Abstract
Thermal impedance spectroscopy (TIS) is a non-destructive method for characterizing thermal properties of entire battery cells. Heat capacity, thermal conductivity and heat exchange with environment are deter-mined by an evaluation of the heat transfer behavior of the battery. TIS measurements are usually conduct-ed with contact-based temperature sensors, such as thermocouples or thermistors, which show drawbacks at higher convection rates and higher temperature differences between battery and environment. To elude drawbacks in these kinds of sensors, an infrared-based temperature sensor system for battery sur-face temperature measurements is implemented. TIS measurements are conducted with this sensor system and with conventional, contact-based temperature sensors. Accuracy and reliability of thermal parameter identification is analyzed for the different sensor systems. Moreover, thermal parameters are identified for different cylindrical 18650 Li-ion cells with capacities between 1.1 Ah and 2.7 Ah. The comparison of different types of temperature sensors shows that contact-based sensors underestimate surface temperatures even at low temperature differences to environment. This causes an error in thermal parameter identification. The TIS measurements performed with contact-based sensors show divergence of 20 - 60 % for heat capacity, 30 - 70 % for thermal conductivity and 20 - 60 % for convective heat exchange with environment. With our IR temperature sensor system, parameter identification is performed for different batteries. Re-sulting values for specific heat capacity are in a range between 900 and 1020 J/kgK and thermal conduc-tivities in radial direction lies between 3.1 and 3.6 W/mK. Our investigations show that IR-based temperature sensors are an effective progression for TIS measure-ments and improve quality of parameter identification at low cost. Moreover, discrepancies mentioned in TIS literature can be explained by our findings.
Highlights
Temperature influences many attributes of Li-ion batteries, such as efficiency, usable capacity, aging, and safety [1]
Our investigations show that IR-based temperature sensors are an effective progression for Thermal impedance spectroscopy (TIS) measurements and improve quality of parameter identification at low cost
Discrepancies between thermal parameters obtained by TIS measurement and calorimetry results mentioned in the TIS literature can be explained by our investigations
Summary
Temperature influences many attributes of Li-ion batteries, such as efficiency, usable capacity, aging, and safety [1]. Evaluating heat transfer behavior for different excitation frequencies yields a characteristic thermal impedance spectrum. This spectrum exhibits the complete thermal characteristics of the battery in question [3]. All TIS publications presented have in common that thermocouples were used for temperature measurements As these sensors belong to the category of contact-based sensors, they have a major drawback: Only one side of the sensor is in contact with the battery‟s surface, whereas the back side interacts with the environment [6]. Radiation and convection at the back side of the sensor causes the sensor‟s temperature to diverge from the battery‟s surface temperature This divergence increases with intensified convection that occurs either with larger differences between battery and environment temperature or with forced air flow around the battery. The battery temperature of 23.6 °C is only 4 °C above ambient temperature , the temperature of the sensor tip (23.2 °C) exhibits a difference of -0.4 °C
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