Foundational Physics

Short Answer: Real batteries have internal resistance because moving charge through their materials and carrying out the chemical reactions that supply electrical energy are not perfectly lossless processes. When a battery delivers current, some energy is transferred inside the battery instead of reaching the outside circuit. As a result, its terminal voltage generally falls when it supplies more current.

Isn’t a battery supposed to push current?

Yes. A battery uses chemical reactions to maintain a difference in electric potential between its terminals. But making that happen involves real materials: electrodes, an electrolyte, and interfaces where chemical reactions occur.

Charges moving through those materials face limitations. The chemical reactions also cannot respond infinitely fast. Together, those effects mean a real battery behaves differently from a perfect source of voltage.

We represent these effects with internal resistance.

Why does the voltage drop when the battery is working?

Imagine measuring a battery with almost nothing connected to it. Very little current flows, so the meter shows a voltage close to the battery’s open-circuit voltage.

Now connect a device that draws substantial current. Some of the available energy is dissipated inside the battery, and its terminal voltage may fall. Your device receives the terminal voltage—the voltage actually present across the battery’s two external terminals while it operates.

The harder the battery is asked to work, the more noticeable the voltage sag can become.

A quick example

Suppose a battery has an open-circuit voltage of about 12 V. When it powers a device, the terminal voltage might fall to 11.8 V.

That does not mean the battery has suddenly lost its remaining charge. It means the loaded battery cannot maintain exactly the same voltage at its terminals under those conditions.

In the simplest model, a battery acts like an ideal source with a small resistance inside it. The internal voltage drop grows as the supplied current grows. Real batteries can behave more complicatedly, but this model captures the basic idea.

Where does that energy go?

Some energy turns into thermal energy inside the battery. Other internal electrochemical effects can also reduce its terminal voltage under load.

That is one reason a battery can warm up when delivering a large current. The energy that is dissipated internally is not available to the external device.

The takeaway

A real battery supplies electrical energy, but delivering current has an internal cost. Internal resistance is the simple circuit model for that cost. It helps explain why the terminal voltage of a working battery can be less than its open-circuit voltage.

When This Breaks

Internal resistance is not always a fixed number. It can depend on the battery’s temperature, state of charge, age, and current. The simple model also does not capture every slow chemical or transient voltage change.

And when a rechargeable battery is being charged, the direction of energy transfer changes; its terminal voltage can be higher than the effective source voltage rather than lower. So “the terminal voltage always drops” is not a universal rule.

Keep Exploring: Current, Resistance, and Ohm’s Law

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