Many older handheld meters, including selenium-cell models, do not need a battery. They use a photoelectric (selenium) cell that generates a small electrical current directly from the light falling on it. That current is enough to drive the meter movement and indicate an exposure value.

So the meter is effectively powering itself from the scene light. In brighter light it produces more current; in dimmer light it produces less, but if the cell is sensitive enough and still in good condition, it can still give usable readings indoors.