AC battery backup without ups possible?

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Discussion Overview

The discussion revolves around the feasibility and safety of using a 7Ah 12V battery as a backup power source for an alarm system that requires AC power. Participants explore the implications of connecting the battery to an AC transformer without a dedicated charger, raising concerns about potential overcharging and fire hazards.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses concern that the setup might lead to overcharging the battery, posing a fire hazard, due to the absence of a charger.
  • Another participant suggests that the battery might have built-in overcharging prevention or a sufficient voltage rating to avoid overcharging.
  • It is noted that AC power would need to be converted to DC for the battery to charge, indicating a potential requirement for additional components.
  • Some participants agree that the alarm system likely runs on 12V DC, implying the need for a rectifier to convert AC to DC for proper operation.
  • One participant humorously declines to bet on the certainty of the alarm's power requirements, indicating uncertainty in the claims made by the alarm company.

Areas of Agreement / Disagreement

Participants generally express skepticism about the alarm company's setup, with multiple competing views on the necessity of additional components like a rectifier and the safety of the proposed configuration. The discussion remains unresolved regarding the specifics of the battery's charging mechanism and safety features.

Contextual Notes

There are limitations in the discussion due to the lack of specific product details, such as model numbers or specifications of the transformer and battery, which could affect the analysis of the setup's safety and functionality.

zefram12
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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?
 
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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.
 
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 :-)
 
I think they are oversimplifying things for you.
 
Averagesupernova said:
I think they are oversimplifying things for you.

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:
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.
 
I won't take that bet!
 

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