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Designing a Battery Backup for a security system

Discussion in 'General Electronics Discussion' started by theturkeydancer, Feb 23, 2018.

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  1. theturkeydancer


    Feb 23, 2018
    First post:

    I am trying to use a relay as an automatic transfer switch for a large 12v battery to power my security system when the power goes out.

    I have worked with relays before, and thought I was comfortable with them. Then I started reading things about mixing AC triggers with DC loads, and voltage spikes damaging upstream components, and I got paranoid about the section of the circuit pertaining to the coil on the relay.

    Can someone please assure me that this circuit will work without issue? Thanks:

  2. Kiwi


    Jan 28, 2013
    My suggestion would be to discard the factory 7.5v supply and just use the 12v battery, charger, and 12v-7.5v converter.
    Charger size should be increased from 500mA to 3A.
    May be a good idea to use a multi-stage charger to get the best battery life.

    Power on.
    The charger will supply the alarm through the converter, and will also maintain the battery charge.

    Power off.
    The battery will supply the alarm through the converter.

    Power back on.
    The charger will supply the alarm through the converter, and will also recharge the battery.
  3. ChosunOne


    Jun 20, 2010
    What's the manufacturer and model of your security system control panel?
  4. twister


    Feb 12, 2012
    Looks good to me.
  5. Bluejets


    Oct 5, 2014
    What is the problem with the original backup battery supply?
  6. ChosunOne


    Jun 20, 2010
    I'd still like an answer about the make and model of your control unit, but I see a potential problem with your design:

    Back in the 1950s, maybe as late as the '60's, professional-grade alarm equipment manufacturers used a similar "switch-over" power-failure protocol. I.e., when 120V line power failed, the panel would use a relay to switch to a battery backup, (That was back in the day that backup batteries were most often primary carbon-zinc, not rechargable).

    The reason they stopped using that system was that it caused false alarms and other erratic behavior every time there was a hiccup in the power grid. A mechanical electromagnetic relay (otherwise known as "a relay") operates very slowly by electronic standards. Most control panels that are left without power for an entire millisecond experience it the same way they'd see an extended power-out, and may go into alarm on all zones when they boot back up. Of course, not knowing the make and model of your system, I can't guess what yours would do.

    Kiwi gave good advice. If you're unhappy with the backup capacity of your existing backup battery, you would do better to disconnect it and use just the bigger one.

    HOWEVER: You do need to know what kind of backup battery your system has, and where it's located. (It's bound to have one somewhere.) For example, if the backup battery is located in the external power supply ( and yes, I've seen systems like that), then a 120VAC line failure won't cause an immediate 2.5V(DC?) loss in the power supply output, and your add-on supply might wind up feeding into your existing battery, depending on how it's configured.

    Again, if I were you, I'd follow kiwi's advice and just replace the power supply instead of augmenting it.
  7. theturkeydancer


    Feb 23, 2018
    thanks for the suggestions. the security system is a simplisafe 2 base station, and it has replaceable backup batteries located in the bottom of the unit: 4 1000ma 1.2v NImh double-As. yes, the power supply is 7.5v.. I can only assume that when power goes out it is able to perform it's duties using 4.8v from the backup batteries, perhaps at reduced consumption. during a blackout, the company texts us to let us know the base station sensed an outage, and those AA's are supposed to last about a day.

    I thought of jumping in a bunch more identical AA's in parallel banks of 4, but I decided against it. It would look terrible in the pantry, and cost as much money for 5 days of backup as an AGM would for 20-30. I would like to have about a month of backup. our normally safe neighborhood got pretty dicey during the last hurricane.

    chosun: since the base station has an integrated battery for power outages, i'm not worried about the long-ish trigger time of a conventional relay. it will "see" something similar to a brownout, and probably won't even text us.

    what I am worried about though are the voltage spikes that I read about online while the coil in a relay de-energizes, and that goes over my head (I'm a DIYer, not an engineer!)

    kiwi: i thought of running the security system just off a charger. I appreciate the elegance of doing it that way, but this setup is at my parent’s house, not mine, so if anything goes wrong, I have to drag myself down there to fix it. This is what led me to think it best to use a relay. that way, when the battery fails (or the dc converter), the system will still work normally.

    I have several concerns, all of which are unfounded (I made them up!).

    1: how reliable (for continuous use) are cheap $7 dc converters? And how well do they regulate voltage? if the voltage isn’t as stable as the stock switching power supply, could that effect the longevity of my base station?

    2: if (when!) the battery fails, what effect will that have on the charger? will it revert to just being a power supply that outputs the float voltage? will the current demands of the base station cause voltage fluctuations from the battery and confuse the charger into switching back and forth between ~14v charge and ~12v float?

    3: if (when!) the battery fails, what effect will that have on the base station? during sporadic periods of maximum current demand (like when it calls the cops), what will happen between the failed battery and confused charger?

    These seem like unknowns to me. Even if they are very likely to be moot.. this is a mission-critical piece of equipment, not a stereo or whatever.
  8. Bluejets


    Oct 5, 2014
    I don't see why you need a "bunch of AA's in parallel. Just put back new of whatever was there before.

    I'm sure the designers knew what they were doing.
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