By Douglas Krantz
The 2007 NFPA72 shows in Table 10.4.4, Item 6(d)(1), that the sealed lead-acid batteries used for battery backup in fire alarm systems
need to be replaced within 5 years of manufacture. The NFPA wants the batteries replaced because the battery capacity is down to about 80% by that time.
Battery capacity, the amount of amphours in a battery, changes over time. In the first few months after manufacture, the amphour capacity of the average battery increases a few percent. For several years, this capacity doesn't change much. Near the end of the battery's useful life, the amphour capacity
starts to taper off. At 5 years, it's down to about 80% of rated capacity.
If the NFPA requires replacement at 5 years, why do most fire alarm service companies replace the batteries after 3 or 4 years?
The answer is timing.
At about the same time each year, fire alarm systems are inspected. Because the battery's stamped date code
is the manufactures secret, the service company has to go by the date of installation, not by the date of manufacture.
The trouble is, after manufacture, it's usually a month or two before the batteries are installed in the first place. This month or two gets added to the NFPA's 5 years, making the total time more than 5 years from the date of installation.
Rather than figuring out the date code stamped on the battery, and then trying to time the replacement at exactly 5 years, most fire alarm service companies decide to replace the batteries after 3 or 4 years of service. That way, usually, the batteries are replaced before they go bad.