Douglas Krantz - Technical Writer - Describing How It Works
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How Can I Tell if the Batteries are Bad?

The only really accurate tester you can use to see if the batteries will work in a fire alarm system for the 24 hour blackout and then sound the alarms function of the batteries is the fire alarm system itself.

When checking the batteries, the hardest test, but the test that requires the least interpretation, is to see how much voltage is on the batteries after a 24 hour blackout

Greetings Douglas,

Ok. So, after conducting an annual fire alarm Inspection on a Edward EST-3 system, the main fire alarm panel batteries (2-12 volt 55 amp hour batteries, installed 2018) were reading 22 hrs. The customer believed the fire panel charger is not working properly because it not charging up the batteries. He tested the connections inside the battery cabinet on the input side and he got 1.6 Volts. The question is what's the normal voltage he should be reading?

Thank you, K F

I've worked with EST3s before.

The EST3 power supply will not give you a voltage reading if batteries are not connected. Never. Ever. Your 1.6 volts is the best voltage you'll ever get out of the charger without the batteries being connected. This is because of the protection circuitry inside the power supply.

Question, did you mean the batteries were reading 22 volts? If that's the case, it could mean that one or more cells in the batteries are bad, or it could mean the power supply is bad.

Question: Were the batteries warmer than the cabinet they were in?

If they were warmer, that means that the charger in the power supply was working overtime trying to charge the batteries to 27 volts. The warmth comes from a constant excessive charging current from the power supply.

If they weren't warmer, then it could be a bad charger in the power supply.


If the batteries are hot for any reason, replace the batteries anyway. Even if the charger is bad, the batteries are compromised whether the charger is working properly or not. Even though the battery cost seems high, replacing the batteries is cheaper than replacing the power supply, and still have to be regularly replaced whether or not the power supply is bad.

After replacing the batteries, check the voltage immediately when the batteries are connected, and after a few minutes. If the charger is working, the voltage should have increased at least a little.

Battery warmth. Remember that anytime batteries in any fire alarm power supply are warmer than the cabinet they're in, the batteries are bad. Replacing the batteries often saves a lot of time troubleshooting.

Douglas Krantz

Further Question

Mr. Krantz

Thank you very much for such a quick response.

So, when I first checked the batteries they read 12.65 Volts and 22 Amp Hours. I checked the batteries with a battery tester, not a multi-meter. After the low amp hour reading, it was recommended they replace the batteries due to low amp-hours. According to NAPA 72, those batteries need to be replaced, even though they haven't gone past the 3 Years since they were installed.

However, the customer's argument was that he replaced the batteries two years ago and that they shouldn't be reading so low. When we checked the voltage today inside the battery cabinet, we got 24 volts on the input side with everything connected. We got the 1.6 Volts when we removed the fuse and batteries. You are correct about getting little or no voltage when batteries were disconnected. We did go ahead and replace the batteries with 2 new 12-volt 55-amp hr batteries. I will keep you posted on what's going on with that job.

Thank you, K F

Things to remember about testing batteries:

False Low Reading - The Amp Hour Tester works by testing the conduction of the battery. To measure the conduction, it allows a high current to pass from the battery to the tester. I have tried this. If the connections to the battery are even a little bit less than perfect, the Amp Hour Tester will show much lower amp hour readings because the reduced conduction of the battery terminals decreases the total conduction of the battery.

Since larger amp hour batteries have to provide greater current, the tester has to draw greater current. More current means the connections have to be better. Make sure the tester is making very good connections with the batteries.

Calibration of the Amp Hour Tester - In the instructions for the tester, you are supposed to use a brand-new battery of the same exact make and model of the battery you are testing. Make sure the new battery is fully charged. Measure its amp hour rating and adjust the amp hour tester to read the proper amp hours. Life in the real world does not allow you to keep new 55 Amp Hour batteries with you - especially batteries that are the exact make and model of the batteries you are testing. That means you will never be able to make a proper measurement.
  • If you are testing 7-amp hour batteries, you have to calibrate the meter using a brand new 7 amp hour battery
  • If you are testing 8-amp hour batteries, you have to calibrate the meter using a brand new 8 amp hour battery
  • If you are testing 18-amp hour batteries, you have to calibrate the meter using a brand new 18 amp hour battery
  • If you are testing 25-amp hour batteries, you have to calibrate the meter using a brand new 25 amp hour battery
  • If you are testing 33-amp hour batteries, you have to calibrate the meter using a brand new 33 amp hour battery
  • If you are testing 55-amp hour batteries, you have to calibrate the meter using a brand new 55 amp hour battery

In other words, you, the technician, cannot count on the meter to be accurate. You, the technician, have to interpret what you read on the meter and decide if the batteries are good.

Accurate Testing

During a power blackout, the batteries are designed to keep the fire alarm system working for 24 hours without an alarm, and then sound the alarm for 5 minutes. Then again, if this is a voice evacuation system, the system has to continually sound the alarm in the whole building for 15 minutes.
  • The Amp Hour Tester you are using will only give you a general idea of how good the batteries are. You have to interpret the readings.
  • Methods suggested in the NFPA are just as poor; you still have to interpret the readings.

The only really accurate tester you can use to see if the batteries will work for the "24-hour-blackout-and-then-sound-the-alarms function of the batteries" is the fire alarm system itself. If, at the end of the whole test, the batteries still have 22 volts, then the batteries are good.

If, at the end of the 24-hour-blackout-and-then-sound-the-alarms test, the batteries are less than 22 volts, there are three possibilities:
  • Probable - The batteries are bad and need to be replaced

  • Possible - The batteries are sized too small and need to be sized bigger

  • Rare - The charger is bad


Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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