I have a circuit that allows me to add a load to a battery and measure the voltage, current and temperature of the battery. I am trying to derive what is the best method for measuring the internal battery resistance using the simplest DC load method. In theory that method is not complicated, but the problem is that the results I am obtaining are far from the battery resistance specified in the datasheet and measured by Hioki meter for the same battery. So far, I tried the following method:
- I measure open circuit voltage
- I apply load.
- I wait for 1 second.
- I measure the closed circuit voltage.
- I disconnect the load.
- 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.