Internal Resistance Calculator — Battery & Source Resistance
Every real battery, cell or power supply has an internal resistance that causes the terminal voltage to drop below the EMF when current flows. Enter the open-circuit EMF, the loaded terminal voltage and the current to find the internal resistance and the power wasted inside the source.
V
V
A
Resistance inside the battery or source opposing current flow
- 1
Voltage drop across internal resistance
EMF − V_t = 12 − 11.4 = 0.6 - 2
Internal resistance
r = 0.6 ÷ 3 = 0.2000Ohm's law on the internal drop: r = (EMF − V_t) / I.
How does this calculator work?
Internal resistance r = (EMF − V_t) / I, where EMF is the open-circuit voltage, V_t the terminal voltage under load, and I the load current. Power wasted inside the source is I² × r. Lower r means more of the EMF reaches the load — fresh, high-quality cells minimise it.
Formula
How this is calculated
A real battery is modelled as an ideal voltage source (the EMF, ε) in series with a small internal resistance r. When no current flows the terminal voltage equals the EMF. As current I is drawn, Ohm's law produces a voltage drop I × r across the internal resistance, so the terminal voltage falls to V_t = ε − I × r. Rearranging gives r = (ε − V_t) / I — the formula this calculator uses.
The power dissipated inside the battery is P_lost = I² × r, and the useful power delivered to the external circuit is P_out = V_t × I. Efficiency is simply P_out / (P_out + P_lost), expressed as a percentage. A brand-new AA alkaline cell has an internal resistance around 0.1–0.3 Ω; as it discharges or ages the resistance rises, explaining why old batteries dim under load even though their open-circuit voltage looks acceptable.
Note: this model assumes a purely resistive, DC-steady-state scenario. Real sources exhibit non-linear behaviour (concentration gradients in electrochemical cells, temperature dependence) that this simple model does not capture.
Frequently asked questions
Measure the open-circuit voltage (no load) and record it as EMF. Then connect a known load resistor, measure the terminal voltage and the current with a multimeter. Plug the three readings in — or calculate r = (EMF − V_t) / I directly.
More current through the internal resistance causes a larger voltage drop I × r, reducing the terminal voltage. This is why battery-powered devices can dim or reset when a high-current load is applied even though the battery still has charge.
Fresh AA alkaline: 0.1–0.3 Ω. Lead-acid car battery: 0.002–0.02 Ω. Lithium-ion 18650 cell: 0.02–0.1 Ω. Discharged or old batteries can be 10× higher, severely limiting usable power delivery.
Also known as
TG we-Calculate Editorial Team. (2026). Internal Resistance Calculator — Battery & Source Resistance [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/internal-resistance-calculator
TG we-Calculate Editorial Team. "Internal Resistance Calculator — Battery & Source Resistance." TG we-Calculate. 2026. https://we-calculate.com/calculator/internal-resistance-calculator.
TG we-Calculate Editorial Team, "Internal Resistance Calculator — Battery & Source Resistance," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/internal-resistance-calculator
@misc{wecalculate_internal_resistance_calculator, title = {Internal Resistance Calculator — Battery & Source Resistance}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/internal-resistance-calculator}}, year = {2026}, note = {TG we-Calculate} }
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