Installed new battery backup for home alarm

[Dave Plowman]

There are diodes available that do this automagically...

[BigWallop]

The alarm panel will only have a transformer that supplies just enough to keep things going at their designed working ratings, so making it work harder can, and will, make it overheat inside the casing. This rise in temperature inside the casing can, and will, have bad effects on other components.

Most panel instructions will give a maximum battery capacity allowed for that design of power supply, so to increase this capacity can, and will, have effects on the design of the system and will void any warranty.

The charger system may well be a voltage / current source with a bit of resistance, but you have to take into consideration the environment that the source and resistance are working in. The system expels heat into its surrounding space. The space within, and the connected heat dissipation appliances, should be enough to safely exhaust that heat away from the appliance. Increase the heat, or close in the space, and you have to consider what effect these change will have within the working parameters that have already been designed into the system.

The increase in the "HUM" sound heard, probably means the power supply is working harder to keep the increase in battery capacity up to the system ratings. So this means an increase in heat produced by the power system. This increase is probably not designed for by the makers, so it can, and will, effect the designed for working state of the system.

The power supply is rated to take what the makers allow for in the design of the panel, so making changes to these parameters will affect what is allowed for in the design. If the power supply has to work harder at charging the battery, then it is probably not supplying other parts of the system with the correct power source they need to work properly.

[usenet]

My Gunson charger says it's fine to put two batteries in parallel on a float charger (assumes that neither has any disastrous sort of fault of course).

[PJO]

More s**te from the philosopher above!

We're talking a 1A psu here... not a car or a caravan.

I have seen overloaded PSU's burn out after months but burn out they do - fact.

[PJO]

Fine BUT we're talking low current capability alarm here - nit car.

[Sparks]

But is that refering to two batteries of the same capacity?

I would strongly advise th OP not to connect the two batteries together here...

Sparks...

[The Natural Philosopher]

There are relays that do it 10000% better automgically. Diodes DON'T work BTW. You need full volts on charge, not full volts -0.6v or whatever.

[The Natural Philosopher]

Yeah. Mire s**te from aan electronics designeer of some 320 years experience, and who has spent more time charging batteries in teh last two years than he is prepared to mention.

Piss off.

So? The principles are the same. They aen't to YOU, of course, beacuse your understanding is totally limited.

I have seen things burn out in micro seconds, millisecond, seconds, hours, minutes days, months and years. All for different reasons. Most of which I ecventually identified, since it was part of my job to do so. Your statement says nothing.

[The Natural Philosopher]

So what?> The principles - that its fine to slap two lead acid batteries in parallel to both charge and discharge, holds.

Just examine the voltage current characterists of lead acid accumulators, under both carge and discharge, do the maths, and work it out for yourself. If you are capable.

[The Natural Philosopher]

No. Whast happens in practice is that the voltage obviously settles around a common point. However you have to over voltage to charge, and under discharge, the volts drop, so there is no huge explosion when you e.g. stick jump leads on a car to start it, even if you use a tractor battery to do it (provided its not a 24 or 48v battery of course).

Under charge, the voltage rises...the more discharged battery will hog current till both batteries are at an eqqual state of charge, concomitant with the applied voltage. At that point they appear as a simple larger single battery. And stay that way until seperated.

The only remaining issue is one of peak current. But any abttery has a pretty low impedance, so teh charger will be designed to supply a certain current to a certain voltage that reflects teh discharge state of teh battery. It doesn't KNOW how big the battery is. It just KNOWS that a 60% flat battery will be at - say - 12.5V under charge, and it will deliver its rated current to that voltage whatever.

The cfact that the OP's charge hummed a bit merely shows the battery was fairly discharged. Unless you are into super fast charging (one hour or less) currents will never be high enough to risk battery damage, and the current into a larger battery will be no higher.

The greater danger is over currenting a small battery.

[Dave Plowman]

It has some form of limiting built in to restrict the charge current to what it can cope with. A simple lead acid battery charger is a DC source of around 16 volts open circuit with a suitable series resistor. Of course modern ones will have some electronics.

Didn't say it was - I said it had a very low internal resistance - and any practical lead acid battery would take more than a burglar alarm power supply could provide without some form of limiting.

[Dave Plowman]

Wonder why they're sold expressly for this purpose, then?

[Sparks]

OK, say this happens...

The small battery is flat

The large one isn't

The large battery will charge the small one until they are at equal states of charge

Trouble is, there is no current limiting in between the two batteries, so the small one will be charged at "cor blimey" amps, which is not good for the battery, or the wires interconnecting them.

Caravans and boats have a special split charging system, so this doesn't happen.

Look in any multi battery UPS, and you will see the batteries are all connected in series, not parallel because of this.

As people have said before, just connecting the big battery will be OK if the charging circuit is of a suitable quality, prolong periods of higher current charging *WILL* cause the voltage regulator to heat up more than it would with a smaller battery

Things are made cheaply, so they will just about do the job they were made for, but no more - I recently bought a cheap electric car tyre inflator, there are warnings you need to only use it for a shot while, then turn it off to cool, then you can use it again - yes it will inflate a tractor tyre, but not in one hit - it would overheat - the same may well go for the charger!

Sparks...

[The Natural Philosopher]

Actually there is a considerable amount of current limiting. Due to the fact that a battery that is off charge is - say - at 12.5v, where as it takes over 14V to charge it at any sort of current.

Go and do some real world tests on voltage versus current in both directions - charge and discharge - and then come back with figures to justify your statement.

Its not ideal to parallel batteries agreed, because its cheaper to get bigger ones and put them in series instead. However, it works.

No, that agian is untrue. At a given discharged satte, the chargingf curent on a big, or little, battery will be almost identical. Unless teh charger is rekyung in thermal inertia for its sole source of cooling, it will rise exponetntially to a temperature concomitant to its power dissipation and the rate it can get rid of heat.

A mass of concrtete surrounded by a ploystryrene box may take longer to get hot, than an empty box, but it doesn't get any less hot for the same continuous power input...

So you reckon that the alarm charger hasn't got fully hot in an 20 hours of trickle charging its battery?

Strange world you live in.

[Dave Plowman]

If this was the case, there would be a large spark when you coupled them together, but since the actual open circuit voltage difference is small this doesn't happen.

[Sparks]

If the flat battery is proper flat, at say 11v or lower, then applying a charged battery sitting at 12.8v or whatever will cause a lot of current to flow. - over 14v - why do alarm PSU's charge at 13.8v then?

OK, when I get a chance I will discharge a 1.2AH SLA battery, then connect it to a 38Ah fully charged SLA, and let you know...

Not ideal however it works.....with batteries of the same capacity

You are missing the point here...

Here is an example...

You place an empty saucepan on the stove You turn the stove on for 2 minutes The pan gets hot, but no damage is done.

You place an empty saucepan on the stove You turn the stove on for 2 hours The pan is now glowing red, and the handle may well have fallen off

The heat on for the extended period of time has caused damage, the same may be true of the components in the PSU if they are not designed for the prolonged high current charging cycle.

When the PSU is trickle charging, the components are doing less work, so don't get as hot, however long they are left on, so no.

I live in the real world, how about yourself?

Sparks...

[BigWallop]

From a fully charged battery to a lesser charged battery, there is a spark when you connect them together. The fuller charged battery then starts to recharge the lesser charged battery until they both become equalised.

Has anyone seen a charger unit that works from batteries, or is it just me. We have two units that we use to recharge remote batteries. They both work from two lead acid vehicle batteries connected in series to give an output high enough, and a duration long enough, to recharge the 12 volt units in the control panels. They have voltage regulators that only allow through

14.5 to 15 volts to the charger system. They are both capable of charging, from full, at least half a dozen 7 A/hr units on long life battery packs.

They both create a spark when they are connected.

[PJO]

320 years eh! Christ, that's before electricity itself was tammed!

Yes, please do. Piss off and learn some manners.

Oh. So why then do manufacturers of PSU's and chargers specify the maximum size of cell to be used??!

Here's one... take a flat car battery and place it on charge. What will the meter on the charger tell you? 3A?, 4A? Then it calmes down as the battery charges.

So, this guy places his 17Ah battery in parrallel and the mains fails for a prolomnged period. The batteries are used and then the power is restored. What typical current are you expecting the batteries to draw on initial charge?! Let me tell you it'll be more than the 1.2A that is the typical fuse rating in the charging circuit!

[usenet]

Any self-respecting battery charger will limit the maximimum current it delivers to a safe (for it) amount. It thus won't care whether you've connected a 10Ah or a 1000Ah battery to it, the maximum charging current will be the same. It'll just take a long time to charge big batteries.

[usenet]

It's referring to dissimilar batteries I would think as it's suggesting that if you have a number of battreries that you want to keep in good condition over the winter (for example) you can float charge them in parallel.

The battery capacity doesn't affect the voltage at all, a fully charged lead acid battery is the same voltage whether it's a 1Ah one or a 100Ah one.