Non-Linear Bipolar Pulse Characterisation of Lithium-ion Batteries

Impedance characterisation of lithium-ion batteries is important for understanding the capacity and power fade of the system. Here, the bipolar-pulse (BIP) equivalent circuit model is proposed to capture overpotentials, non-linear open-circuit voltage effects, and discrepancies between charge and discharge polarities using a 2 minute charge-discharge pulse. Unlike traditional equivalent circuits, each BIP parameter has physical significance; and in contrast to electrochemical impedance spectroscopy, the BIP model does not require a linear time-invariant system assumption. The model is validated using experimental data from 9 nickel-cobalt-aluminum cells subject to different cycling conditions and temperature with a mean absolute percent error bounded by 0.7%. Pulses are disaggregated into their constituent parts to show quantitatively how the transient components vary with SoC, SoH, and polarity. The BIP thus provides detailed diagnostics from a simple pulse perturbation.