Pulse resistance effects due to charging or discharging of high-power lithium-ion cells : A path dependence study

Battery life estimations and state-of-health projections for commercial applications such as hybrid electric vehicles are highly dependent on accurate resistance monitoring. This study examined discharge/charge hysteresis (path dependency) effects on measuring resistance using two different lithium-ion cell chemistries. Cells were either discharged or charged to a target voltage, followed by a rest at open-circuit for electrochemical and thermal equilibration, immediately prior to a resistance measurement using a high-current pulse profile. Results show that a voltage hysteresis effect has an impact on cell resistance measurements, depending on the direction a target voltage is reached. Specifically, charging to a target condition yields different and less consistent resistance measurements compared to discharging to that same condition. Further, using slower rates to approach the target condition has a small impact on resistance on the discharge curve but does give a noticeable improvement on the charge curve. Unfortunately, slow charging and discharging are generally not practical for hybrid electric vehicle applications due to the rapidly changing power demands of the driver. Consequently, these results indicate that life estimates should be primarily based on resistances determined from pulses on the discharge curve.