Installed new battery backup for home alarm

[usenet]

Pardon, how can the charged battery (at 12.8 volts) cause "over 14v" to go into the discharged battery at 11v ?????

[Dave Plowman]

13.8v is correct for a gel type battery - you charge those at constant voltage.

[Dave Plowman]

But two batteries in series isn't the same thing. The reason you're doing this is to provide more volts to allow one '12v' unit to be fully charged. It's the same as a mains charger in principle.

If you *only* connect one fully charged battery in parallel with a flat one, the flat one will never be fully re-charged.

[The Natural Philosopher]

Your guess is as good as mine there Dave. Maybe for the same reason that Saniflo pumps are sold...

[Sparks]

Sorry, I have no idea how that got in there!

Sparks...

[The Natural Philosopher]

Dunno. Spent to long around Electrical and electronic stuff to know I guess :-)

[The Natural Philosopher]

Yes, but its not a huge current. If the capacities are similar, its really quite small. In the OP case it was a 7A/h and a 15A/h battery. Very close capacity wise. I have jumped a totally flat ride on mower from a big truck battery. Yes, there is a spark, no, it does not hurt.

Thats not the same. There is both a higher voltage in the unit, and a charger in between.

[The Natural Philosopher]

None of mine do. They do specify the minimum capacity tho.

Correct. That shows you the state of the battry not its capacity tho.

Otherwise a totally flat 10000000A/h battery would take a zillion amps wouldn't it.

In fact, it takes...3 or 4A...

No, it will be almost preciesly he same no matter what battery is on the end.

[Dave Plowman]

They specify both a maximum and minimum, and the maximum is somewhat over twice the minimum, so 7 and 17 would be ok as in this case with the correct charger.

Car batteries are not sealed gel types. You need a special charger for these.

Perhaps you don't know how to design a current limited charger?

[The Natural Philosopher]

He asked teh right question, but came up with teh wrong answer.

It takes 13.8v to charge a battery that will, on being charged, end up and about 12.5V. Batteries are not linear, and that's why it all works.

A battery under charge will show up to 14.4v roughly. The moment the charger is removed, it will be a tad over 12v. That drops away down to at worst about 10V for a battery discharged to the point of being wrecked if discharged any further. So the worst case of a fresh battery onto a flat one is 2v differental.

NOW you have to ask at what current a flat (10V) battery will charge, at an applied 12v. The answer is almost bugger all. It needs more like

13V to get really going. Thats why you don't get a huge flash and melted jump leqads when you jump star a car. You get a small flash and about (on a typical setup) 5 -10A flow on 80A/h cells. On a 7A/h setup that would be maybe 0.5A - 1A. Well within safety limits.

The charger now, will send current into the pair, and the more discharged batery will absorb teh bulk of it, since it needs less volts to charge. Eventualy things equalise, and it all looks like one big battery. Current is limited by the charger itself. They are not constant voltage devices. They are current limited constant voltage devices with a deliberate resistance in them, because that is what suits lead acid charging. They are designed to work into a load that looks like at worst, 10v in series with a couple of back to back diodes in paralellel...that being teh sort of voltage drop that an 'on charge' battery looks like versus an 'off charge' battery. If they blow up, its because someone shoved a dead short or worse opn their backsides, not a flat big battery.

[Crooksie]

cor a lot a folk have chucked there dolly out of the pram over this one, i stick bye what i said earlier it is not a good idea to parrallel the batteries. reason being that they do indeed equalize the voltage after connection, although possibillity of large current flow between them until this happens. the problem lies with the batteries connected in this way off charge, they will discharge each other until flat. this happens because of the slight differences of internal resistance and temperature effects. this always happens even with identical batteries.

however the larger battery will be ok to use due to the charger being a constant voltage type with current limiting. meaning the larger battery will take longer to charge, therefore the charger will be putting out its max current for longer. this should not be any problem.

the only valid point that has been made against the use of the single 17Ah is the possibility that the internal charger is of dubious quality i.e it may not stand up to the prolonged time it has to spend at max current. bob

[PJO]

No, I haven't a clue. What I do know though, after many years in the security systems business, that if you overload an alarm panel battery by installing one of larger capacity that the manufacturer states the fuse will likely pop or the PSU will burn out. I have seen this so many times and in some cases the damage to the panel has been beyond economical repair.

So I'm not basing my comments on calculations or a supreme knowledge of electronics but on over 20 years experience in the field and I think that matters more.

Control panel PSU's/charging circuits are generally nothing fancy and usually just incorporate the bare minimum of components to give a charge. OK, on some of the top end panels the PSU and charger are top dollar and will react to abuse accordingly. Some even have status reporting via the LCD. I think though that someone who is posting here will have something more basic and will therefore be running the risk of a burned out PSU.

[Peter Parry]

To be fair - nor do most alarm panel designers. Some of the designs are very crude and simply rely upon the resistance of the transformer windings. Whilst this is usually adequate for the battery specified if called upon to operate in this mode for a protracted period heat related failures are common.

[Dave Plowman]

But they'll never be off charge but be on float, apart from when in use during a mains failure. And paralleling lead acid batteries is quite sucessfull - it's Ni-Cads that tend to discharge rather quickly if you do.

[Dave Plowman]

Peter, if you try an charge a gel type battery - as all the alarms I've ever seen use - with the method you describe, it will have a very short life indeed. At the very least it needs a constant voltage charge, and since it is left permanently on charge a way of dropping this to a 'float' amount. And ICs to do this very job cost pennies these days.

[Sparks]

Capacity quite simalar?

I suppose a tad over 114% more is similar isn't it

[The Natural Philosopher]

Paralleling Nicads is even more successful. Its done as routine in aeromodelling circles.

[The Natural Philosopher]

Yes. a lot more similar than 500%. Which is wehat you get when you stick 200A/h battery

nto a 40A/h one to jump start it.

[Dave Plowman]

It will work, but they self discharge very quickly. Not a problem with a model which will be straight off the charger and into use.

You won't see a power tool etc with parallel connected batteries.

[The Natural Philosopher]

?

Why?. They are at the same voltage after all..

Its actually done in avionics supplies, for redundancy. Not on the motor side. If a pacck goes open circuit - and they do in IC powered models - wires fractrure etc - the second pack still allows control.