6.28.21 – SSI – Bob Dolph
While it is important to have adequate backup power for all systems, it is especially important for life-safety systems such as fire alarms.
An important area for all best practicing techs to understand is applying proper battery backups. This topic is often overlooked until, when? You guessed it — the systems power fails and the backup batteries only worked for a few minutes instead of hours.
The system appears to be working fine on a daily basis so how can this be? Let’s take a few minutes this month to drill down on the technology of battery backups.
First, what type of battery are we talking about, one that starts a car? While they are physically big and have a lot of starting power, they are not best suited for alarm systems. Let’s look at sealed lead acid batteries (SLA). Being sealed allows for locating in alarm cabinets and not always positioned upright.
While it is important to have adequate backup power for all systems, it is especially important for life-safety systems such as fire alarms. This is why I will take a brief look at some of the backup battery requirements for fire alarm systems in standard NFPA 72.
In this standard, it is noted that the batteries will be able to supply ample backup power in a nonalarm state for 24 hours. After the 24 hours the batteries should still be able to provide power for at least five minutes in an alarm status.
One of many requirement options is that the system may be required to show, in calculations, a 20% safety margin. To become more familiar with this standard I recommend you visit the NFPA website where you can now view NFPA 72 and other standards for free.
First, remember that many alarm system and SLA battery manufacturers have some very good reference materials. One battery vendor that has some great documentation is PowerSonic, check them out.
A bit of battery tech trivia — how many of you mark newly installed batteries with the date that you installed them? If you do, you are being diligent, but not exactly correct. You should place the date the battery was manufactured.
What about the NFPA 72 comment on showing standby power calculations of at least 20%? One common formula to help calculating battery size is C min = [(I1 x T1) +I2] x 1.25. I1is standby current of alarm panel, T1 is standby time (24 hours for NFPA 72) and I2 is full alarm current of system.
Notice the aging factor of 1.25 at the end of the formula to give you that safety margin of 25%. Your answer will be the amp-hours requirement for your battery selection.
PLEASE NOTE – If you find your calculation requires a battery larger than the normal usage of the alarm panel. The existing panel charger may my not be adequate and you will need to check with the panel manufacturer.
Here’s a battery myth – “A digital multimeter (DMM) is all you need to test an SLA battery.” Not exactly the whole story. A DMM can be used to test a battery charger output by disconnecting the battery and reading charger output, about 13 volts.
A DMM can be used to check the normal voltage level of a disconnected battery not under load. The battery must have been fully charged. It should read greater than 12 volts for a 12V SLA battery. Less than that, it should be replaced.
A DMM can also check a battery under load, either from the panel or induced by a 5-10 Ohm resistor (HOT!). You should see a slow voltage decay rate that will bump up a bit after a few minutes. The bump level should be greater than 12V. A fast decay rate and a bump level less than 12V is a good indicator the battery should be replaced.
Most alarm SLA batteries have a normal lifespan of about three to five years. Many techs just automatically replace the batteries every three years. This should not excuse a tech from battery tests as a battery cell can always fail at any time.
Not every tech can afford to have a special battery tester in their toolkit. This is why I gave the above DMM guidelines. However, if you can swing it there are several special testers for SLA batteries that can save techs time and money. These testers (see Tool of the Month below) have built-in loads and onboard computers that calculate the quality of a battery very quickly and accurately.
Tool of the Month
This month I have selected an SLA battery tester that is very popular among techs. It is the Battery Life Tester from ELK Products.
Sometimes referred to as the Mhos Meter as it measures the battery’s internal conductivity (Mhos) versus internal resistance (Ohms). This is the best indicator of a battery’s health and life expectancy.