Douglas Krantz - Technical Writer - Describing How It Works
Go to the Fire Alarm Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Go to the Science Article Map Page of Douglas Krantz - Technical Writer
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works

Fire Alarm -- Maintenance

Two batteries in series, one of them drains to zero while the other still has a charge. Will the one with a charge be able to drive the one that hit zero negative?
The batteries are there for backing up the system when power drops out. Both batteries have to keep the panel in a standby mode. They both should totally discharge at the same time.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






Do You Have to Replace Fire Alarm Batteries in Pairs?

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.

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.

Was this
helpful?
Yes   No

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.







Douglas Krantz

Describing How It Works
writer@douglaskrantz.com
Text
612/986-4210

View Douglas Krantz's profile on LinkedIn



Ask
The
Technician

Readers Questions



Short Circuit
Free Subscription
I'll Send You the
Twice-Monthly
Fire Alarm
Newsletter

Get Short Circuit


Articles

How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More

How is a Buffer Relay Wired Into a Door Holder Circuit? -- Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut. The door holder looks simple and innocuous enough... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More



Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.