How to Size a Standby Battery for an Alarm Panel

Why this matters

Most panels ship with a battery size printed in the install guide, and most installers fit that size without doing the math. That works until the job carries a cellular communicator, three extra motion detectors and a second keypad, at which point the "standard" battery quietly covers a fraction of the outage it was meant to ride through. The customer finds out during a storm, when the panel goes dark at 3 a.m. and the station sees nothing more from it. Sizing a battery takes ten minutes with the spec sheets in hand, and this card is the shop's method for it. Sibling cards that mention battery size point here.

The relationship everything follows from

A battery is a tank of amp-hours. The panel drains it at one rate while nothing is happening (standby) and a much higher rate while sounding (alarm). The battery must hold:

Required capacity = (standby current x standby hours) + (alarm current x alarm hours), then a margin on top, then rounded UP to a battery size you can actually buy and the panel can actually charge.

Every term in that line has a source, and the method is mostly about getting each term from the right place.

Step 1: build the standby load from the device list

List every device the panel's power feeds: the panel board itself, each keypad, each powered detector (motion, glass-break, smoke on a four-wire loop), each module (wireless receiver, zone expander, communicator), and anything else on the auxiliary output. Take each device's standby current from its maker's spec sheet. Use the published figure, not a guess, and use the figure for the mode the device will actually run in: a keypad with its backlight set to stay on draws more than the same keypad dimmed.

Why from spec sheets and not a meter alone: a meter reading taken on install day measures today's devices in today's state. The sheet tells you what each device needs at its worst steady state, and it lets you size the battery before the job is wired. The meter comes later, as the check.

Step 2: build the alarm load

In alarm, every standby device keeps drawing, some draw more (keypads light up and sound, the communicator transmits), and the sirens come on. The alarm current is the whole lot together, not just the sirens. Take the siren current from the siren's spec at the voltage the panel delivers.

Step 3: set the two durations

Alarm time is how long the sirens run before the panel's bell cutoff stops them. Read it from the panel programming; 5 minutes is a common setting, and some local ordinances cap how long an exterior siren may sound.

Standby time depends on what kind of system it is:

• Fire. Where the panel carries fire detection, NFPA 72, in the edition your authority having jurisdiction has adopted, sets the secondary power duration. For protected-premises systems that is commonly 24 hours of standby followed by 5 minutes of alarm; household fire alarm systems have their own chapter with its own alarm duration, so check the edition in force. Fire side work is often separately licensed; see related: Combination Fire and Burglary Panels and the License Line.
• Burglary. The minimum comes from the panel's listing and any local ordinance or insurer requirement that applies. Many dealers standardize on 24 hours anyway, because real outages after storms routinely outlast a short minimum. That is a dealer standard, not a universal rule; write down which one you used.

Step 4: the worksheet

Here it is for a real-sized residential job: hardwired panel, two keypads, four motion detectors, a cellular communicator, one interior and one exterior siren. Device currents are illustrative values of the kind found on spec sheets; use your own devices' sheets.

STANDBY BATTERY WORKSHEET   ACCT 2290

STANDBY LOADS          QTY   mA EACH   mA TOTAL
Panel board              1      80         80
Keypad, idle             2      25         50
Motion detector          4      10         40
Cellular communicator    1      30         30
Standby total                  200 mA = 0.20 A

ALARM LOADS
All standby devices, in alarm             250
Interior siren           1     350        350
Exterior siren           1     400        400
Alarm total                   1000 mA = 1.00 A

A  Standby  0.20 A x 24 h            =  4.80 Ah
B  Alarm    1.00 A x 5 min (0.083 h) =  0.08 Ah
C  A + B                             =  4.88 Ah
D  C x 1.20 margin                   =  5.86 Ah
E  Next standard size above D        =  7 Ah

Line by line:

A. 0.20 A for 24 hours is 4.8 Ah. Checked the other way: 4.8 Ah divided by 24 hours returns 0.2 A.

B. Five minutes is 5 divided by 60, or 0.083 hours. 1.00 A times 0.083 h is 0.083 Ah, shown as 0.08. Checked: 1 amp for a twelfth of an hour is a twelfth of an amp-hour.

C. 4.80 plus 0.08 is 4.88, call it about 4.9 Ah.

D. 4.88 times 1.20 is 5.86, about 5.9 Ah. The 20 percent covers what the arithmetic does not see: the battery's capacity fading with age, and a little temperature and tolerance slack. Many makers treat a battery at about 80 percent of its rated capacity as at end of life. Strictly, a battery that has faded to 80 percent needs a 1.25 multiplier, not 1.20, to still cover the load, which is one more reason step E always rounds up: the 7 Ah chosen below still holds 0.8 x 7 = 5.6 Ah at end of life, against 4.88 Ah needed.

E. The common sealed lead-acid sizes around this point are 4, 5 and 7 Ah. The first one at or above 5.86 Ah is 7 Ah, which leaves 7 minus 5.86, or 1.14 Ah, beyond the margin already built in.

What the worksheet teaches. The alarm portion is 0.08 of 4.88 Ah, under 2 percent. Standby dominates. That means the small continuous draws matter far more than the loud ones: the 30 mA communicator alone costs 0.03 A times 24 hours, or 0.72 Ah, which is 15 percent of the 4.8 Ah standby line. Doubling the exterior siren would barely move the answer; adding four more 10 mA motion detectors would add 0.96 Ah before margin.

Step 5: check the battery against the conditions its rating assumes

A battery's amp-hour rating is published at a stated discharge rate, commonly the 20-hour rate, and at room temperature. A 7 Ah battery at the 20-hour rate delivers its rating at 7 divided by 20, or 0.35 A. This job's 0.20 A standby is below that rate, so the rated capacity is available or slightly better, and the brief alarm draw at 1.0 A is too short to matter much. Two conditions break that:

• Heavy continuous draw. A system whose standby current is several times the 20-hour rate gets noticeably less than nameplate capacity. Use the maker's discharge table at the actual current.
• Cold. A panel in an unheated garage or outbuilding works from a battery whose capacity falls as the temperature falls. The 20 percent margin was not meant to cover a winter garage; use the maker's temperature curve and size up.

Step 6: check that the panel can charge what you chose, and that it fits

A bigger battery is not automatically better. The panel's charger is current-limited and its manual lists the largest battery it is rated to recharge, often with a recharge-time requirement attached for fire use. A battery larger than that may never fully recharge after an outage. The enclosure also has a physical limit, and a battery that will not let the door close is not installed. Fit the size the worksheet calls for, within what the panel manual allows.

When the answer outgrows the panel

Run the same house after a remodel adds a wireless receiver, a third keypad and four more detectors. Standby rises to 0.32 A and alarm to 1.12 A.

• Standby: 0.32 A x 24 h = 7.68 Ah
• Alarm: 1.12 A x 0.083 h = 0.093 Ah, call it 0.09
• Total: 7.68 + 0.09 = 7.77 Ah
• With margin: 7.77 x 1.20 = 9.32 Ah

Now the next common size above 9.32 Ah is 12 Ah. Say this panel's manual lists 7 Ah as the largest battery its charger is rated for. The fix is not a 12 Ah battery the panel cannot recharge. It is one of:

• A listed auxiliary power supply with its own charger and battery, carrying the keypads and detectors, so the panel's own battery only carries the panel and communicator.
• Reducing standby load: keypad backlights dimmed, a device with a lower standby draw where the maker offers one.
• A panel whose charger is rated for the load, if the job is growing that much.

Split the worksheet by power source when you add a supply, and size each battery against only what it feeds.

How to verify it

Measure standby current on the finished install. With the account on test, unplug the transformer so the panel runs on battery, and read the current out of the battery with a DC clamp meter suited to low current, or a meter in series with one battery lead. Compare it to the worksheet's standby total. On this job the reading should be near 0.20 A. A reading of 0.27 A means something is drawing that the worksheet missed, often a keypad backlight left at full brightness, a module nobody listed, or a device someone added later. Find it and put it on the sheet; a worksheet that is off by a third is a battery that is short by a third. Then plug the transformer back in, confirm the AC trouble clears at the keypad and the station has the AC restoral, and only then take the account off test.

Keep the worksheet. Attach it to the account record with the battery date. The next tech who adds a device updates it, and the tech replacing the battery reads it to know what size goes in; see related: How to Replace an Alarm Panel Battery and Load Test It.

References

• NFPA 72, National Fire Alarm and Signaling Code, in the edition adopted by your authority having jurisdiction: secondary power duration for fire systems.
• Panel manufacturer installation instructions: charger rating, largest approved battery, device current table.
• Battery manufacturer data sheets: discharge rate tables and temperature derating.
• See related: How to Replace an Alarm Panel Battery and Load Test It
• See related: Combination Fire and Burglary Panels and the License Line