How long does an AOV battery backup have to last?

BS EN 12101-10 sets a standby period of 72 hours as the default for smoke and heat control power supplies, with the battery required to carry the maximum standby current for that whole period and then still drive the system's operational load. The 72 hours can be reduced to 30 hours where faults are supervised and reported so they get attended to quickly, and to 4 hours where spares, repair personnel and a standby generator are permanently available on site. Any reduction is a design decision that has to be justified and recorded, not a default. Check the published standard and the building's fire strategy before sizing to anything shorter than 72 hours.

How do you size the batteries for a smoke control panel?

Add the total quiescent current of the panel and everything it powers, multiply it by the standby period in hours, then add the operational current multiplied by the operating period expressed in hours. Apply a derating factor for ageing and temperature, typically around 1.25, and select the next standard capacity above the result. Standby current dominates the answer almost every time, because 72 hours is a long multiplier and vent operation lasts minutes. Size the panel's output rating separately, against actuator running current and inrush.

How often should AOV standby batteries be replaced?

Sealed lead-acid standby batteries in smoke control panels typically give three to five years of service, and many maintenance regimes set a fixed replacement interval within that window rather than waiting for a fault. Service life is heavily temperature dependent, and a battery sitting in a warm riser or a roof plant room will fall short of one in a cool cupboard. A battery can also fail a capacity test well before its interval expires, which takes priority over the calendar. Replace both batteries in a 24V pair together, from the same batch, and never mix an old one with a new one.

How do you test an AOV panel's standby batteries?

Record the resting terminal voltage and the charger output first, then isolate the mains and measure the battery voltage under the system's actual load, including a full vent operation. A healthy 24V set holds up well under load, while a failing one sags sharply the moment the actuators draw current and recovers as soon as they stop. The panel's own battery test function is a useful indicator, not a capacity test, because it applies a load for seconds rather than hours. Where the standby period genuinely matters, use a proper discharge or impedance test and record the readings.

AOV Battery Backup: Sizing, Testing, Replacement

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Publishing Sunday 18 October 2026

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How AOV panel standby batteries are sized under BS EN 12101-10, why undersized sets still pass the weekly test, and how to test and replace them properly.

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