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Every battery has a small internal resistance Rint (often milliohms) that causes the terminal voltage to sag under load. Model the battery as an ideal source Voc in series with Rint: under a load current I the terminals read Vload = Voc − I·Rint. Rearranging gives Rint = (Voc − Vload)/I. That resistance also burns power as heat, P = I²Rint, which limits how much current the battery can usefully deliver.
| Quantity | Formula |
|---|---|
| Internal resistance (drop) | Rint = (Voc − Vload)/I |
| Internal resistance (2-point) | Rint = (V1 − V2)/(I2 − I1) |
| Power lost as heat | P = I² × Rint |
| Max (short-circuit) current | Isc = Voc / Rint |
Internal resistance rises as a battery ages, gets cold, or discharges, so it is a key state-of-health indicator. A new lithium cell may be a few milliohms; a worn one several times that. Lower resistance means less voltage sag, less heat, and higher deliverable power.
It is the small resistance inside a battery that opposes current flow, causing the terminal voltage to drop under load. It is modelled as a resistor in series with the ideal cell voltage and is usually a few to a few hundred milliohms.
Measure the open-circuit (rested) voltage, then apply a known load current and measure the voltage again. Internal resistance is the voltage drop divided by the current: R = (Voc − Vload)/I.
Instead of needing the open-circuit voltage, you measure the terminal voltage at two different load currents. The resistance is the change in voltage divided by the change in current: R = (V1 − V2)/(I2 − I1).
It depends on size and chemistry. A healthy small Li-ion cell may be 20–50 mΩ, a large lithium pack cell a few milliohms, and a car lead-acid battery only a few milliohms. Lower is better.
It sets the voltage sag under load, the heat generated (I²R), and the maximum power the battery can deliver. High resistance means weak performance under load and more wasted energy.
Yes. Resistance rises sharply in the cold, which is why batteries feel weak in winter, and falls somewhat when warm. Measurements should note the temperature.
Yes. As cells age and cycle, chemical changes and electrode degradation raise the internal resistance. A rising resistance is one of the clearest indicators of declining battery state of health.
Any current through the internal resistance dissipates power as heat, P = I²R. At high currents this heat can be significant and must be managed to avoid thermal runaway.
DC internal resistance is measured from a step in load current, as here. AC impedance (measured at 1 kHz or via EIS) probes the cell at a specific frequency and gives a slightly lower value; both are useful health metrics.
The terminal voltage falls as I×R, so at very high currents the voltage collapses and useful power drops. The theoretical maximum is the short-circuit current Voc/R, but batteries should never be operated near it.
Yes. Cells in series add their resistances; cells in parallel divide it. A pack of N series and M parallel cells has Rpack = N/M × Rcell.
The open-circuit voltage must be the true no-load value; if the battery has just been charged or discharged it needs to settle first. Otherwise the calculated resistance includes transient effects and is inaccurate.
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