Burglar alarm Ni-MH battery

[Tim Lamb]

Rather than hi-jack the existing thread....

Some weeks ago our burglar alarm triggered, possibly a spider. Unfortunately, we were away so the strobe continued and possibly ran down the local battery.

I am now in dispute with the alarm company as their call out man replaced the entire sounder board rather than simply replacing the battery (7.2 V 300mAh Ni-MH) 3xAAA.

Battery renewal is an annual maintenance job according to the agreement so I don't see why I should be stuck for the £112 cost of the Strobe and Sounder card.

I need to test the SaS card to prove my point. (with the sounder unplugged:-)

I had planned to cobble together a charger from my various redundant phone charger wall warts but got nervous after reading the battery charger shelf thread.

Any suggestions? I suppose I could knock up a temporary supply from 6 Ni-Cads.

[Bill Wright]

Use five partly used non-rechargable AA cells. They will produce about

7.2V. You could take two tired ones out of a torch and add three new ones.

Bill

[Peter Andrews]

Why should the strobe run down the local battery? Surely it's powered by the mains supply via the control panel or had that failed as well.

Peter

[Peter Andrews]

Another thought, had the strobe failed due to being left flashing for a 'long' period. I don't know what the expected life of a strobe is!

Peter

[Martin Brown]

Doesn't the local battery only play a part if the mains power is cut?

3xAAA would be more like 3.6-4.2v @ 800mAh depending on state of charge. Sounds a bit puny for an active alarm backup to me. Mine has great hunk of about 7Ah lead acid cell on float charge replaced when it fails a stress test or after four years whichever happens sooner.

If it is a 300mAh cell then you can't do it much harm if you charge it for twelve hours at the nominally C/10 rate 30mA. Choose a PSU with enough voltage and a suitable series resistor to keep the charging current under 30mA and leave it to charge for half a day.

NiCads have low internal resistance so can source a lot of current. You must current limit when charging batteries or they will cook.

The steam that comes out if it vents can be boiling hot caustic - not nice!

[Dave Liquorice]

That's what I would expect but, IIRC, there are two main types of sounder. "SCB" Self Contained Bell and "SAB" Self Activated Bell. SCB sort of implies that it runs from it's battery no matter the state of the supply, where as SAB only uses it's battery if needs to make noise without supply.

TBH I've never quite got my head around how sounders work, it's not as simple as "make a noise if the alarm wire is in x state". Local tamper comes into it, which is also linked to the panels tamper circuit and there is also a "hold off" and supply detection.

Not a lot of help, but perhaps enough for Mr Lamb to feed google and get sensible information.

[dennis]

Mine has 12v going to it, that charges the battery. The alarm will self activate if the 12V is removed. The tamper circuit will trigger the alarm if broken and the control box removes the 12V to sound the bell. The strobe is a separate 12V supply that just goes on when the panel activates it. It doesn't have a battery as the power consumption is quite high.

[Tim Lamb]

Umm... Perhaps the charge rate is less than the load? Perhaps the battery is OK and the strobe has failed. This is what I hope to discover. I am not an alarm engineer!

[Dave Plowman (News)]

My alarm has a battery inside the panel in case the mains fails, and a second one inside the sounder/flash housing - so it will sound if the cable etc going to it is cut or interfered with.

[Tim Lamb]

Oh! I might be looking up the wrong tree:-)

I can put 12V DC on the card input and see if that charges the battery. As found, the strobe was not running so something had failed.

[Martin Brown]

Likewise although the latter is only inspected about every five years or if the bell/strobe fails to work when on test without mains. (this has never happened so it gets swapped by default timeout)

It might be a ~2Ah lead acid in the bell housing but I think it is larger than that. The main control box has a 7Ah cell in it.

[The Natural Philosopher]

If you flatten NiMh, they seldom recover, unlike NiCd.

[cl]

All of which is pretty irrelevant nowadays, who takes any notice of alarms, except to moan about the noise?