1. I measure open circuit voltage

2. I apply load.

3. I wait for 1 second.

4. I measure the closed circuit voltage.

5. I disconnect the load.

6. Calculate the IR with formula: (V_open - V_closed) / (I_closed - I_open).

The result will highly depend on the load application time, as it can be seen in the scope capture graph. Loading the battery for a longer period of time, will make the voltage drop even more. I tried a different method as well in which I take under consideration not the open voltage but the regeneration voltage:

The calculation results are similar and still far from the reference.

I am fully aware of the fact that the plain DC method is not very precise, but based on the fact that my results are ~70 % off (IR is too high compared to the reference), there has to be something I am doing wrong.

I would appreciate all help!