Comprehensive electro-thermal model of 26650 lithium battery for discharge cycle under parametric and temperature variations

Ambient temperature affects the electro-thermal performance of lithium iron phosphate (LiFePO4) batteries in electric vehicles. This paper proposes a combined electro-thermal state-space model for estimating the surface and core temperature of the battery cell and stack. A detailed experimental setup is used to determine the internal resistance and resistor-capacitor (RC) model of the electrical battery cell model and relationships between state-of-charge (SOC), open-circuit voltage and terminal voltage values at different ambient temperatures. The coupling between the electrical and thermal model provides estimation of voltage, core and surface temperatures under thermal uncertainties using measurable voltage, current and ambient temperature. The open-circuit voltage remains quite independent with ambient temperature at SOC value between 0.3 to 0.95 with a higher variation at 5 degrees C and 15 degrees C. The thermal parameters are identified as 2.23 k/W, 71.5 J/K and 4.35 k/W, respectively. The different between the surface and ambient temperature is around 3.6 K as compared to core and surface temperature of around 2 K. The mean square error of the surface temperature between measurement and simulation of the battery stack is around 3 degrees C.