Find battery or source internal resistance from open-circuit and loaded voltage. Includes power loss, efficiency, short-circuit current, and maximum power transfer analysis.
Every real voltage source — batteries, generators, solar cells — has internal resistance that causes the terminal voltage to drop under load. Understanding this internal resistance is crucial for predicting battery performance, sizing power systems, and diagnosing aging or damaged cells.
The internal resistance is measured by comparing the open-circuit voltage (EMF) with the terminal voltage under a known load current: r = (EMF − V_load) / I. This simple measurement reveals how much power is wasted as heat inside the source and how the terminal voltage will sag under increasing load.
This calculator computes internal resistance, voltage drop, power loss, efficiency, short-circuit current, and maximum power transfer conditions. It includes a voltage-vs-current table showing how the terminal voltage decreases as load increases, and reference data for common battery types. Battery technicians, electronics designers, and physics students will find this tool essential for source characterization and circuit design. Check the example with realistic values before reporting.
Internal resistance is not printed on battery labels but profoundly affects performance. A battery with high internal resistance cannot deliver high currents and wastes energy as heat. This calculator determines internal resistance from easily measurable quantities and predicts behavior across all load conditions, helping you choose batteries, diagnose degradation, and design efficient circuits.
Internal Resistance: r = (EMF − V_load) / I_load Terminal voltage: V = EMF − I × r Power loss: P_loss = I² × r Efficiency: η = V_load / EMF = R_load / (R_load + r) Short-circuit current: I_sc = EMF / r Max power transfer: P_max = EMF² / (4r) (at R_load = r)
Result: r = 40 mΩ
A car battery reads 12.6V open-circuit and 11.8V at 20A: r = (12.6 − 11.8) / 20 = 0.04 Ω = 40 mΩ. Internal power loss is 20² × 0.04 = 16W. Efficiency is 93.7%. Short-circuit current would be 315A.
Internal resistance is the most reliable indicator of battery health. New lithium-ion cells have internal resistance of 20-50 mΩ, which gradually increases over hundreds of charge cycles. When resistance doubles from its initial value, the battery is typically considered end-of-life for demanding applications. Battery management systems (BMS) monitor internal resistance trends to predict remaining useful life.
The maximum power transfer theorem states that maximum power is delivered to a load when its resistance equals the source internal resistance. While this maximizes output power (P_max = EMF²/4r), it is only 50% efficient. In practice, most systems operate with R_load >> r for high efficiency. The exception is RF and audio circuits where impedance matching is critical for signal quality.
In series-connected battery packs, the total internal resistance is the sum of all cell resistances. If one cell has abnormally high resistance, it limits the pack's current capability and may overheat under load. This is why battery pack manufacturers carefully match cells by internal resistance (cell balancing) and why BMS systems monitor individual cell voltages.
Internal resistance is the inherent resistance inside a voltage source (battery, generator, solar cell) that causes the terminal voltage to drop when current flows. It is caused by electrode and electrolyte resistance in batteries, or winding resistance in generators.
Measure the open-circuit voltage (no load), then measure the voltage under a known load current. Internal resistance = (V_oc − V_loaded) / I_loaded. Dedicated battery testers use AC impedance methods for more accurate results.
A healthy car battery typically has 5-15 mΩ internal resistance. Values above 20-30 mΩ indicate aging. Above 50 mΩ, the battery may struggle to start the engine, especially in cold weather.
Maximum power is delivered to the load when R_load = r_internal. However, this condition is only 50% efficient — half the power is wasted inside the source. Power systems aim for R_load >> r for high efficiency.
Yes. Internal resistance is lowest at full charge and increases as the battery discharges. Near the end of discharge, resistance rises sharply, causing voltage to collapse under load.
I_sc = EMF / r. For a car battery (12.6V, 10 mΩ), this could be 1,260A — enough to melt wires and cause fires. This is why fuses and circuit breakers are essential. Never short-circuit a battery.