The quality and durability of an alarm panel’s secondary power supply – in this case rechargeable batteries – is one of the most significant of the many aspects of fire detection and life safety that concern fire technicians. Without a reliable source of back-up power, detection and response cannot be assured. Dealers need to understand how to best use and store their batteries.

Back-Up Power to the Rescue!

The secondary power supply maintains the operation of a fire alarm panel when the public electric bus fails. This is especially true when a power outage lasts for an extended period.

Section 1-5.2.6 of NFPA 72, National Fire Alarm Code, requires that the fire alarm panel automatically switch from primary to secondary power within 30 seconds after the public electric bus has failed. In this case, when switchover occurs, there must be a ready and reliable supply of back-up power available. Of course, it is the fire technician’s job to see that this takes place without a hitch.

The same portion of the code requires that these secondary batteries sustain the system in standby for a period of 24 hours in protected premises, central station and proprietary systems; 60 hours in auxiliary and remote station systems.

After the standby period has elapsed, the same batteries must then sustain the system for a subsequent ring time of 5 minutes. In the case of an emergency voice/alarm communications service system, these batteries must maintain the system for 2 hours in emergency mode or 15 minutes in evacuation mode, all of this after 24 hours of standby.

Safely Store Lead-Acid Types

The first consideration for fire technicians, especially those who run relatively large operations, is the stockpiling of lead-acid batteries. This ensures that service and installation crews will always be able to do their jobs without delay.

Lead-acid batteries are one of the easiest rechargeable batteries to store for extended periods of time. If they are charged when storage begins, they can realize a significant work potential at the end of the storage period. This makes the lead-acid battery inherently better for panel use than lithium, nickel-cadmium and other battery types.

To realize long storage times with a relatively high output charge, lead-acid batteries must be 80-percent to 100-percent charged when storage commences. The reason for this is the lead-acid battery’s low self-discharge rate. On the other hand, discharged batteries will not last very long before they reach the point of no return.

All batteries, both primary and secondary, have a tendency to lose their charge over a period of time. In the case of the primary battery, discharge can take years to occur; in secondary batteries, the time period is considerably less. Gradual discharge takes place because of chemical reactions that continue to take place within the cells of a battery during storage.

For all these reasons, fire technicians should check the charge on their lead-acid batteries before they commit them to long-term storage. The higher the charge when they are placed in storage, the more likely they will survive. In addition, the higher the charge at the beginning of the storage period, the more likely the batteries will contain viable charges when they are taken out of storage.

The traditional method of checking the charge contained within a flooded lead-acid battery is to measure battery voltage. However, this is not always an accurate way of determining capacity. The better method is to use a battery tester, such as the STC-612 battery analyzer manufactured by Stone Technologies Corp. of Austin, Texas (see opening photo).

Another aspect associated with long-term battery storage relates to battery voltage after a period of time. In general, battery voltage cannot be allowed to drop below some determined minimum. The ramifications could be partial or permanent damage. This minimum voltage may vary from one battery make to another, but the most common open-circuit voltage is 1.8V per cell.

“1.7V or lower per cell is where there is a significant increase in the chemical/mechanical damage to the battery and its long-term ability,” confirms Art Stone, president of Austin-based Stone Technologies. “When the per-cell voltage drops below 1.6V, permanent damage occurs to the battery. That is why a completely discharged battery rarely comes back alive after even a day or two.”

Temperature is important when storing batteries because the rate of self-discharge is inherently associated with ambient temperature. For example, at room temperature, the typical storage period of a sealed, lead-acid battery is three years before recharging is necessary. For every 10º C higher than room temperature, the capacity drops 50 percent.

Plan to Exceed Life Expectancies

How long should a lead-acid battery last? In most cases, battery manufacturers warranty their sealed, lead-acid products for a period of five years – but everyone knows that many of these batteries will last a lot longer.

Of course, end-of-life and effective capacity are two separate issues. In reality, when most batteries have reached their half-life, they have also reached the point where their rated capacity has been halved. Thus, it may be prudent for fire technicians to install secondary battery supplies that contain more capacity than necessary.

For example, where the use of two 7AH, 12V batteries would normally fulfill a common 24VDC fire alarm system’s needs, perhaps it might be wise for the dealer to up the AH rating to 14 or 18.

Another aspect of battery life that fire technicians need to be aware of relates to how often their batteries are actually used. Probably the most harmful thing for lead-acid batteries is to sit in a fire alarm panel without ever being called upon to do any work. Eventually, such batteries become unable to perform when needed.

“Exercising [lead-acid] batteries is one of the most important things for long battery life. Most panels made today do a daily load test. It should be at least 3 minutes in duration,” adds Stone. “Checking the charge voltage is another must-do. Excessive charging voltage can lead to early battery failure.”

When checking series-connected batteries, the fire technician must be sure to check the charge voltage across each battery, not both of them in series. A shorted cell in one battery puts that much extra voltage on the others. This, in turn, will overcharge them and lead to a complete battery package failure.

Probably the saddest part of such a catastrophic failure is that such a condition could have been avoided by early detection and replacement of the original defective battery with the shorted cell. A continuous charge voltage of greater than 14VDC at room temperature is detrimental to a lead-acid battery’s life expectancy.

I Stand Corrected …

In the sidebar that appeared in the May “Fire-Side Chat,” the voltage shown at the power supply terminals was 12V @ 14AH. It should have read 24V @ 14AH.

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