Why Replace Batteries in Pairs?

Batteries are usually wired in series in a fire alarm panel. Because they're wired in series, if one battery is old and the other new, they won't be matched, they won't have the same capacity. The one will still have voltage when the other has run out. The one battery will then reverse charge the other battery.

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?

The words used in the Fire Alarm Code isn't the issue; the issue is fire and life safety. 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.

Zero Volts

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

When the batteries are matched, they discharge at the same rate, reaching zero volts at approximately the same time. The current stops so neither battery is reverse charged.

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.