Multi-Cell and Wide-Frequency In-Situ Battery Impedance Spectroscopy

The use of dc-dc converters for in-situ electrochemical impedance spectroscopy has been investigated by several works in recent years, with different implementation strategies and promising results. There are, however, two important limitations that still hinder a commercial application of this technique: first, the need to deal with the battery discharge during the measurement, particularly critical at very low frequencies; second, the difficulty of accurately measuring the small ac voltage response of several cells in a pack, with common-mode dc voltages that can be five (or more) orders of magnitude higher. This article addresses both challenges, from an instrumentation and measurement perspective, presenting a solution for impedance measurements down to 10 mHz, on a system composed of 16 lithium-iron-phosphate cells or modules connected in series. A dc-dc boost converter is used to inject a multisine current perturbation on all batteries, with closed-loop control, and all cell voltages are conditioned to optimize the measurement resolution and accuracy of their ac components. Suitable signal processing compensates for the voltage drift caused by the battery discharge, and evaluates the residual distortion in the signal, to assess the validity of the impedance estimate. Experimental tests confirm that the obtained results are sufficiently precise (or repeatable) to allow detecting impedance variations occurring during the battery discharge or after repeated charge/discharge cycles.