Impedance calculation using FFT from time, voltage and current battery data

Hello,

I have battery pulse test and pseudo OCV measurements (time, current and voltage) in time domain and I want to obtain plot similar to EIS in freuqnecy domain using Fourier Transform.

Is it possible? If yes, then please explain the approach

Thankyou

[ekagra gupta]

? I know using FFT but it would be great if someone would rpovide some code to follow up

Answer 1

[Mathieu NOE]

hello

should be possible using tfestimate function

alternative (older) code :

x and y are your time domain data (x = input of the system , y = output)

you need to specify the fft buffer length, sampling freq, type of window and overlap (if you want)

%%%%%%%%%%%%%%%%%%%%%%%
function [Txy,Cxy,f] = mytfe_and_coh(x,y,nfft,Fs,window,noverlap)
% Transfer Function  and Coherence Estimate 
    
% compute PSD and CSD
window = window(:);
n = length(x);		% Number of data points
nwind = length(window); % length of window
if n < nwind    % zero-pad x , y if length is less than the window length
    x(nwind)=0;
    y(nwind)=0;  
    n=nwind;
end
x = x(:);		% Make sure x is a column vector
y = y(:);		% Make sure y is a column vector
k = fix((n-noverlap)/(nwind-noverlap));	% Number of windows
					% (k = fix(n/nwind) for noverlap=0)
index = 1:nwind;

Pxx = zeros(nfft,1); 
Pyy = zeros(nfft,1); 
Pxy = zeros(nfft,1); 

for i=1:k
    xw = window.*x(index);
    yw = window.*y(index);
    index = index + (nwind - noverlap);
    
    Xx = fft(xw,nfft);
    Yy = fft(yw,nfft);
    Xx2 = abs(Xx).^2;
    Yy2 = abs(Yy).^2;
    Xy2 = Yy.*conj(Xx);
    Pxx = Pxx + Xx2;
    Pyy = Pyy + Yy2;
    Pxy = Pxy + Xy2;
    
end

% Select first half
if ~any(any(imag([x y])~=0))   % if x and y are not complex
    if rem(nfft,2)    % nfft odd
        select = [1:(nfft+1)/2];
    else
        select = [1:nfft/2+1];   % include DC AND Nyquist
    end

    Pxx = Pxx(select);
    Pyy = Pyy(select);
    Pxy = Pxy(select);
    
else
    select = 1:nfft;
end
Txy = Pxy ./ Pxx;                   % transfer function estimate 
Cxy = (abs(Pxy).^2)./(Pxx.*Pyy);    % coherence function estimate 

f = (select - 1)'*Fs/nfft;

end

[ekagra gupta]

Thank you for the try, can you briefly tell me what should I select nfft, window and noverlap as?

A little walkthrough the code would be great

[Mathieu NOE]

Hello again

can you share some data ? then I will try to find the best parameters and give some advices how I did it

[Mathieu NOE]

hello

problem solved ?

[ekagra gupta]

not exactly, but thank you for helping

[Mathieu NOE]

Still need help ?

[ekagra gupta]

No thank you! Got it covered.

Thank you for ,your efforts and time

[Ruben Messner]

hi,

could you share your final solution by chance? I'm trying to do the same calc...

Thx in advance!

[Florian Bittner]

Hi, guys i also try to analyse the battery voltage using fft. Do you guys found a working code/solution?

Thanks!