Internal Resistance Calculator

Every practical battery, power supply, generator, and electrical source behaves as though a small resistance sits in series with its ideal voltage source. This internal resistance is not usually a separate physical resistor. It represents the combined effects of electrode materials, electrolyte conductivity, contacts, windings, semiconductor paths, and other losses inside the source. When no current flows, a voltmeter reads the open-circuit voltage. When a load draws current, some voltage is lost internally, so the terminal or loaded voltage falls. This calculator uses a simple Thevenin-source model. A known load resistance is connected to the source, and the loaded voltage is measured across that resistor. The load current equals loaded voltage divided by load resistance. Internal resistance then equals the voltage drop inside the source divided by that current. Combining those relationships gives internal resistance = load resistance multiplied by (open-circuit voltage divided by loaded voltage minus one). The model also reports load current and the observed voltage drop so that measurements can be checked. Internal resistance is an important indicator of source condition. A fresh battery normally has lower internal resistance than an aged, discharged, cold, or damaged battery of the same chemistry. As resistance rises, terminal voltage sags more severely under load, heat generation increases, and the source becomes less capable of supplying high current. Automotive starting batteries, radio-control packs, uninterruptible power supplies, and laboratory cells are often compared by repeating the same controlled load test over time. For useful results, measure both voltages with the same calibrated meter and make the loaded reading quickly enough to limit heating or state-of-charge changes. Choose a resistor with an adequate power rating because it dissipates loaded voltage squared divided by resistance. The simple model assumes the source is stable and approximately linear during the test. Electrochemical batteries can have dynamic behavior, so pulse length, temperature, charge level, and recent use may affect the apparent resistance. Do not short-circuit a battery to estimate resistance. A short can produce destructive current, burns, fire, venting, or explosion. Use a suitable known load, fused test leads, and manufacturer limits. This calculator supports engineering estimates and comparisons, but safety-critical battery qualification should follow the relevant test standard and use properly rated equipment.