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Battery tools
Two calculators for standby batteries: size the battery your system needs, and see how the temperature of the battery location affects how long it will last. Open either or both.
Size the standby battery for your system to BS 5839-8, with Peukert correction and cable checks.
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.
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.
Detailed calculation
System details asked; battery behaviour from the built-in catalogue or a two-point Peukert fit.
System
Battery data source
Full manual
Every parameter exposed, including the message-cycle composite duty factor (2013 structure with 2023 factors).
Demand data source
System
Message structure (duty factor)
Battery behaviour
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.
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.
Choose your batteries and the ambient temperature of the battery location. Design life assumes 20 °C; sustained heat shortens it.
Feeds the replacement horizon. Future dates are not accepted.
Predicted service life against temperature
The drawn curve is a derived default, not per-model manufacturer data: service life halves for each 10 °C above 20 °C, the rule published in the Power-Sonic technical handbook and corroborated by Panasonic and EnerSys. Where a battery has two or more of its own published temperature-life points on file, those are drawn for it instead. As a standards cross-reference, BS 5839-8:2023 Annex C notes that at a continuous 35 °C a valve-regulated battery can expect only about 60 % of its specified life.
Sources define end of life differently: BS 5839-8 Annex C works from the battery's specified life, some manufacturers use 60 % of rated capacity, and 80 % conventions exist elsewhere. Whichever definition applies, a standby battery near predicted end of life may no longer hold the capacity the sizing calculation requires; replace before this point, not at it.
Replacement horizon at the chosen temperature
Bars show predicted years of service from the installation date at the chosen ambient temperature, from the conservative end of the design-life range. This view deliberately does not draw a capacity-decline curve: manufacturers do not publish the decline shape, so this tool will not invent one.
Your inputs stay in your browser. Save my inputs downloads a small file you can load again on a later visit; nothing is sent to us. This tool provides a calculation record, not a certificate.
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.