Fire Alarm Standby Battery Calculations: A Worked Example

A panel on mains power tells you nothing about how it will behave when the mains fails. What matters then is the standby battery, and the only way to show it’s big enough is to do the sum. Plenty of engineers skip it, fit whatever batteries were in the van and hope for the best. This guide goes through the standard calculation with real numbers so you can do it quickly on site and explain it in an exam.

If you want more background on the inputs first, read our companion piece on standby battery sizing inputs.

The Two Loads You Need to Measure

Every battery calculation starts with two currents:

• Quiescent current (I1). This is what the system draws at rest: panel healthy, detectors monitoring, no fire. It’s usually small and often measured in milliamps.
• Alarm current (I2). This is what the system draws in full alarm, with every sounder, visual alarm device (VAD) and other alarm load running at once. It’s always higher than the quiescent load, sometimes much higher.

On a new installation you work these out from manufacturer data sheets by adding up each device’s standby and alarm current. On an existing site you can measure them: record the quiescent draw, put the system into full alarm and record it again.

Watch your units. Most mistakes happen when you convert milliamps to amps, not in the formula. 35 mA is 0.035 A, not 0.35 A. That slip makes the quiescent term ten times too big.

The Formula

The standard minimum capacity formula used with BS 5839-1 is:

C = 1.25 × (T × I1 + D × I2 × 0.5)

Where:

Symbol	Meaning
C	Minimum battery capacity (Ah) at the 20-hour discharge rate
1.25	Factor allowing for the loss of battery capacity as the battery ages
T	Standby period in hours
I1	Quiescent (standby) current in amps
D	De-rating factor, normally 1.75, for the reduced effective capacity when a battery discharges at high current
I2	Alarm current in amps
0.5	The 30-minute alarm period, expressed in hours

You’ll see different explanations of the two factors in the field. Some people call the 1.25 a temperature allowance and the 1.75 an ageing allowance. For exam purposes, stick to the wording in the standard. The overall 1.25 multiplier allows for the loss of capacity over the battery’s life. The 1.75 de-rating factor applies only to the alarm term, because a sealed lead-acid battery delivers less of its rated capacity when discharged quickly. In both cases the effect is to make the battery bigger. Cable de-rating works the other way and reduces the current a cable may carry.

What the Formula Is Actually Modelling

The formula describes a specific scenario:

1. Mains power fails.
2. The batteries hold the system in a healthy quiescent state for the whole standby period (T).
3. At the very end of that period, a fire is detected.
4. The system must then run in full alarm for 30 minutes, with every sounder and VAD operating.

That final half hour is the worst case, and the alarm term exists to cover it. A battery that only just survives the standby period and then fails when the sounders start is useless.

Worked Example

Take a small conventional system with these measured figures:

• Quiescent current, I1 = 35 mA = 0.035 A
• Alarm current, I2 = 85 mA = 0.085 A
• Standby period, T = 24 hours

Step 1: Standby term

24 × 0.035 = 0.84 Ah

Step 2: Alarm term

1.75 × 0.085 × 0.5 = 0.074 Ah (0.0744 before rounding)

Step 3: Add them together

0.84 + 0.074 = 0.914 Ah

Step 4: Apply the ageing factor

1.25 × 0.914 = 1.14 Ah

The minimum capacity is about 1.14 Ah. Batteries aren’t sold in that size, so you go up to the next standard capacity, which for this system is a battery in the 2 Ah class. Don’t round down.

Notice how much the standby term dominates on a small system. On a long standby period, even a modest quiescent draw usually outweighs the alarm load. Cutting quiescent current, for example by choosing low-current devices, often shrinks the battery more than anything you do on the alarm side.

Choosing the Standby Period

T is not always 24 hours. The standby period depends on how quickly someone will notice a mains failure and act on it. For example, it matters whether fault signals go to a staffed location or an alarm receiving centre, and whether the building is left empty for long periods such as weekends. Longer periods are needed when a failure could go unnoticed for longer.

Always take the required standby period from the current published edition of BS 5839-1 and the project specification. Don’t assume the figure from the last job applies. For more on how the standard is organised, see What Is BS 5839-1?.

Don’t Just Fit the Biggest Battery

It’s tempting to skip the maths. A panel may have space for a 3.2 Ah battery, and the difference in price is a few pounds. On a small system with a light load, oversizing like that is often fine, as long as you’ve done the calculation to prove the minimum.

There’s a limit, though. Every panel’s charger has a maximum battery capacity set by the manufacturer. The charger has to recharge the batteries within a set time after mains is restored. Put in a battery bigger than the charger is designed for and it may never reach full charge. Then you have a large battery that can’t deliver its rated capacity.

In practice:

• Check the panel manufacturer’s maximum supported battery capacity.
• Make sure your calculated minimum falls inside that limit. If it doesn’t, you may need a larger panel or an additional power supply unit meeting BS EN 54-4.
• Record the calculation. It shows your design decisions were based on evidence, which supports your duties under health and safety legislation, including the Electricity at Work Regulations and CDM where they apply.

Taking Over an Existing Site

When you inherit a system, don’t assume the batteries are right just because they fit. Measure the quiescent and alarm currents, do the calculation and compare the result with what’s installed. Look at the battery date codes as well. Sealed lead-acid batteries in fire systems are commonly replaced every four to five years, and an old battery won’t deliver its label capacity.

If the batteries are undersized or past their life, that’s a legitimate finding to report to the responsible person. Include your measurements and calculation so the recommendation holds up. Put it all in your records. Our commissioning records workflow covers keeping that paperwork tidy.

Exam Tips

Battery calculations come up regularly in design units. To avoid losing easy marks:

• Convert all currents to amps before you start.
• Apply the 1.75 factor to the alarm term only, not the standby term.
• Multiply by 0.5 for the 30-minute alarm period. Don’t use 30.
• Apply the 1.25 factor to the total, after adding the two terms.
• Round up to the next available battery size, and say so in your answer.
• State where T came from. An examiner wants to see that you know why a particular standby period applies.

Online calculators are useful for speed, but learn to do the calculation by hand before you rely on them. Then you’ll spot a wrong input, such as a milliamp figure typed in as amps, before it ends up on a certificate.

How Fire Regs Pro Fits Into This

Fire Regs Pro is built for exactly this kind of exam and design task. The app includes 1462 questions across 71 topics and 7 exam paths, including power supply and battery calculation questions. You can practise the formula until the steps come naturally and you stop making unit mistakes.

The six built-in calculators include a standby battery tool. Use it to check your hand calculations, not to replace them. The nine quick-reference areas give you BS 5839 key points on site. The field records features (certificates, service log and CPD tracking) help you document battery checks and calculations properly. If you’re not sure which qualification to aim for, compare the exam paths or start with the free practice quiz.

As always, check your design against the current published standard, the project specification and the manufacturer’s data. The calculation shows the minimum battery size, but the finished design must also meet the standard, the specification and the panel’s limits.