Standby batteries are used to maintain system operation in the event of a mains failure. The size of batteries required will depend on several factors:-

  • How much current the system takes in the normal condition
  • How much current the system takes in the alarm condition
  • How long the system is required to run in the event of a mains failure.

The general requirement for battery backup is that the system is to be able to run for 24 hours, then run in alarm for 30 minutes with an 80% charge in the batteries. (This is to allow for batteries not being fully charged, battery degradation etc.)

If the building is likely to be unmanned for extended periods, one of the following should be considered:-

  • Fit standby batteries that will allow the system to run for 48 or 72 hours instead of the usual 24 hours
  • Transmit the fault to an alarm receiving centre (via a Redcare box, or similar autodialler)

In general terms, addressable systems tend to have a higher quiescent current than conventional systems, and will usually require a larger standby battery capacity.

The batteries should be replaced as part of the maintenance routine approximately every 5 years.

In order to calculate the standby battery size required, the following formula can be used:-

Battery Size (Standby time in Amp Hours) = 1.25 x [(TALM x IALM) + (TSBY x (IQP + IQD))]


  • TALM= Maximum time in hours required for the alarm [½ hour is most common time]
  • IALM = Total Alarm Current in amps for all alarm devices connected to the alarm circuits
  • TSBY = Standby time in hours for the system after mains failure [normally 24, 48 or 72 hr]
  • IQP = Quiescent current in amps of control panel in fault condition [because of mains failure]
  • IQD = Quiescent current in amps of all devices on the system [detectors, sounders etc]

Typical Battery Usage

Battery Size

Conventional System

Addressable System


Small System

Not suitable


Medium System/Small system - extended standby

Small System


Large System/Medium system – extended standby

Medium System/Small system – extended standby


Large system – extended standby

Large System/Medium system – extended standby

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