Fitting Randles type circuit to EIS spectra using LEVMW

[Ravi Kumar]

Dear All,

I work on thin film anodes materials for Lithium-ion batteries. I have EIS spectra at fully lithiated stage and have been trying to fit Randles type circuit to extract parameters such as surface resistance, charge transfer resistance etc. I have gone through the manual of LEVMW and have tried different combinations of CKT B (Randles Type), but I am still facing several problems.

1. I always get an error saying "BAD CORRELATION MATRIX" and also "Singular Matrix"

2. Some of the parameters don't seem to change at all although I specify them as free parameters, the problem seems to be specific to Warburg impedance element

I am attaching one of the modified TST files from circuit B including my EIS data (f, Zre and Zim). Please remove .txt extension from the file when you download it, and it will become TST file fro LEVMW. I have the following circuit in the program right now R1 - [RA || DE1 (NDE=2)] - [{R2 - DE2 (NDE=9)} || DE4 (NDE=2)].  (- means in series, || means in parallel)

I request suggestion from group members on how to do the right fitting. If you need more information on the data or circuit, please let me know.

Thanks.

Regards,

Ravi Kumar

PhD Student

Brown University

[Macdonald, J. Ross]

I had no trouble fitting the data you provided.
Attached is a text file of the fit and a LEVM graphical fitting result.  The fit was very good.

Ross Macdonald

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Dr. J. Ross Macdonald is William R. Kenan Jr. Professor of Physics, Emeritus; Department of Physics and Astronomy of the University of North Carolina at Chapel Hill, and a member of NAS and NAE. Website: http://jross macdonald.com

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From: impedance-s...@googlegroups.com [impedance-s...@googlegroups.com] on behalf of Ravi Kumar [kravi...@gmail.com]
Sent: Wednesday, April 23, 2014 3:31 PM
To: impedance-s...@googlegroups.com
Subject: Fitting Randles type circuit to EIS spectra using LEVMW

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[Ravi Kumar]

Dear Prof. McDonald,

Thanks for putting efforts in fitting my data. I really appreciate it. For some reason, the graphs you sent me (Zre vs f and -Zim vs f) look different from the data I had attached. It maybe that I am missing something here or by mistake you sent me the wrong file. I am attaching the data again in case there was some problem with the earlier one in text format with (f, Zr and Zim). I am also attaching the graphs of my fit (which gives singular matrix error).

Also, if possible, I request you to send me the modified TST file.

Regards,

ravi

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[Chao Lyu]

Dear Ravi,

I am facing the same problem when I try to fit a Randles type circuit with a GFWE derived from CKT A, to impedance data of a PEM fuel cell. No matter how I change the initial values of GFWE, the fitting result and the GFWE itself seem not working at all.

Have you solved this in your case? Could you please show me how? Thanks!

[Ravi Kumar]

Dear Chao Lyu,

Actually I stopped using LEVMW long time back as it was too complicated for me. I am currently using Zview software for fitting equivalent circuits to my battery EIS data and it does the job pretty well.

I am sorry that I can't help you with this.

Thanks.

Regards,

ravi

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Regards,

Ravi