Thermal impedance spectroscopy for Li-ion batteries with an IR temperature sensor system

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 determined by an evaluation of the heat transfer behavior of the battery. TIS measurements are usually conducted 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 surface 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. Resulting values for specific heat capacity are in a range between 900 and 1020 J/kgK and thermal conductivities 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 measurements and improve quality of parameter identification at low cost. Moreover, discrepancies mentioned in TIS literature can be explained by our findings.