I am working on a project which involves rechargeable batteries. To calculate the variations in the internal resistance of the battery, i used data from the existing experimental results to establish an approximate value for the internal resistance as a function of the SOC, discharge time and the temperature (effectively a 3D lookup table). At any given instant, the power demanded from the battery is known and the current profile (w.r.t time) is also known. The SOC can also be calculated. But I am not sure how to calculate/choose an appropriate discharge time w.r.t which the resistance can be determined. Can someone please help me understand it or point me in the right direction? Thanks in advance.