Internal Resistance Calculator

Every real-world power source, such as a battery or power supply, has some opposition to the flow of electrical current originating from its internal chemical or physical composition. This is modeled as an internal resistance (r) connected in series with an ideal electromotive force (EMF, or E). When a current (I) flows through the circuit, a portion of the total voltage is lost internally, which reduces the usable voltage delivered to the external load resistance (R). The primary governing equations are: internalResistance = (cellEMF / current) - loadResistance, terminalVoltage = cellEMF - (current * internalResistance), and terminalVoltage = current * loadResistance.

Imagine you have a fresh lead-acid battery with a nominal electromotive force (EMF) of 12 volts. When you connect it to a load resistance of 5 ohms, a current of 2.18 amperes flows through the circuit. To find the internal resistance of the battery, we substitute these values into our formula. First, calculate the total resistance of the circuit by dividing the EMF by the current: 12 V / 2.18 A = 5.5 ohms. Next, subtract the known external load resistance of 5 ohms from this total resistance: 5.5 ohms - 5 ohms = 0.5 ohms. Therefore, the internal resistance of the battery is 0.5 ohms, which accounts for the missing 1.09 volts of terminal voltage drop under load.

When measuring or calculating internal resistance, always ensure your current and voltage measurements are taken simultaneously, as battery chemistry can shift rapidly under varying thermal and load conditions. Avoid short-circuiting power sources to find internal resistance, as extreme currents can overheat components and ruin batteries. Keep connections secure to prevent contact resistance from skewing your calculations.