Abstract: The authors found a normal transfer function to calculate online lithium-ion battery temperaturefields
for estimating battery-performances, which strongly influence electric vehicle (EV) performances. The
internal-external temperature differences and the heat transfer parameters were measured for a type of lithiumiron
phosphate-graphite (LFP-G) energy-cell, with a 3-D thermal model then established. The temperature fields
simulated using a Fluent software under natural convection situation were tested by experiments, with a normal
transfer function between the internal heat powers and the internal-external temperature-differences then being
found. The results show that the 3-D model calculation results correspond well with the experimental results
with a maximum error of 0.7 K, with internal-external temperature-differences having an error of 0.1 K when
using the transfer function estimation is compared with using the 3-D model. Therefore, the transfer function
estimation can be used to online estimate the internal temperature field of lithium-ion batteries

Key words: electric vehicle (EV), lithium-ion battery, internal temperature, 3-D thermal modeling, internalexternal
temperature difference, transfer function