Security system battery question?
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  1. #1
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    3 weeks ago when Charlie hit, we were without power for 6 days. My ADT security system has a battery backup, but it is only good for about 12 hours.

    The battery is a small sealed lead acid 12V 4AH.

    I called ADT and asked a 'tech' if I could hook a 12V car battery to it, and I got a response of 'uh...no, it has too many amps.'

    Now I am not the smartest electronics guy in the world by far, but I don't buy his response. The alarm is only going to draw the current that it needs, which is very little, right?

    We are preparing for this hurricane Francis, which is headed straight for us, and I assume that we will be without powere for an extended time again. I don't like having the alarm down.

    12V dc is 12V dc, am I wrong?

    Thanks-Hanz

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  3. #2
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    You are correct, the tech was an android afraid to say anything other than standard issue was ok.

    Or maybe he was just an idiot

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  5. #3
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    The Home Depot now carries those small gell cell batteries. A small garden or motorcycle battery should work fine in the short run

    Randy

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    There is a possibility that a higher amp hour battery could overload the charging circuit in the alarm if the battery was significantly discharged. It will take a 20 amp hour battery a lot longer to come up to voltage than a 4 amp hour and the charger might overheat depending on design.

  7. #5
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    One thing to look out for though is that a battery has internal resistance. If the car battery has less internal resistance than the alarm battery had, it is possible to draw too much current and harm the system. This is a rarity when dealing with lead acid, but it has happened to me.
    If you wish to be on the safe side, a current mirror circuit (with a switch system to allow higher current when the alarm is triggered)is the perfect way to ensure that your system will not be overloaded. But then again, if you do not have power now, it would be a pain in nevermind to build and tweak it.
    You should be fine with the car battery, and you could hook up two in parallel to increase the amp hours.
    You could probably even use your car to charge up the previous battery by hooking it to your cigarette light and charging it while you drive (when the system is 13.whatever volts) It would take a while, and I cannot remember the equation off the top of my head for evaluating correct charge time, but the slower the better. You could probably just hook it up and drive around for a while, check the voltage of the battery out of circuit until it has a nice steady 12 to 13 volts.

    (OT: Oh yeah, stock up on supplies for your hurricane party. We used to make bets on which tree would go down in x-time. A few bottles and no worries! Having the Eye pass right over you is a pretty sight not to miss.)

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    Thanks guys for reassuring me.

    Evan, what I will do to prevent overloading the charger is disconnect the 110 going to the box (if we do loose power,) and then when the power comes back on I will manually reconnect it.

    Jimm- what I did for the first couple days was take the ADT battery home and charge it with my auto trickle charger, and bring it back to the shop in the evening and reinstall it. But then when the gas shortage came and I was getting low I decided to conserve what gas I had.

    This one looks like it could be really bad.

    Hanz

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    The power supply of the alarm will work fine with the car batt. One word of caution would be that car batteries are seldom 12vdc on the nutt. They vary b/t 14 (or higher) on the high to 10 on the low. I don't know if the power supplies regulator can adjust for the variation in voltage but my guess would be yes. But as for current draw the power supply wont know the diff.

    Evan: Unless I missed something, what are you talking about re: charging system. I am assumin the charger for the back up batt. Is it a recharging batt. Is there a charger there? If so, make sure the 120v house line is physically disconnected from the alarm so the charger wont be trying to "fill the car batt. JR

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    One quickie, If after the hurricane, security companies may get the message, but not be able to get a police dispatch out. I would invest in heavy locks and chain and bar the windows. Most looters would not be concerned with police response and would do a quick smash and grab. But if you make it physically impropable for them to get in, they will most likely just bypass your shop.

  11. #9
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    I am talking about the charging system in the alarm. It keeps the backup battery trickle charged while the alarm is running from AC. If the backup battery is used due to line power failure the gell cell backup battery is discharged to operate the alarm. The lower the voltage of the lead-acid gell cell the greater the charge current will be when the AC comes back on. If the alarm charging system for the backup battery doesn't regulate this current actively then it may be possible to damage the charging circuit by attaching a larger battery. The larger battery will take much longer to come up to full charge after a failure of AC. This could overload the alarm charging circuit, as I said, depending on design.

    Lead acid batteries all have the same nominal cell voltage, two volts DC per cell. Under charge they develop a so called "surface charge". A nominal 12V battery will go up to around 13.5 volts when fully charged with charging current removed. Under charge it will go up to around 14.7 volts. The closer to that voltage the lower the charging current. A discharged battery will appear as a short circuit to the charger until the voltage rises. How much of a problem this is depends on how "smart" the charging circuit is. It wouldn't suprise me at all if the alarm charging circuit is designed expressly to use only a 4 amp hour battery.

  12. #10
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    There's no problem with the voltage of the battery, regardless of type, as others have said. What will happen is that the larger A/H battery's voltage won't rise to as high a level as the smaller gel cell's will under the same charge current. This can mean that the charger will attempt to output a higher current level into the larger battery, and the battery will accept that for a longer time, so some overheating of the charger could occur. If the charger is designed to feed some maximun current until it's battery comes up to full charge, then it will output the same for the larger battery as well. The difference is it will take longer to charge the larger battery. I would expect that if it takes an hour for the gel cell to charge fully, then the charger is capable of continuous operation at that charge current level, and it will be safe to charge the larger battery as well. It will just be working for a longer period of time.

    What you can do to be on the safe side is instal a rectifier/resistor combo inline to the positive lead to the larger battery. Use about a half-ohm resistor in parallel with a rectifier (diode). The rectifier feeds the current around the resistor when it's powering the emergency lights, and the resistor simulates the internal resistance of the smaller gel cell, so the charger is protected from outputting too large a current into the larger battery. With it's current level under control that way, it can safely operate to charge the battery for a longer peoiod without overheating or damage, and you won't have to know the charging characteristics of the unit. The small loss of voltage caused by the rectifier when the battery is powering lights is not going to make much difference, the lights will just be a little dimmer. The rectifier is oriented to pass current from the battery to the circuit board, from where a relay sends power to lights. (the arrow on the rectifier points away from the battery positive lead). It blocks charging current, so that must flow through the limiting resistor. An alternator diode would be suitable, as long as it doesn't overheat while powering the lights. Some small heatsink will do. A half ohm, 5 watt resistor should be easy to find.

  13. #11
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    Also I want to clarify my above statement. When I say "power supply" I am referring the power supply of the alarm's electronic circuit not the charging circuit. Didn't know if I confused the issue. JR

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    Clarify, yes, mee too. Partway through my encyclopic explanation, I started to assume it's a power failure system you're dealing with, and not just an alarm system. Right you are, there's not much current draw from an alarm system, the bulk of it being the sirem when it's sounding. Less than an amp total, in almost every case. My idea of the limiting resistor will work fine, but you can eliminate the diode, as it's function is to prevent voltage loss to the circuit during a large draw from the battery, as would be the case with an emergency lighting system. What I have suggested is something I have done to extend the duration of an emergency light system, which is also an alarm system in my case. For anyone considering adding a larger battery, or replacing the existing one on any alarm or emergency light system, the above description of a safe way to do it still stands. The only difference is whether to include the diode or not.
    You could always just keep a charged battery standing by, wired to the system using just an on-off switch. Then you can control charging that battery any way you normally would. Once it's charged up, adding it to the system by switching on won't tax the alarm circuitry at all. In fact, the internal battery may get some of it's charge from the added battery. Wire the resitor in anyway to keep that current under control.

  15. #13
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    I am late joining you but i have the same problem, with pg$e cutting off the power my new battery on the alarm only works for twelve hours, i wanted to put in a car battery and the answer was it has too much amperage, meanwell makes a 12vdc charger and power supply, it switches automatically when the power goes off to battery power and charges when power comes back on, and it is true 12vdc, i just looked for hiher amperage supplies and this popped up, im going to try this out, any suggestions on batteries? Deep cycle? Thanks
    https://www.amazon.com/SUPERNIGHT-Sw...2218037&sr=8-8

  16. #14
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    As long as the alternative battery has matching voltage and equal or greater amperage, the battery will work. I even went the extreme of connecting up a 12VDC power supply which was fed from my very large computer UPSs to my fire and burglar alarm panels and it worked fine, this gives me many days of backup.


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