12 battery isolation in live circuit

Hi all. I need a circuit to isolate a 12v battery from overcharging whilst the power supply ALSO supports my home security system via a 12v to 240v inverter. What I have is:

. a 12v battery also connected to the charger such that should the charger fail the battery will continue to support the inverter in real time.

I was thinking of a diode on the battery positive and a second smart charger charging the battery but I guess the battery will still feed into the inverter and thus the second charger will still continually chage the battery and it will eventually still overcharge.

Any help with a circuit diagram would be most appreciated.

I think you can consider buying/making a battery charge controller circuit.

Thanks for that. Indeed your correct but I have no idea what to buy/make hence my question here.

Please suggest something to purchase by brand name/model or offer a circuit diagram so I can make one.

I think I have been misunderstood given the only response clearly did not address my concern.
If forum members took the time to read and UNDERSTAND my issue you would realise it isn’t as straightforward and the one response I received suggests.
I can solve my problem by putting a timeswitch on the power to the charger and partially running down the battery but this is just a workaround hence my request for help here.

You’ll get more replies with honey rather than the saltiness, however, it’s not very clear what problem that you are trying to solve.
A schematic would help your story greatly and give everyone an idea as to what the issue is.

If you have a ‘proper’ 12v charger then it wont overcharge the battery, but without you going in to more detail as to:

  • what your current setup is
  • what the outcome that you want to achieve
  • what problem that you are trying to solve

then it is going to be next to impossible for anyone to help.

FYI, you’ve just posted your first post in a relatively small community, there’s simply not hundreds of people waiting around to solve every issue that someone raises.
Others will be by in time, you have to give them more than a week sometimes.

Brilliant, thank you for that.

I thought I described my problem fairly well but I agree, it’s hard to visualise from words alone.

My setup uses a smart charger but it is the permanent power supply to my security system AND it charges the battery thus the charger always sees the load and the battery is thus always on charge. I use this arrangement do that if the power grid fails, the battery keeps the security system running.
I would have thought this a common arrangement with circuits that have a battery backup. They dont overcharge so there must be a circuit out there.

Sorry if I posted in the wrong place.

Hi Wally
That is a bit clearer.

Andrew’s statement is true.
And your statement is equally true except for

Have never heard of one that doesn’t use a simple switch.
I think the solution is to get the correct type of battery for a start. A Lead Acid flooded battery like an automotive type WILL NOT do this job. You need a deep cycle battery that has been designed for constant float charging. Then a charger to suit that will accommodate the particular chemistry of the chosen battery. It is possible your existing smart charger will satisfy a number of different battery types and will do the job.

You are correct here, this is a common arrangement for all different size situations and if the correct battery AND charger is used there will be no overcharging.

I (we) don’t know exactly what your change over arrangements in the event of a power failure but there would be a couple of ways to do this. Switch to the battery/inverter combination when the power fails which will have a slight interruption while the switching takes place. Or have the battery/inverter running the security all the time which will be a no break system. If the former the battery/inverter will be isolated while mains power is available and as stated with the correct battery and charger there will be no over charging. If the latter there is one thing that is sometimes overlooked. When the battery has been in use after a mains interruption the charger will have to supply the power to operate the system PLUS the power to replenish the battery. This requires a somewhat larger charger than in the first scenario
Cheers Bob

OK, but doesnt the security system have this capacity built in?
I dont think that I have ever seen one that didnt have a ‘gell cell’ battery.
These are so ubiquitous to a security setup that they are literally called ‘security batteries’ as a colloquial name.

The small 12v 7Ah sealed ‘gel cell’ batteries were designed for this task, however Ive never seen a security system with a manual switch.

Basically you need a 12v UPS

Hi Andrew

I haven’t either but is about the only way I can think of off hand to do this.

All systems I have seen use your “proper” battery and charger.
Cheers Bob

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Thank you Bob.

My usage is the latter of your scenarios ie " If the latter there is one thing that is sometimes overlooked. When the battery has been in use after a mains interruption the charger will have to supply the power to operate the system PLUS the power to replenish the battery. This requires a somewhat larger charger than in the first scenario"

My charger is able to supply 30 amps and so can do this comfortably.

I am using a lead acid car battery (actually out of a D9 dozer) but, as I indicated in an earlier email, I now have the charger itself on a timer thus the battery can at least not be on a constant charge but be available at all times to run the inverter.

Thanks again and regards

OK that finally sunk in. This is why I asked for a schematic or basic drawing of what you have and how it is connected.
Why are you running an inverter to power the security system?
Surely it a 12v type?

With that battery you are clearly expecting it to have to run for extended periods of time without mains power, however it is not the best suited to the job.
Equipment lead acid batteries are expected to deliver a LOT of AMPS quickly, such as in a cranking situation and not very good at running for extended periods of time with low load.

I think that you need to re-think the application, design and execution of this setup as it seems that no part of it is fit for purpose.

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Hi Andrew

I would have to agree with that.
Personal experience with auto type batteries float charged to operate radio sets back in 1971.
All these different built for purpose were not around or very common then and the best we could do was about 6 months or in one case 3months use under these conditions.

Only tip I can give you here with that set up is test on a routine basis. Or you may find that the power has been switched off or failed and someone breaks in and your system has run out of battery unbeknown to you.
Cheers Bob

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Thank you all for your help.

A UPS is probably best but a small one won’t support the recorder for long enough. I’m hoping to get 20 or so hours uptime out of it. If only I could get my hands on the circuit used in one of thos to isolate it’s battery.

As for gel batteries in the recorder, there are none. I have a cheap Aldi 8 cam recorder set-up so it’s all pretty basic really.

Hi Wally
Terminology, ah what a minefield. Common use for a UPS is a supply to hold up a computer or such long enough to allow safe shut down, usually up to 15 or 30 minutes. That is by understanding anyway.

A back up supply is designed to power a system for some extended period, hours, days or even weeks. This is the type of system you are after I believe.

Some systems can be switched while others are required to be “no break”. The switched systems will have a short outage while the switching takes place while the other is self explanatory.

The largest “no break” system I actually installed was a Tropposcatter system of 2 transmitters and 4 receivers. The supply was 24V battery float charged. The requirement was to power the system 24/7 for 2 weeks without mains power.This consisted of 2 battery banks 24V each @ 2000Ahour each, total 24V @ 4000Ahour. Float charged with an enormous Westinghouse charger. Battery banks were made up of 2V cells each 2000Ahour, each cell held 3 GALLONS of electrolyte.

Your system of course is nothing like that but similar points apply.

Does your equipment operate off 240V only or can it be powered with the 12V, that is does it require 240V straight in or is it powered by plug packs. This can make a difference to what you can do.

In deciding a battery size you will need to ascertain the equipment power requirements (Watts @ what volts), Also what, if any inverter you have as efficiency will be lost here. The battery size depends so much on how you need to set this up as well as actual power requirement.

The battery and charger are most important and if you want some semblance of reliability do not go on the cheap and nasty path here. Experience tells me you will be very disappointed somewhere down the track when things don’t work when you expect them to.
Cheers Bob

Hi Wally
If that security system from Aldi is the “Cocoon” model it requires 12V @ 3A for the cameras and 12V @ 2A for the recorder provided it seems from 2 plug packs. I found the manual for this one on line.
If this is the case you should be able to run the whole thing from your 12V battery and float charge it without any 12 to 240V inverter.

BUT. Allowing a worst case of 5A you will need in excess of 100Ahour battery to run 20hours which for a “proper” battery could be quite expensive.
Cheers Bob