Pseudo-random sequences for low-cost operando impedance measurements of Li-ion batteries

Operando impedance measurements are required for monitoring batteries in the field. In this work, we present pseudo-random sequences for low-cost operando battery impedance measurements. The quadratic-residue ternary (QRT) sequence is known to possess special properties related to eigenvectors of the discrete Fourier transform matrix; it is proven in this paper that these properties extend to direct-synthesis ternary (DST) sequences derived from the former sequence. A method is proposed to employ these properties to efficiently compensate for drifts and transients while detecting nonlinearities in operando impedance measurements. Practical considerations, such as the computational load, memory requirements, and choice of measurement parameters are discussed. An experiment is performed on a commercial Li-ion battery cell during fast-charging from 20 to 80% state-of-charge to illustrate the feasibility of the proposed technique. The impedance is successfully measured at 20 different state-of-charge levels across a charging time of 35 minutes. Low-cost hardware requirements, fast measurements, and simple data-processing make the method practical for embedding in battery management systems.