ELECTROTHERMAL MODELS · BEGINNER

Battery voltage sag and incandescent lamp inrush

A real battery has internal resistance, and a filament lamp changes resistance as it heats. Together, those effects explain why the startup current can be much higher than the running current.

Open the lamp example

Battery under load

A battery's terminal voltage is lower than its open-circuit voltage while it supplies current. The voltage drop is approximately the load current multiplied by internal resistance.

V_terminal = V_open − I × R_internal

Higher current, lower state of charge and colder conditions can make the effect more visible. This is why a device can reset even when an unloaded battery appears healthy.

Why the lamp draws an inrush current

Tungsten has a much lower resistance when cold. Right after switch-on, current is high and the filament heats rapidly. As temperature rises, resistance rises and current settles to a lower operating value.

Try it in resistor.dev

Open the lamp and switch example, then run the transient simulation.
Observe the current at switch-on and compare it after the lamp warms up.
Change battery internal resistance and compare terminal-voltage sag.
Change ambient temperature to see how starting resistance and heating change.

Use ratings for the real component

The model teaches the trend, but a real lamp and battery need manufacturer ratings, wiring limits and suitable protection. Design switches, fuses and power supplies for startup conditions, not only steady-state current.