Calculate the internal resistance of a battery or power source.
Voltage & Current Measurements
Calculation Results
Enter measurements to calculate (Note: V_oc ≥ V_load)
About this calculator
Quick Reference
V_ocOpen-circuit Voltage
V_loadMeasured voltage under load
IMeasured current
rInternal Resistance
Overview
Real-world power sources like batteries have an internal resistance (r). When you draw current from them, some voltage is 'dropped' across this internal resistance, causing the output voltage to decrease.
💡
Pro Tips
Open circuit voltage is measured when no load is connected (I = 0).
Internal resistance increases as a battery ages or discharges.
High internal resistance causes the battery to heat up under load.
!
Fun Facts
"Ideally, a power source would have zero internal resistance."
"Internal resistance is why your phone can feel warm when running intensive tasks while charging."
"The 'Maximum Power Transfer Theorem' says you get max power when load resistance equals internal resistance."
