By Douglas Krantz
The question is asked: In a fire alarm system
, where backup batteries are wired in series, can the batteries be changed out one at a time, or do the batteries have to be changed out in pairs?
Remember that the fire alarm system
is a life safety system. Backup batteries for the fire alarm system are meant to allow the system to keep working even when there's an electrical blackout. They keep the fire alarm system working for either a 24 hour period (one day) or a 60 hour period (over the weekend). After that, they still should have enough of a charge left in them
so the system can sound the evacuation.
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Looking at the picture from another point of view, the question can be worded: Do the batteries have to be matched? Or another way of asking the same question, as the batteries are discharged, do both batteries have to reach zero volts at the same time?
Unmatched Backup Batteries
Batteries are all different. Each one will discharge at a different rate from another. Different manufactures will have greatly different discharge rates, and even different runs of batteries within a single manufacturer will have a different discharge rate. The best one can do is to have the same manufacturer and date codes on both batteries being used.
Remember, these batteries are wired in series -- the current going through one of them also goes through the other. If the batteries are not matched, as they are being used up one of them will reach the totally discharged point (0 volts) while the other still has a charge. The one that still has a charge will continue to push current through the totally discharged battery, causing it to reverse charge.
This is not a good thing.
To start with, even a little reverse charging of a battery damages it. Continued reverse charging can rupture it, cause it to start a fire, and in a few cases cause it to explode.
Matching the Backup Batteries
The batteries need to be matched so they both discharge at the same time.
Of course, the fire alarm system is dead when the AC is gone and the batteries are dead. But at least while the alarm system isn't working, the batteries themselves won't cause a fire.
Backup Battery Cost
The fire alarm system is a life safety system. Compared to most of the rest of a fire alarm system, batteries are cheap. Don't skimp and try to save a few bucks, replace both batteries because so much is at stake.
You can see how the fire alarm system works by ordering the book "Make It Work - Conventional Fire Alarms
" by Douglas Krantz.