Fire alarm back-up batteries are batteries that provide power to a fire alarm system in the event of a power outage. They are designed to ensure that the fire alarm system continues to operate and provide warning signals to the building occupants in the event of a fire, even if the primary power source is lost. They are typically used in commercial and industrial buildings, as well as in residential buildings with hardwired fire alarm systems.

Back-up batteries come in various sizes and capacities, depending on the needs of the fire alarm system they are powering. They are connected to the fire alarm control panel and are continuously monitored to ensure they are charged and ready to provide power in case of an emergency.

Back-up batteries are typically replaced every 3-5 years, depending on the type of battery used. They are also tested regularly to ensure they are in good working condition. It's important to note that the fire alarm system should be tested on a regular basis to ensure that the back-up batteries are able to provide power in case of an emergency.