Online identification of internal impedance of Li-ion battery cell using ternary-sequence injection

Abstract

The internal impedance of a battery has been shown to change as a function of state-ofcharge (SOC) and state-of-health (SOH). Therefore, online impedance measurements can provide useful information for SOC-and SOH-estimation algorithms. Broadband injections techniques such as pseudo-random-binarysequence (PRBS) are attractive alternative for replacing conventional electrochemical-impedance-spectroscopy (EIS) which is very slow for online battery impedance measurements. However, non-linearities of batteries have a negative impact on the measurement results obtained by the PRBS for which the identified system is assumed to be linear. This paper demonstrates the use of ternarysequence excitation signal for battery impedance measurements. Appropriately designed ternary-sequence can efficiently capture the linear component of the impedance by canceling out the distortion caused by the non-linear parts. The presented method can be used for rapid impedance measurements and thus, as a valuable tool in online SOC-and SOH-estimation algorithms. Experimental measurements are shown from a Lithium-Iron-Phosphate (LiFePO4) battery cell.

Fast Approach for Battery Impedance Identification Using Pseudo-Random Sequence Signals

“…As a drawback, the operation of the PRBS is limited for identification of linear systems, which is a challenging aspect for battery impedance measurements due to the nonlinear behavior of batteries [20], [26]- [28]. However, PRS signals with three levels (ternary sequence) can be designed in a way that the nonlinearities at specific frequency harmonics can be reduced yielding to improved accuracy of the nonlinear system measurements [28]- [30], [32]. The generating algorithms of both PRS signals, the PRBS and the ternary sequence, are carefully studied and presented in the following sections.…”

“…However, the excitation performance for nonlinear systems can be improved with multilevel PRS signals [28]- [31]. From such PRS signals, a three-level ternary sequence have been studied in [32] for battery impedance measurement, where it showed to provide accurate results under battery nonlinearities.…”