Battery Pack Internal Resistance Calculator | BatteryPackCalc

How to Use the Internal Resistance Calculator

Enter the series count (S), the parallel count (P), and the internal resistance of one cell in milliohms — take the DC value from the cell datasheet or from an IR meter reading. Then add the load current your pack must deliver and its nominal voltage. Resistance accumulates along a series string and is shared across parallel groups, so the pack figure is S × R_cell ÷ P: a longer string raises it, while each extra parallel group divides it down.

With the default 13S4P layout and 20 mΩ cells, pack resistance works out to 13 × 20 ÷ 4 = 65 mΩ. Drawing 40 A through 65 mΩ drops 40 × 0.065 = 2.6 V, so a 48.1 V nominal pack actually presents about 45.5 V at its terminals under that load. The results panel lists all four derived figures — pack resistance, voltage sag, voltage under load, and internal power loss — so you can see at a glance how much of your nominal voltage survives a hard pull.

Internal power loss follows I²R: the same 40 A example dissipates 40² × 0.065 = 104 W as heat inside the pack instead of at the motor. Heavy sag also makes the BMS low-voltage cutoff trip earlier than the remaining capacity alone would suggest, which shows up as lost range under aggressive throttle. Keep in mind that cell resistance is not a fixed property — it climbs as cells age and when they run cold — so it is worth re-running the numbers with a measured IR value rather than relying only on the new-cell datasheet figure.