PSO-Based Identification of the Li-Ion Battery Cell Parameters

Abstract

The article describes the results of research aimed at identifying the parameters of the equivalent circuit of a lithium-ion battery cell, based on the results of HPPC (hybrid pulse power characterization) tests. The OCV (open circuit voltage) characteristic was determined, which was approximated using functions of various types, while making their comparison. The internal impedance of the cell was also identified in the form of a Thevenin RC circuit with one or two time constants. For this purpose, the HPPC pulse transients were approximated with a multi-exponential function. All of the mentioned approximations were carried out using an original method developed for this purpose, based on the PSO (particle swarm optimization) algorithm. As a result of the optimization experiments, the optimal configuration of the PSO algorithm was found. Three different cognition methods have been analyzed here: GB (global best), LB (local best), and FIPS (fully informed particle swarm). Three different swarm topologies were used: ring lattice, von Neumann, and FDR (fitness distance ratio). The choice of the cognition factor value was also analyzed, in order to provide a proper PSO convergence. The identified parameters of the cell model were used to build simulation models. Finally, the simulation results were compared with the results of the laboratory CDC (charge depleting cycle) test.

HPPC Test Methodology Using LFP Battery Cell Identification Tests as an Example

“…The first was a measurement based on the discharge characteristics [44,48,49]. The second, proposed by the authors, was the use of current waveforms recorded during HPPC tests [63].…”

“…The HPPC pulse length used in practice is therefore a compromise between these two requirements. In the case of nickel manganese cobalt (NMC) cells, the first two time constants are relatively short [63] and do not exceed a dozen or so seconds, so pulses from 9 s to 18 s are sufficient. Usually 10 s pulses are used [16,[43][44][45].…”

“…Its coefficients a to f were obtained by optimization with the particle swarm method (PSO) described in [63]. The resulting function plot and its coefficient values are given in Figure 9.…”

“…Choice of the appropriate approximating function is a further problem. Several types of functions were tested, but a log-linear exponential (LLE) function [11,75] gave the best result [63]. The LEE function has the following form: 𝑈 OC (𝑆𝑂𝐶) = 𝑎 + 𝑏 ln(𝑆𝑂𝐶 + 𝑐) + 𝑑 𝑆𝑂𝐶 + e 𝑒(𝑆𝑂𝐶−𝑓) .…”

“…The article describes the issues that are a continuation of the research described in the article [63].…”