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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).

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Resistance is easily calculated by the array size like 6S2P =6/2 * Cell resistance as you have done.

But good Li Ion 18650 cells might be 33 mΩ +/- 50% when fully charged and 10x this < 10% SoC so when mAh is mismatched there is increasing risk and accelerated wear on the weakest cell with the lowest mAh getting charged or discharged early thus the under or overcharged resulting in higher ESR aging. So tolerances are critical for mAh and ESR with cutoff voltage and improved lifespan with balance protection IC's.

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