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

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ekagra gupta 2022-8-23

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评论： Florian Bittner 2023-9-21

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

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ekagra gupta 2022-8-23

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

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Answer 1

Mathieu NOE 2022-8-24

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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