Battery Internal Resistance
Battery internal resistance is the effective opposition inside a cell or pack that produces a voltage change and heat when current flows.
What the measurement means
Under load, internal resistance contributes to voltage sag during discharge and voltage rise during charging. Resistive loss increases with the square of current, so resistance helps limit acceleration, regenerative braking, and fast-charging power. Rising resistance can also be one sign of battery aging.
Internal resistance is not one fixed physical resistor. A reported value depends on temperature, state of charge, current direction, pulse duration, cell chemistry, and the measurement method. Direct-current pulse tests and electrochemical impedance spectroscopy can therefore produce different values because they probe the battery over different timescales and frequencies.
It is also distinct from isolation resistance. Internal resistance describes behavior within the battery's current path; isolation resistance describes electrical separation between the high-voltage system and the vehicle chassis.
For degradation mechanisms and resistance growth, see Battery Degradation. Battery controls use this information when estimating State of Power (SoP).