Alarm going off after power cut

[F]

We've had a couple of power cuts recently (up to two hours) while we were out and the alarm was set. Neighbours have said that the alarm has gone off each time the power has come back on. While the power was off none of the panel lights were lit and when the power came back on and we reset it 'Day' and both zones were lit.

Is this expected behaviour?

I thought the battery in the control panel and the one in the bell box were supposed to deal with power cuts. Could it be that one or both of the batteries needs to be changed?

The alarm is a Scantronic 9448.

[Owain]

Suggests that the battery in the panel is dead and the one in the bell isn't.

Owain

[F]

Should have added that, according to the neighbours, the sounder sounds fairly 'wheezy' and underpowered.

[Bob Minchin]

Replace both

Bob

[Phil L]

I bought new batteries about 12 months ago, they were about £12 each IIRC, yours won't be much different in price. Word to the wise, don't order them off the internet - the P&P doubles the overall cost - any decent electrical outlet will have them in stock

[Alan]

In message , F wrote

When this happened to my alarm it was the battery at fault, even though it was still giving a 12v reading. The batteries don't last forever and after a few years should be changed.

I suspect that old batteries don't hold a full charge and while they can maintain the alarm for a short period whilst the mains is off they drag down one of the power supply lines when the mains comes back on. This glitch on the internal supply line triggers the alarm. You can test this yourself by switching off your mains power at the 'fuse' box for a couple of minutes.

The battery in the bell is used to sound the alarm when someone cuts the wires to the main control unit.

[Al]

Sounds like the panel one needs replacing, and possibly the bellbox too.

They are *supposed* to be changed regularly, whne the system is 'serviced'. Frankly, nobody bothers ;-)

Al.

[Andrew Gabriel]

Battery life depends heavily on the quality of the charging circuit, and the number of times you've had it run too flat. (A larger capacity battery is less likely to run flat during a typical power cut.)

Commercial alarms often include a daily battery test cycle, and can include circuitry to protect against running the battery too flat. I don't know if these features are found in domestic alarms, but I suspect certainly not cheaper ones. It could also be that commercial alarms may have better quality charging circuits.

[Stuart Noble]

Are they easy to change without triggering the alarm?

[Andrew Gabriel]

IME, they are vastly more expensive in local electrical outlets than on the Internet. Just make sure you order enough to get free postage.

I suspect that depends heavily on the alarm. On mine, I have to put it into Engineer Test Mode so I can open the box without it going off, and then doesn't care about you disconnecting the battery, as long as the mains stays on. (If left disconnected, it generates a dead battery alert, but continues working fine otherwise.)

Might want to be careful and check instructions though. I could imagine a cheap charging circuit might rely on the battery for voltage stablisation, and the internal electronics supply voltage might go out of range if the battery is disconnected.

[Alan]

In message , Stuart Noble wrote

The battery is easy to change - it will have two pull off spade terminals, perhaps with the aid of a pair of pliers.

Access to the alarm panel will trigger the alarm. It will have an anti tamper switch.

If you turn off the mains and then remove the battery the alarm may/will forget all of it's settings so have all the instructions to hand. Otherwise, leave the mains supply on, remove the case of the panel (triggering the alarm), quickly change the battery, refit the cover with one screw and reset/disable the alarm in the normal way.

[Frank Erskine]

On 19 Jul 2009 10:56:46 GMT, [e-mail] (Andrew Gabriel) had this to say:

I got a replacement battery locally (an auto electrical factor) cheaper than anywhere I'd seen on the 'net. It was a decent quality one too - not some unnamed brand. He didn't have the exact size one I wanted, but it took only a day or two to get the right one. Sadly the proprietor died a while ago and the business is closed down.

[Stuart Noble]

Thanks. We don't actually set the alarm any more (inherited a cat and can't be bothered to change the sensors) but I gather it may still go off in the event of a power cut if the battery is naff

[Mike Harrison]

Probably not what you're seeing, but I once had a faulty PIR which false-triggered when its supply voltage dropped slightly when the panel switched over to battery power. Took 2 Police callouts before tracing this one....!

[Dave Plowman (News)]

At the cheaper end aren't most backup batteries an option?