7.2 Batteries for Alarm Systems

Why batteries are a license topic, not a hardware footnote

When the dedicated branch circuit in Section 7.1 dies, the secondary power supply is what keeps smokes powered, communicators up, and horns able to sound. On almost every FACU and security panel you will service in New York, that secondary source is a sealed lead-acid battery, usually described as valve-regulated lead-acid (VRLA) — absorbed glass mat (AGM) or gel. Independent OpenExamPrep material here applies Chapter 6’s ampere-hour preview to real batteries. Full secondary-power arithmetic, including a 20% factor as industry/NFPA practice, is Chapter 9. This section teaches chemistry, float charging, aging, and a simple AH = I × t example you can do closed-book.

19 NYCRR 195.1 is explicit. Installation includes placing and connecting equipment such as batteries. Maintaining includes inspecting devices such as batteries. Servicing includes the repair, troubleshooting, or replacement of malfunctioning, failed, or damaged equipment such as batteries, and also testing and work on the client’s control panel. Replacing a pair of 12 V blocks in a FACU is not a hardware-store errand. It is licensed servicing. Line-voltage landing of the charger’s AC feed remains the 195.2 electrician side of the wall (Section 7.1).

What you will actually see in the cabinet

Most fire panels use two 12 V VRLA batteries in series to make a 24 V bank. Series means the same current through both; voltages add (Chapter 6). Most security panels use one 12 V block on a 12 V bus. Capacity is marked in ampere-hours (Ah) at a stated discharge rate (often a 20-hour rate on the label). A 7 Ah and an 18 Ah battery can look similar in a plastic case. Read the label and the panel’s listed range.

Sealed means you do not add water. The valves relieve pressure if the charger overcharges; they are not a fill cap. Do not take a sealed battery apart. Do not assume “sealed” means zero hydrogen ever — heat and a runaway charger can still vent. Keep metal tools off the terminals: a wrench across 24 V at hundreds of cold-cranking-style amps is an arc-flash in a can.

Those float windows are typical manufacturer practice. Use the listed charger voltage in the installation instructions. A meter reading of 26.0 V on a 24 V pair with AC present often means the charger is low, a cell is failing, or you are already on battery with AC failed — Section 7.3 is how you tell which.

Standby current versus alarm current

Standby current (quiescent current) is what the system draws while it is watching: panel electronics, two-wire smoke standby, communicator idle, keypad backlight, auxiliary detectors. It is usually tens to a few hundred milliamperes on a modest FACU, more if you have hungry communicators, many four-wire detector bases, or a loaded security bus.

Alarm current is what the system draws after it goes into alarm: NACs (horns, strobes, speakers), door holders dropping out or magnetic locks releasing depending on design, dialer going off-hook or cellular radio transmitting, relays transferring. Alarm current is often amperes, not milliamperes. Chapter 6 already showed that six 75 mA appliances are 450 mA at the loads alone; add the panel and you are in the ampere range quickly.

The battery must cover standby for a long time, then still have enough left for alarm for a short time. That is why a panel that is idle but hungry kills batteries faster than a loud five-minute NAC.

Simple ampere-hours — AH = I × t

Ah = I (amperes) × t (hours)

Convert minutes to hours before you multiply. 5 minutes = 5/60 = 0.0833 h. 15 minutes = 0.25 h. Convert milliamperes: 250 mA = 0.25 A.

Worked example — 24-hour standby (simple)

A teaching FACU draws 0.25 A (250 mA) of standby from the 24 V battery.

Ah_standby = 0.25 A × 24 h = 6.00 Ah.

0.25 A × 24 h is not 0.25 Ah and not 24 Ah. The trap is dropping the 24 or treating 250 mA as 2.5 A (60 Ah).

Worked example — 5-minute alarm (simple)

Same system, alarm current 1.80 A for 5 minutes.

Ah_alarm = 1.80 × (5/60) = 1.80 × 0.0833 = 0.150 Ah.

Standby (6.00 Ah) still dwarfs alarm (0.15 Ah) because 24 hours is long and 5 minutes is short.

Rough combined draw before any safety factor: 6.00 + 0.150 = 6.15 Ah. 6.15 Ah is not a finished battery size. Chapter 9 adds every NAC, auxiliary, and communicator load you actually connected, then applies a 20% factor as industry/NFPA practice. Do not multiply by 1.20 on this chapter’s exam items unless the question asks for it.

Preview — common NFPA 72 secondary-power times

Common NFPA 72 concepts (confirm the adopted edition — New York’s 2025 Uniform Code references NFPA 72—22):

• Many fire alarm systems: 24 hours of standby (quiescent) operation, then 5 minutes of alarm notification.
• Emergency voice/alarm communications (voice evacuation): still 24 hours standby, then often 15 minutes of operation at maximum connected load rather than 5 minutes.

If the same 1.80 A alarm load ran 15 minutes instead of 5: Ah_alarm = 1.80 × 0.25 = 0.450 Ah. Three times the 5-minute alarm energy, still usually smaller than 24 hours of standby. Occupancy exceptions and generator-plus-battery combinations belong in Chapter 9. Do not invent a 60-hour rule on this exam unless a cited edition in front of you actually states it for that system type.

A related charging-system concept in NFPA 72 is that the charger should be able to recharge the batteries within a stated time (commonly taught as 48 hours) after a deep discharge. Oversized batteries on an undersized charger fail that test. Again, Chapter 9 and the listed power supply.

Float charging, aging, and temperature

Alarm panels use float charging: with AC present, the charger holds battery terminal voltage at a constant value slightly above open-circuit so the bank stays full and draws only a small maintenance current. That is not the same as a car’s cyclic charge, and it is not a “trickle” wall charger you bought in bulk. Use the listed charger in the panel.

When AC fails, battery voltage sags under load and the bank supplies standby, then alarm, current. After AC returns, the charger must refill what was taken without cooking the cells.

Temperature works two ways. Cold reduces available capacity — a garage or loading-dock panel in January delivers fewer Ah than the label written near room temperature (often about 20–25 °C on manufacturer data). Heat accelerates grid corrosion and shortens life — an attic, boiler room, or unventilated can above a kitchen. Do not treat a “five-year battery” as a code guarantee. Follow the manufacturer replacement interval and the inspection/testing rules in Chapter 17. A battery can show a healthy float voltage and still collapse under alarm load because internal resistance has risen.

Never mix casually:

• Old and new in the same series string. The weaker unit limits the bank and can be reverse-charged.
• Different Ah ratings in series.
• AGM with gel, or lead-acid with lithium, nickel-cadmium, or a random laptop pack, unless the listed power supply is designed for that chemistry.
• A “temporary” automotive starting battery on a FACU float charger.

Replace both 12 V blocks on a 24 V FACU as a matched set: same type, same rating, same approximate age. Record date codes. Observe polarity; many cabinets put a fuse in the battery lead — replace it with the listed type, not a nail. Dispose of lead-acid as required; do not trash them with cardboard.

Exam traps

• Reporting 6.00 Ah as 6.00 Wh (missing the volt factor from Chapter 6) or as 0.25 Ah (dropping the 24 hours).
• Using 5 minutes as 5 hours in AH = I × t.
• Treating the Chapter 9 20% factor as already included in 6.15 Ah.
• Mixing a new 18 Ah AGM with a five-year-old 7 Ah gel “to average them.”
• Calling battery replacement unlicensed because “it’s just a battery,” contrary to 195.1.

Section 7.3 is how you prove with a meter that the charger is floating, the pair is open, or a ground fault is dragging the bus down.