Sometimes fitting software just doesn’t seem to work properly and the fit values aren’t even close to the measured values. This can be due to bad starting values for the fit. In this section some guide lines to find good starting values for fitting are given.
Often a fit will be pretty good with the first iteration. Sometimes, a fit just won’t deliver a curve which overlaps with the measurement. This is due to the way a fit is calculated. The computer tries to find values for the variables that deliver a curve with the smallest difference to the measurement (Least Square fitting).
A drawback of the Complex Non-linear Least Squares fitting algorithm is that when given initial values for parameters that are too far off the absolute minimum it can get stuck in a local minimum and return a suboptimal fit.
Unfortunately, the default initial values for the resistors and capacitors are sometimes too far off from the optimal values for some systems. The default values have been selected to be optimal for the most common types of electrochemical cells.
In order to obtain a correct fit when modelling coatings/corrosion with equivalent circuits the following steps are recommended, if a clear RC system is visible (semi-circle in Nyquist plot).
Although these steps have been quite specific for RC systems, they can be used analog for other circuits. First, values are estimated from the measurement and afterwards a fit with these parameters fixed is performed to determine the values of the other parameters. Afterwards a fit with no fixed parameters is performed.
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