Installed new battery backup for home alarm

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Sparks wrote:

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.

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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

if
it
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.

made
it
tyre,
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...
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Pardon, how can the charged battery (at 12.8 volts) cause "over 14v" to go into the discharged battery at 11v ?????
--
Chris Green ( snipped-for-privacy@x-1.net)

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wrote:

current
Sorry, I have no idea how that got in there!
Sparks...
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snipped-for-privacy@isbd.co.uk wrote:

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.
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13.8v is correct for a gel type battery - you charge those at constant voltage.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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You can't really "charge those at constant voltage", if you applied a constant voltage source of 13.8 volts to a discharged battery you would get a very high current flowing which could damage the charger and/or the battery. All battery chargers current limit by some means or another. If designed correctly they current limit at a level which will neither charge the battery too fast nor damage the charger. Then, when the battery approaches full charge, they become constant voltage sources of 13.8 volts (hopefully temperature compensated) and keep the battery charged with a 'float' charge.
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Sparks wrote:

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

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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.
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wrote:

so
for
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.
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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.
--
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BigWallop wrote:

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.

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wrote:

spark
to
Capacity quite simalar?
<sarcasm> I suppose a tad over 114% more is similar isn't it </sarcasm>
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Sparks wrote:

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.
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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.
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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
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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.
--
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Dave Plowman wrote:

Paralleling Nicads is even more successful. Its done as routine in aeromodelling circles.
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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.
--
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Dave Plowman wrote:

?
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.

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