For obvious reasons, fire alarm systems are required to have two sources of electrical power sources according to NFPA 72 (1999 edition), section 1-5.2; a reliable primary source and a standby secondary source. There is an exception to this requirement for two sources, where the primary source is connected to an emergency power system where a person specifically trained in its’ operation is on duty at all times. This could be an emergency power generator used to deliver the primary power full time such as co-generation with the commercial utility company.
Where a standby power supply is provided, it is required to provide power to the fire alarm system for a minimum of 24 hours when the fire alarm system is functioning in a non-alarm condition. At the end of the 24 hour period, the standby power supply must provide power to the fire alarm system operating in emergency mode for 2 hours, and the notification appliances (strobes, horns, chimes, etc.) for 15 minutes.
You can use batteries to meet this 26 hour power supply requirement, or you can use the emergency power generator, life safety branch already provided in your hospital or nursing home. If you choose to use the emergency generator power as your standby source, then you must have batteries that are capable of operating the fire alarm system in a non-alarm condition for four (4) hours, then operate the system in an emergency mode for 15 minutes.
That is why you may observe smaller batteries in the hospital fire alarm control panel as compared to the batteries in a non-healthcare occupancy location, such as a clinic or office building. Those batteries would have to be sized to operate the fire alarm system for 26 hours, as compared to the 4 hours if the system had emergency generator power as a standby source.