State of health estimation of lithium-ion batteries based on the regional frequency

The state of health (SOH) estimation of lithium-ion batteries at low data sampling frequencies has important practical significance in engineering application. This paper proposes a new SOH evaluation method for lithium-ion batteries under the framework of probability density function (PDF). The concepts of regional voltage and regional frequency are introduced, and the SOH models are established based on the experimental cycle data of the nickel-cobalt-aluminium (NCA) battery cell and lithium iron phosphate (LFP) battery module. The results show that the SOH is a simple linear function of the regional frequency in the obtained model, and the fitting R-squared (R2) of two battery SOH models can reach more than 0.99 with appropriate regional voltage. This method does not directly use the characteristic parameters of PDF and is insensitive to the sampling frequency as low as 1/60 Hz. It is effective for accurate online SOH evaluation in occasions where the sampling frequency cannot be too high, such as large energy storage power station. Moreover, this method is more suitable for module SOH evaluation, which can overcome the poor SOH evaluation caused by the inconsistency of cells in a module.