*3.1. Cell SoH Estimation*

The measured 1 kHz impedance in Figure 3 is used to estimate the cell SoH. Figure 5 shows the average of the 1 kHz impedance at each SoH and the fitted line.

The fitted line in Figure 5 is obtained through linear regression of these 1 kHz impedances measured at each SoH, and it is expressed as Equation (7).

$$Z\_{\rm f2} = -0.1355 \cdot \text{SoH} + 64.2 \tag{7}$$

The measured 1 kHz impedance can be used to estimate the cell SoH using Equation (7). Here, root mean squared error (RMSE) is 0.21 mΩ and R<sup>2</sup> is 0.99. R2 represents the goodness of fit of the estimated model. The closer this value is to 1, the better the estimate matches the target value.

#### *3.2. Cell SoC Estimation*

The cell 1 Hz impedance is greatly influenced by not only the cell SoC but also the cell SoH and cell temperature. Therefore, to be used for cell SoC estimation, the 1 Hz impedance is adjusted as cell temperature in Section 3.2.1 and normalized as cell SoH in Section 3.2.2.
