AC battery backup without ups possible?

  1. Hi,

    A local alarm company has given me a quote, and they want to connect the power to the alarm as follows:

    The 30VA 16VAC transformer is connected to a 7ah 12V battery.

    The battery apparently can run in AC mode, this was news for me.

    The alarm requires AC power, and is connected in parallel with the battery.

    They claim the alarm will run from the mains power if mains is on, but the battery will provide backup power when the mains are off.

    What concerns me is there is no charger for the battery in the setup. I don't know about AC charging, but wouldn't this setup cause the battery to permanently overcharge? Potentially be a fire hazard?
     
  2. jcsd
  3. mfb

    Staff: Mentor

    The battery could have something to prevent overcharging, or have a sufficient voltage rating to stop charging at some point.
    Just a guess, I don't know the devices.
     
  4. Ok, thanks for the response, I guessed that may be the case. The way I understand it the AC needs to be converted to DC for the battery to charge anyway.

    I unfortunately only have the description of the items, not actual skus or model numbers.

    Also, one would hope that their standard setup isn't something that could explode :-)
     
  5. I think they are oversimplifying things for you.
     
  6. davenn

    davenn 3,816
    Science Advisor
    Gold Member
    2014 Award

    Agreed.
    there has to be at minimum a rectifier on the output of the transformer to convert to DC
    Then if its a standard 12V 7Ah sealed LA or gel battery, there's also likely to be overcharging prevention circuitry

    cheers
    Dave
     
    Last edited: Jan 28, 2014
  7. Willing to bet the alarm REALLY runs on 12V DC....but needs AC (via the transformer) for continuous operation - there is a small rectifier in there somewhere.
     
  8. I won't take that bet!
     
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