Capacity Fade Estimation of LiFePo Cells Based on Improved Impulse Response Method: Experimental Results

The paper presents an Improved Impulse Response (IIR) method for the identification of capacity fade in LiFePo pouch 40Ah cells. The impulse response (IR) method for capacity-fade identification consists of building a pre-calculated table in which the cell voltage response to a input pulse current is measured and stored at a given State of Charge (SoC) for several iterations of full charge/discharge ageing cycles. Each stored impulse response can be used to calculate the voltage response to whatever input by means of convolution integral. However, since the same method can be also used for on-line SoC identification, it is necessary to investigate on the correlation between the cell impulse response, SoC and residual capacity, and the temperature. To this aim, an IIR method is here proposed that allows to evaluate the accuracy of IRs at different conditions of SOC, capacity fade, and temperature. A significative planning of charge-discharge cycles and IR measurements has been carried out to this aim. At present, only some rows of the IIR matrix have been already characterized.