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Does your alarm or a neighbour's alarm start ringing when there's a power cut?

If so, here's the reason why.

All alarm systems have a main control box (not necessarily the keypad used to arm/disarm the system) but a larger (often white) metal/plastic box. These are usually fixed to the wall in an under stairs cupboard, utility cupboard or inside a wardrobe.

Inside this box is a large lead acid battery, used as a backup power supply.

98% of the time an alarm rings during a power cut is because this battery is dead! Therefore when the mains is removed, the main control panel battery can't keep the system running so the external sounder starts ringing to inform you there's a problem.


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How does the external siren ring if there's no power to the alarm?

A good and common question!

Inside the external sounder is another smaller battery. This battery powers the sounder only in the event of a mains fail, or if the cable to the sounder was cut.

How to stop alarm

If your external alarm siren is not ringing during a power cut but the keypad (or main alarm box) inside your proprty is beeping or sounding a solid tone then you can silence the tone/beep by simply entering your code. This notifies the alarm that you have aknowledged the power cut and, at this point, the alarm is being powered by the back-up battery.

If your external alarm siren is ringing during a power cut, then it means your internal alarm battery (inside the main alarm box) needs replacing. More on how to do this below:

The external ringing alarm should only continue for 20 minutes, then cut out.

If the offending alarm is a neighbours alarm, why not click the Facebook "LIKE" button below. Your neighbours may then see this article in their Facebook news feed and actually do something about it :)

Replacing the main battery

CAUTION

This task should only be carried out by a competent DIY person or electrician. If you have a maintenance contract with the alarm company you should let them do this.


Before unscrewing the screws on the control box, please be aware this box contains a live 240v feed, so make sure you isolate the supply first. This could be as simple as switching off the plug top which feeds the main control box, or flicking out a fuse if fitted with an un-switched fused spur.

Once isolated and safe, you can unscrew the lid of the control box.


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Be aware, as soon as you remove the lid a tamper switch is released and activates the internal sounder. You should be able to stop this sound by cancelling the tamper alarm by entering your code into the keypad.

The control panel will be full of many coloured wires or none at all if your system is a wireless alarm. You can ignore these wires as you're only interested in the battery.

The main battery has two fly leads connected to the terminals on the top of the battery, which terminates onto the main PCB.

The lead acid security alarm battery will probably look like one of these below:

Make sure you buy the exact replacement battery, check it's voltage (12v) and match the ampere hour (Ah).

Disconnect the two fly leads from battery terminals. The fly leads (often red and black) are connected to the battery terminals with spade connectors, so hold the spade connector firmly as you pull/wiggle each lead off.

Reconnect your new battery to the fly leads.

Screw the lid back on your main control box. If the tamper switch is accidentally pressed again, don't forget you can silence the alarm by entering your code.

Once the lid is securely on, switch back on your mains supply.

At the keypad press the reset button (to reset those tamper activations).

Your alarm is now healthy!

The next time the mains power goes off, your alarm siren should not make a sound.

Alarm batteries should last ~5 years before needing to be replaced.

Is your alarm over 15 years old?

Qet a free no-obligation quote for an upgrade.

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