Mains power instead of battery?

[R D S]

We have a wireless alarm at work, i'm sick of replacing the batteries in the sensors*. I presume to alleviate this bother i'd just need to attach a suitable wall wart to the battery terminals?

Also it would be better in cases where the mains have failed to have battery backup, so if I was to use something like this and have a rechargable battery in there too, would that be ok?

*And more concerned about finding dead sensors and wondering how long they've been that way.

[alan_m]

You may need a way to limit the charging current to the batteries once fully charged or you may find that the battery is still the source of failure. Maybe a much reduced trickle charge current?

Make sure that when the mains fails that your wall wart doesn't have any circuits that may drain the battery almost instantly. A diode or two in circuit should prevent this.

Make sure that your wall wart is fully regulated and provides a nice smooth dc voltage.

[Theo]

It really needs something designed for the purpose because you need it to do several special things. One is to maintain battery health for a battery which is almost never discharged, compared with a normal battery charger where you remove it when it's done. The recent discussion here on cordless phone batteries relates the problem.

Another is providing standby current to power the sensor which could confuse a battery charger that sees the battery taking current and thinking it's still charging. That standby current depends on the specific load being used, and it could even by pulsed or some other unfriendly pattern.

One setup that could work is two sets of batteries. One set powers the sensor, the other set is being charged in a trickle charger designed for standby use. When the sensor batteries get low, a relay swaps the sets over. You'll kill the batteries eventually, since they're going through regular cycling, but it should be more reliable and less likely to confuse the equipment.

Theo

[Chris Green]

Wouldn't it be cheaper and certainly more reliable and safer simply to get a mains powered (possibly with battery backup) alarm?

If it's a smoke/fire/CO alarm I think tampering with it in any way would likely leave you open to all sorts of post disaster problems.

[Robin]

If this is part of an alarm system you are /required/ to have for fire safety in the workplace then I think what would in effect be a DIY alarm risks difficulties. Is there no compatible mains powered/battery backup alarm - not even second-hand?

[R D S]

We've a 'proper' smoke/fire alarm setup that I will be leaving well alone.

This is a cheap GSM intruder alarm that sends a text if alarmed. The PIRs are the 9v type and quite expensive and they don't seem to last very long. It will take wired sensors, but it's going to be a challenge to do a tidy job.

[Robin]

Thanks; understood.

Occurs to me an ideal ideal power supply might be a time-expired mains power/battery backup smoke alarm. But I've never exploded one to see how easy it is to find and patch onto the supply.

[Bob Eager]

I have several GSM alarms that are similar. They run off 3 x AA cells (that last about 3 months) but also have a power socket on the side if you want to use a wall wart. I think they were about £70, and might be a simpler (but possibly more expensive) solution.

(they also call a set of specified numbers if required; in our case the phones ring with an 'SOS' cadence!)

[Harry Bloomfield, Esq.]

R D S formulated the question :

Point is, how long do they work for on battery?

Might a regime of regular battery replacement al round, solve the problem?

[Owain Lastname]

Better quality batteries, bought in bulk from eg CPC, rather than popping out to the poundshop each time?

Owain

[Max Demian]

Maybe use rechargeable batteries and recharge them externally. What size do they use, "PP3" (or whatever they call them nowadays)?

[Clive Arthur]

I'd consider a 9V USB rechargeable Li-ion battery. Keep a USB charger plugged in. I don't know if they still supply 9V when on charge though.

Or a 9V supercapacitor with a 9V mains supply (and a blocking diode), assuming the mains outage isn't too long.

[Fredxx]

In various tests, alkaline batteries bought from Poundland have performed rather well. There might be lithium variants with extra capacity, at a price.