Installed new battery backup for home alarm

[Redtag]

Following help in previous postings I have finally installed my new 17Ahr Yuasa battery in parallel to the existing 7Ahr battery in my alarm box. (£38 including delivery from Maplin!)

All looks fine, but can I check on one thing - When connecting the new battery there are a increased hmmmm coming from the box. This is just the transforming starting to charge my new (and presumably not fully charged) battery, isn't it? Its nothing to worry about is it?

Because I have added the extra battery it is just going to take longer to charge, but there is no other consideration for the charger is there?

Just checking :-)

[Spider]

Hi Redtag,

I don't know the reasons why or what panel the 17Ah battery is connected to, but I would disconnect it now!!

If you are referring to a domestic intruder alarm panel, most only have the ability to charge a 7Ah battery, also, you have connected in parallel batteries of differing capacity which must not be done due to issues of equalisation of charge etc.

Regards

[Grunff]

Hehe - I like your thinking.

The 17Ah battery will have a much lower internal resistance than your 7Ah. It will draw a much higher current when charging (unless the charger is constant current, which would be strange for lead acids).

Having them both in parallel almost guarantees that your charger (the thing that's humming) will die soon.

Remove the 17Ah, and just have the 7Ah as was. If you really need more capacity, you need to build a suitable charging circuit.

[SantaUK]

I agree with the other two - 7Amp max m8

[Crooksie]

i must agree with the person who said that putting batteries of this type in parrallel is not a good thing for the stated reasons. however the internal charger in the alarm panel will be a constant voltage type with current limiting, designed to float charge the battery, this means that the current will reduce to a small float level (trickle) when the battery has fully charged. so i would say disconnect the small battery and the large one will be ok (just take longer to attain the full, float charge voltage so the charger will be putting out its max current for longer) regards bob

[Redtag]

Ok, thanks everybody. My new 17Ahr is now disconnected :-) Hope it was not too late!

So, how should I do this? I want to add more capacity for those long winter power cuts we get out here in the country and I thought dropping in a new battery would be just the job.

How should I add my new battery? I don't mind buying a new charger but how should I connect it in relation to the other battery and charger.

Thanks

[Peter Parry]

This assumes the charger circuitry lasts that long. A lot of alarm panels are of dubious quality and I suspect the charge circuit is designed to sustain its maximum output for only a relatively short time. Making it work at max output for several times the expected time may well cause things to overheat and expire.

[Crooksie]

well peter the internal charger will definatly have some form of current limiter circuit and 'should' be quite capable of sustaining the max current indefinatly. it matters not how big the capacity of the battery is, this max current is fixed. the only thing that will vary is the charging time. however if the unit is of dubious quality as you suggest who knows bob

[PJO]

OK, You can either do it properly or do it cheaply. The proper way would be to have a proper rated PSU/charger in a proper steel cabinet with the battery. Quite expensive.

The cheap way would be to have a large battery like your 17Ah or a car battery (that way you can go as big as you like!) and have it on a trickle charger - although the best way is to simply charge it up ready for use as it'll hold a charge for many months without trickle.

Now then PLEASE don't connect it to you alarm panel cos your alarm panel will go bang. Instead you can wire in a relay so that if your mains fails the relay will switch from alarm battery to car battery. The problem is that during switch over the panel may activate (alarm) due to PIR's etc. dropping out of circuit. The SAB too may operate causing a tamper fault. The real proper way would be to have a control circuit which would monitor the panel O/P voltage and bring in the car battery at a suitable level. All very messy.

If I was in your situation where you are expecting lots of power cuts I would probably do away with the panel battery and disable the panel charging circuit (usually a simple matter of cutting a link or resistor - speak to the manufacturer) and then just connect a fully charged large battery to the panel making sure a proper fuse arrangement exists in the battery feed. That way the panel won't need to charge anything but will have a damn good standby power source in the event of a mains failure. You do need to ensure that the cutting of any charging links does allow the battery to power the panel. Older panels had a marked link on the PCB for dry cell batteries but that was many moons ago.

Remember too that any short circuit of a large battery will melt the wiring so please fuse well.

[Redtag]

Pete, I am fine with doing it properly - it will still be cheaper than the £228+VAT that the alarm people wanted!

If I do source the properly rated charger (a 3A one should do it) then how to I connect this? I would still like to keep the existing 7Ahr battery, and add this in parallel but if I do this then how to I charge it? I want to do as little as possible in "their" cabinet and to add my own with this new battery.

Cheers

[roger]

Peter Parry writes

I suspect the extra hum from your kit may be due to some metalwork getting slightly re-positioned by accident, and is nothing to worry about. Or there could be a problem not associated with the new battery that does need worrying about! It is unlikely IME that fitting the 17AH battery as well as or instead of the 7AH will have any adverse effect. [as long as it is not faulty] If you are worried about it, just put a 10 ohm 5 watt resistor in the

+ve lead between the new battery and the rest of the system.

I really wouldn't go for separate chargers etc

[BigWallop]

Get hold of a Power Supply Unit / Charger that will comfortably take your 17 A/hr battery, and comes complete with a relay switch that operates on mains failure.

The relay should be wired with the open no voltage contacts connected to your alarm panel. The voltage side of the relay will be connected from charger to battery when mains power is present, and then drop from battery to appliance if a mains failure occurs.

These people should be able to point you in the right direction:

[PJO]

You really do need to read properly (!!). You CANNOT connect the large battery in parallel with your 7Ah - period.

If you do as some say and use a standby PSU with a relay (I mentioned this too) then the alarm will probably activate on change over as the PIR's etc will go into alarm, the SAB will operate and the panel may go into "power up" in which case it'll activate itself too!!!!

If you do away with your 7Ah and just use the large battery in "dry cell" mode then all will be well. Reading between the lines... is you alarm on a maintenance contract? because if it is then you obviously can't cut the charging link out although you could always cancel the contract and come here for advice when it fails!

Others have suggested a caravan battery - brilliant idea. A boat battery is another. These are designed to hold a charge for many months without use and a gel filled as opposed to liquid acid.

You can buy a simple "battery maintainer" from the likes of halfords for around £20 and that will keep the battery topped up OR, if you're into playing, buy a small windmill charger like boat owners use and use that to keep the battery up to scratch!

In any event you need to discard the 7Ah!

[Dave Plowman]

It's common with caravans. You use a diode between them to stop one discharging the other.

[The Natural Philosopher]

I really think everyne overreacted.

Lead acid chargers are just a voltage source in series with a resistance. Using one that delivers less merely means the float charge is slower, and teh batteries take longer to recharge out of an outage.

My guess is you have panicked needlessly.

[The Natural Philosopher]

May, but not will.

But I doubt it.

What a partially dicsharged battery looks like is a low voltage source in series with a bit of resistance - fairly negligible in comparison to the charge currents being used. What happens if you stick a bigger battery in the vcharge circuit is that it will run at the same charge current (cos its voltage is the same) for longer.

I have xcharged car batteries with trickle chargers from one amp up to

15 amps. The battery is no different. The little chargers will still charge the bigger batteries. Just takes a couple of days, is all.

Any charger is going to run full charge on all butteh smallest cells for at least an hour...and if it hasn't blown in an hour, it won't blow in three...its up to working temp in that hour.

[BigWallop]

And I wouldn't let you look after my alarm system. :-))

[The Natural Philosopher]

Don't listen to him. He is seriously talking through his arse.

If your charger didn't go bandg withing 5 minutes of hooking

up the bigger battery, I would lay you odds of 50:1 that it

will never go bang in a day or five, or however long it takes.

If you are concerned, charge the bigger battery up elsewhere, and then comnect it fully charged.

[The Natural Philosopher]

At last, some common sense!

Not especially, but not as dire as you think.

Overkill, but a neat idea.

[The Natural Philosopher]

You can actually.

Lots of things like cars and caravans regularly connect the batteries in parallel. They charge just fine. They even discharge just fine, but its obviously advantaegous in e,g. a camper to discnnect teh primary engine battery from teh camper to ensure you can start the engine the next day if you flatten the lesiure cells.

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