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alarm killing battery

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pierguppy

Marine/Ocean
Oct 31, 2001
4
hello
im trying to power a 120vac alarm system for our workboat.
the alarm realy needs 16vac (plug in transformer)
the current set up they want to use is as follows.
group 24 12vdc truck battery (with charger)
small power inverter (12vdc to 120 vac)
wall plug in trans (120 to 16 vac)
well all this is killing the battery overnight.
any ideas to get 16vac to the alarm board without all the ups and downs wasting our battery away?

john t
 
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Does the alarm system have its own 12V battery, like most intruder alarms? What does the alarm use the 16V AC for? I bet it rectifies it straight away and regulates it to 12 or 15V DC for the alarm circuitry. In which case, cut out all the (lossy) intermediate steps and either connect your battery to the alarm's battery input via a series diode to prevent backfeed, or modify the alarm to accept a power input from a DC/DC converter, where again I'd suggest a diode in series with the converter output to prevent backfeed.



-----------------------------------

Start each new day with a smile.

Get it over with.

 
now i just have to crack it open and decipher the board
fun fun.....
 
try adding replacing your battery with one that has more amp hours.
 
The total load wattage is not listed. It is 120VAC at how many watts? You say it is a "plug in" transformer so I am guessing it is "Class 2" or a 100VA power supply if you are in North America. Alarm systems require very little power to operate. Even big systems don't require huge sums of power. A class 2 system limited at 100VA output takes away a lot of liabilty and safety issues for the manufacturer of an alarm system. This is why I am guessing your power supply is Class2 type.
So 100VA/16V=6.25A AC. Let's say for arguement sakes that the DC current is approximately equal to 6A. The max. rectified DC voltage you can get with a simple full wave bridge rectifier is about 10V. So I guess that your power alarm system uses 9VDC@6A although the true current draw is probably less than this figure. To have capacity to operate for 12HRS you need about 6A*12HRS=72A of battery capacity. I am guessing that you might be able to get away with 50A-HR of battery capacity. A typical 6V "lantern" battery has about 25A-HR of capacity so you can imagine how much capacity a typical "car battery" has. So why is it that you can't run your system on 1 "truck" battery? Should be lots of capacity in 1 battery. Why is it you need 24 batterys? Am i missing some key information here?
Open up the alarm system and find out what DC voltage your system needs. Hopefully you can locate the rectifier portion of the alarm cct board.
Use a single truck battery for your system. Install a 12v to 9v convertor and connect your battery to your alarm system and you should be ok. Ensure that the converter uses low power to perform it's task. You don't need to supply unwanted power losses.
The problem I think you will have is charging the battery properly. With a 50% duty cycle the battery may not have enough time to recharge or recover fully. Batterys are like people - they need adequate recovery time. If they don't get this recovery time they will eventually collapse and die from fatigue. I don't know if you can get a battery that can fully recharge in 12 hrs AND have a long and useful life. I bet such a battery would be expensive. And no, a "deep cycle" marine battery won't work in this application either. They die as well. I know from experience.
The easier way would be to use 2 batterys and rotate them on a daily basis. Simple timers could switch over the batterys from charge to discharge states evry other day.
Right now all you are doing is supplying power losses to rectifiers and inverters and what not. Not to mention the internal power losses due to the "paralleling" of 24 batterys.
There is an easier way.
 
Oh yes. Really good commercially available cheap "trickle" chargers exist for deep cycle marine type batterys. They monitor and check the "health" of your battery and charge them at rate that gives maximum life to the battery. These are "smart" chargers that only charge your battery when they need it. Use them. They save you a fortune in batterys and they are generally inexpensive to purchase. If you charge your batterys "manually" with a car charger they will not last as long as when use an automatic "trickle" charger.
 

i m not sure of the wattage needed by the alarm yet. its been a while since ive been down there...
oh ..it not 24 batteries we have a "group 24"battery about the size of 2 standard car batteries
it has a trickle charger that operates during working hours (8-9 hours a day) maybe not enough charge time..
it realy sounds like ive got to get to the board for more info
thanx ill let you guys know....
 
I bet this alarm takes way less than 1/4 amp untill it has to set off an "alarm." I would cut the wall transformer cord and just hook that up to the battery. You can't do any damage and chances are that it will work just fine. You never stated if the alarm itself has a backup battery.
 
Yes you are probably right Operahouse. The rectifier will just appear to be fwd bias diodes and I am sure there will be some sort of voltage regulator (zener) in the alarm cct board itself. Great simple, idea and nothing to lose.
 
S'funny, I think the first reply said more-or-less that. Deja vu or what?

-----------------------------------

Start each new day with a smile.

Get it over with.

 
I should have added to the statement, "can't do any damage" that the battery should not be supplying power to anything else. If this battery is also supplying power to lamps it is also likely grounded to the boat, if conductive. The alarm would also likely have a ground connection. Most systems are negative ground so in almost all cases that will not cause a problem. If the battery was randomly connected positive ground, that could damage the rectifier by creating a short. If the electronics and battery negative were grounded, connecting both wires together of the transformer wire and connected to the battery positive could be the prefered method by reducing diode drop.

I have a wireless patio speaker that has a 16 vac transformer and internal 12V battery. Obviously, the prefered power source is the transformer to provise higher audio output from higher supply voltage.
 
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