Internal Resistance Calculator

Every real battery has internal resistance that drops its voltage under load.

How the Math Works

The Internal Resistance Calculator uses Ohm's Law principles to determine a battery's internal resistance. The formula r = (EMF - V) / I calculates resistance by finding the difference between the battery's electromotive force (EMF) and its terminal voltage (V), then dividing by the current (I) flowing through the circuit. This works because when current flows, internal resistance causes a voltage drop, making terminal voltage lower than EMF. The larger this voltage difference and the higher the current, the greater the internal resistance.

Practical Applications

To use this calculator for practical analysis, measure the battery's EMF using a voltmeter when no load is connected, then measure the terminal voltage while the battery is powering a device, and finally measure the current with an ammeter in the circuit. For example, if a battery shows 12V EMF, drops to 10V when loaded, and carries 2 amps, its internal resistance is 1 ohm. This information helps engineers design appropriate charging circuits, predict battery performance under load, and diagnose deteriorating batteries before they fail.

Day-to-Day Use

Understanding your battery's internal resistance helps you make informed decisions about when to replace it, whether it's suitable for high-drain devices like smartphones or power tools, and how it will perform in different conditions. A car owner can use this to verify if their battery truly needs replacement or if the issue lies elsewhere. It also helps optimize device design - knowing your battery's characteristics allows you to choose the right load or predict how long it will last in your specific application, potentially saving money and preventing inconvenient failures.

Worked example

1.5 V EMF dropping to 1.3 V at 0.5 A → 0.4 Ω.

FAQ