Free tool

PAVA Standby Battery Calculator

Standby battery capacity for voice alarm systems to BS 5839-8:2023 Annex C, with derating factors derived from Peukert's law. Three levels: a quick estimate from two questions, a detailed calculation from system data, or full manual control of every parameter.

Which level to use

Level 1 gives a first estimate from two questions and lists every assumption it makes. Level 2 asks for real system data and suits most design work. Level 3 exposes every parameter, accepts site measurements from existing systems, and handles unusual message cycles. All three give the same style of result, so start simple and refine: a Level 1 answer is a starting point, not a design, and more detail usually lowers the battery requirement rather than raising it.

Level 1 · Quick
Level 2 · Detailed
Level 3 · Full manual

Quick estimate

Two questions; everything else is assumed and listed with the result so you can refine it in Level 2 or 3.

The DC voltage of the backup battery supply. Common values: 24 V (many conventional VA panels); 12 V (a single 12 V battery, e.g. Bosch PRAESENSA via its multifunction power supply); 48 V (four 12 V batteries in series, e.g. Bosch PRAESIDEO amplifiers). A 48 V bank is four matched 12 V batteries in series (same make, model, capacity and age), replaced as a set; this tool sizes one battery in that string. Check the equipment manufacturer's figure for your system.

Result
Verifies the Peukert engine and reproduces the 2008, 2013 and 2023 Annex C worked examples.
How this calculator works
BS 5839-8 needs a battery derating factor it admits you often cannot get. This works it out from the datasheet, and always shows you where the number came from.

What it calculates

The minimum battery capacity for a voice alarm system to BS 5839-8:2023, Annex C: enough to run the system quiescent for the standby period (normally 24 hours), then broadcast at full alarm load for the alarm period (minimum 30 minutes), with the standard's 1.25 allowance for ageing and temperature. The result is the capacity at the 20 hour rate (C20) that the battery bank must equal or exceed.

How the battery requirement is calculated Two parallel streams, system demand and battery behaviour, converge on the BS 5839-8:2023 Annex C formula, followed by battery selection, guard rails, and the calculation record. SYSTEM DEMAND BATTERY BEHAVIOUR 1 · Choose demand data source Measured › kit list › estimated 2 · Gather system inputs L, η, V, I1, IF, monitoring duty 3 · Amplifier input current IA = 100 L / (η V) 4 · Duty factor f Speech 1/8 · tone 1/4 · cycle mix 5 · Alarm load current I2 = I1(1+o) + IF + f·IA measured I2 skips steps 2–4 1 · Choose battery data source CC table › CP table › Peukert fit 2 · Rate-dependent capacity Peukert: t = H (C / I H)ᵏ 3 · Derating factor D = C20 / (I · t) 4 · Evaluate both durations D1 at 24 h (=1) · D2 at 30 min BS 5839-8:2023 Annex C Cₖᵢₙ = 1.25 × ( D1·T1·I1 + D2·T2·I2 ) Select battery Smallest catalogue C20 ≥ Cₖᵢₙ Guard rails Never below VACIE certified minimum · charger limits Calculation record Inputs · every term · provenance tags · margin Both data-source steps carry a provenance tag through to the record: the result always says where its numbers came from.
How the battery requirement is calculated Single-column version for small screens: the system demand steps, then the battery behaviour steps, both feeding the BS 5839-8:2023 Annex C formula, followed by battery selection, guard rails, and the calculation record. SYSTEM DEMAND 1 · Choose demand data source Measured › kit list › estimated 2 · Gather system inputs L, η, V, I1, IF, monitoring duty 3 · Amplifier input current IA = 100 L / (η V) 4 · Duty factor f Speech 1/8 · tone 1/4 · cycle mix 5 · Alarm load current I2 = I1(1+o) + IF + f·IA measured I2 skips steps 2–4 BATTERY BEHAVIOUR 1 · Choose battery data source CC table › CP table › Peukert fit 2 · Rate-dependent capacity Peukert: t = H (C / I H)ᵏ 3 · Derating factor D = C20 / (I · t) 4 · Evaluate both durations D1 at 24 h (=1) · D2 at 30 min BS 5839-8:2023 Annex C Cₖᵢₙ = 1.25 × (D1·T1·I1 + D2·T2·I2) Select battery Smallest catalogue C20 ≥ Cₖᵢₙ Guard rails Never below VACIE minimum · charger limits Calculation record Inputs · every term · provenance · margin Every result says where its numbers came from: both data sources carry a provenance tag to the record.
How the calculator arrives at the battery requirement

The number the standard won't give you

Annex C requires derating factors taken "from the battery manufacturer's data", then admits in the same breath that this data is often not available. That is the number this calculator supplies. It derives the factors from whatever the datasheet does publish: the manufacturer's constant-current tables where they exist, Peukert's law where they do not, and it always tells you which source it used. The answer is only as good as its source, so the calculator ranks them for you, best first: manufacturer table, then a Peukert fit from rating points near the alarm duration, then a fit from long-duration points (floored for safety), then a conservative default.

The Peukert rated-capacity method used by this calculator follows the work of Chris Gibson of SmartGauge Electronics, whose writing on Peukert's law also shaped our own understanding of the subject. His original articles are well worth a read at smartgauge.co.uk.

Which level to use, and when to move up

Start at Level 1. Two questions, a first estimate, and every assumption it made listed beside the answer, so you can see exactly what it guessed on your behalf.

Move up to Level 2 once you have the real system data: loudspeaker load, quiescent current, battery voltage. This is the level most design work needs.

Use Level 3 when you have the full picture: site measurements from an existing system, an unusual message cycle, or a specific battery whose datasheet you want the calculation built on.

You do not have to choose the right level first time. Start simple and climb as the information arrives; each level carries the same result forward and takes it further.

One thing worth knowing before you specify a battery. The lighter levels protect you by assuming the worst: Level 1's guesses are deliberately conservative, so its answer usually sits above the real requirement. Feed in measured data at Level 2 or 3 and the number often comes down, sometimes to a smaller, less expensive battery that still meets the standard in full. Refining is not only about a more accurate figure; it can stop you fitting more battery than the system actually needs.

What does 11/11 mean?

The calculator checks its own maths whenever you run the self-tests: 11 scenarios with known correct answers, including the standard's worked examples, are recomputed and compared. 11/11 means every check passed. If it ever shows less, do not rely on the results, and please let us know.

Before you rely on the result

The output is a calculation record, not a certificate. Verify the inputs against the installation, check the battery manufacturer's published data, and never size below the voice alarm equipment manufacturer's certified minimum battery or outside the charger's limits. Battery standby capacity is a life-safety calculation and final responsibility rests with a competent person. Questions, or want us to run the calculation with you? contactus@proaudium.com.

Before you rely on a result

This calculator produces a calculation record, not a certificate. Battery standby capacity for a voice alarm system is a life-safety calculation. Results must be verified by a competent person against the battery manufacturer's published data and the voice alarm equipment manufacturer's certified minimum battery requirement before use in any design, quotation or installation. Method: BS 5839-8:2023 Annex C with derating factors derived from Peukert's law.