Once a BMS is enabled with impedance measurement capability, identifying the battery parameters that correlate with the impedance becomes the focus.

A Nyquist plot is a common technique to visualize impedance over frequency. Figure 2-3 shows the real impedance on the x-axis and the imaginary impedance on the y-axis. Each line comprises a sweep of frequencies from 0.1Hz to 1kHz, with the lowest frequency on the right and the highest on the left. The line color in Figure 2-3 indicates the temperature of the battery during the sweep.

In the plot, the impedance curves change shape versus temperature. Furthermore, the black circle, which represents a single impedance at 500Hz, moves as the temperature changes. Designers can use this EIS-based temperature data to identify the core temperature of the battery, monitor spikes in temperature during charging, and reduce the number of temperature sensors needed in the system.