing discharge rate from 1C to 4C caused heat generation to double for all operating temperatures due to the increased ohmic heating. Heat generation was highest where the thermal gradient was largest. The largest gradient, near negative electrode current collector to external tab connection and was evaluated using IR thermography to be 0.632 °C/mm during 4C discharge with passive room temperature natural convection air cooling. Battery designs should utilize a greater connection thickness to minimize both electrical resistance and current density which both drive the dominant mode of heat generation, ohmic heating. Otherwise cooling solutions should be concentrated on this region to minimize the temperature gradient on the battery.
Presents basic and advanced techniques in the analytical and numerical modeling of various heat pipe systems under a variety of operating conditions and limitations. It describes the variety of complex and coupled processes of heat and mass transfer in heat pipes. The book consists of fourteen chapters, two appendices, and over 400 illustrations, along with numerous references and a wide variety of technical data on heat pipes.