What Does Battery Internal Resistance Tell Us?

Internal resistance, often quoted as DCIR or AC impedance, is one of the most useful health indicators of a lithium cell. It describes the opposition the cell offers to current flow inside its electrodes, electrolyte and connections.

What it causes

Under load, internal resistance creates a voltage drop (V = I x R) and dissipates heat (P = I^2 x R). A low-resistance cell delivers its rated voltage even at high current and stays cool; a high-resistance cell sags, feels warm and may trip out under heavy load.

Why resistance rises over time

As a cell ages, the electrolyte dries out, reaction products build up on the electrodes and contact interfaces degrade, all of which increase resistance. This is why an old battery may still read a fine open-circuit voltage yet collapse the moment a load is applied.

Practical meaning

For power tools, drones and high-rate packs, low and consistent internal resistance is as important as capacity. Compare cells of the same chemistry and measurement method, because values depend on state of charge, temperature and test frequency. Rising resistance is an early sign that the pack is approaching end of life.