Nonlinear Characterization of Lithium-Ion Batteries With Bipolar Pulsing

Applying a bipolar charge and discharge current pulse to a lithium-ion battery cell engenders a complex voltage response governed by linear overpotentials and nonlinear open-circuit voltage and hysteresis behavior. These dynamics are analyzed with the proposed bipolar pulse (BIP) model. With ten parameters, it disaggregates the cell response into its constituent parts. The BIP characterizes thousands of pulses collected from 11 nickel-based cells, and the nonlinear contribution is quantified with respect to state of charge and state of health. Modeling error is bounded by 1% and is shown to be a valuable signal in its own right. Components of the BIP model are examined with ridge regression to show that nonlinearity may help with state prediction at low levels of charge, and may be strongly linked to the battery degradation level. Sensitivity analyses also show the benefit of pulsing for state estimation beginning from a 0.05% net change in state of charge.