Let's assume you have a battery bank not connected to anything and we are dealing with perfect ideal values. You just want to calculate the total battery internal resistance of your battery bank.
To calculate the total resistance of a battery bank, can you simply take the internal resistance of the battery and replace the battery with the resistor value and proceed to find the total equivalent resistance as you normally would in a simple circuit that just has resistors?
For example, would 2 batteries in series with an internal resistance of 2mΩ simply equal 4mΩ? 2mΩ+2mΩ=4mΩ? Or is there some physics law I am violating due to it being a battery?
Then assuming you have another 2 batteries in parallel which equals 4mΩ, the total resistance to this circuit is 2mΩ?
Battery 1 + Battery 2 = 4mΩ
Battery 3 + Battery 4 = 4mΩ
(Battery 1 + Battery 2) || (Battery 3 + Battery 4) = 2mΩ
All of this is assuming ideal values (ignoring temperature flux, jumper resistance, health of battery, etc, etc).