Battery Internal Resistance

Rint = (Voc − Vload) / I

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Result
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Formula

R_int = (V_oc − V_load) / I

Description

Battery internal resistance causes a voltage drop proportional to discharge current, reducing the terminal voltage under load. It can be measured by comparing the open-circuit voltage (no load) to the voltage under a known load current. Internal resistance increases as the battery ages, as temperature decreases, and as the state of charge decreases. It is a key indicator of battery health (state of health, SoH) and determines the maximum current the battery can deliver before the terminal voltage drops below the minimum operating voltage.

Variables

  • R_int — Internal resistance (Ω)
  • V_oc — Open-circuit voltage, no load (V)
  • V_load — Terminal voltage under load (V)
  • I — Load current (A)

Practical Notes

Typical internal resistances: 18650 Li-ion 20-80 mΩ (new), lead-acid car battery 5-10 mΩ, coin cell CR2032 10-30 Ω. A new 18650 cell at 50 mΩ drawing 3 A drops 150 mV. When internal resistance doubles from its initial value, the battery is generally considered end-of-life. AC impedance measurement (EIS) at 1 kHz provides a more accurate and faster measurement than DC methods.

Related Formulas

Battery Runtime Battery Energy Battery Loaded Voltage Battery Charge Time Battery Self-Discharge Battery Temperature Derating

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