Internal Resistance and Terminal Voltage

A battery is not a perfect source of voltage. Inside it, chemical reactions push charge around, but the materials carrying that charge have some resistance too, called internal resistance, r. Think of the battery as an ideal emf source with a small resistor stuck in series with it, hidden inside the case. You cannot get a probe inside to measure across just the emf, you can only measure across the whole battery's terminals.

When current I flows, some of the emf gets used up driving that current through r before the charge even leaves the battery. What is left over to drive the external circuit is the terminal voltage, V = emf - I r. The larger the current, the more voltage gets lost internally, and the lower the terminal voltage sags. A battery sitting on a shelf with no current flowing shows its full emf, because with I = 0 there is no drop across r. That is why the number printed on a battery is really its emf, not a promise of what you will measure while it is working under load.

This connects directly to why adding more bulbs in parallel dims every bulb slightly, and why a car's headlights dim when the starter motor cranks: the starter draws a huge current, that current flows through the same internal resistance as everything else on the battery, and the resulting voltage drop lowers the terminal voltage available to the headlights too. As external resistance drops, current rises toward a ceiling set entirely by r, and terminal voltage collapses toward zero, since nearly all the emf is spent inside the battery itself.