Hi Fethi,

I would do the following to calculate this resistance you mention:

Create a difference quotient like (U(t) - U(t-1)) / (i(t) - i(t-1)) where U is the measured cell voltage at t and t-1 time and i is the currents at the same time where the voltage was acquired. You can do this the whole data series to get the resistance for the whole. The first resistance data must be 0 or a guess.

There are some other solutions but it could work.

I hope I could help you.