Battle damage, equipment failure, power conservation, short circuits, and other sudden events can render portions of the interior of the ship without power or light.

These battery operated lamps automatically come on whenever the main power to the area is interrupted. They can last for many hours and are constantly recharged from the main power supply.

Not all areas of the ship are equipped with these battle lanterns...only the most important areas. Heads (bathrooms) are usually not equipped with these. For that reason, experienced sailors often carry a small pocket flashlight with them at all times. In the event of a power failure, even if they happen to be in an unlit compartment, they can continue their activities or leave the compartment easily.

Source: Operational Medicine 2001,  Health Care in Military Settings, NAVMED P-5139, May 1, 2001, Bureau of Medicine and Surgery, Department of the Navy, 2300 E Street NW, Washington, D.C., 20372-5300