low battery shutdown for alarm?

I have a python 991 alarm in our vehicle which is sometimes parked for extended periods of time.

I was wondering if anyone had isolated their alarm system so it would be completely shut down when the battery voltage dropped below a certain threshold, to prevent killing the battery?

I'm sure it has been done....


Some use a 2nd battery (AGM, typically a 7AH for ~AUD$35) connected to the main alt/battery only when charging - eg, the "UIBI" - a common relay ~$3-$5 controlled by the charge-lamp; plus 2 circuit breakers (AUD$7 each).

Though that too should have LV protection.
Although circuits can be built (including an AUD$22 kit that only uses 50uA except when toggling its 80A latching relay), I'll commonly suggest the ~$20 MW728 "Battery Protector" that isolates at 11.2V (or higher with added diodes). But I've forgotten its standby/sensing current, and "connected" current. Either may be too high.   

MOSFET circuits may otherwise be the best for minimal added load, but that adjustable kit with 50uA stand-by current (or 500uA with its indicator LED) is hard to beat (see here).


Not that I know of alarm-specific solutions that others hereon would know about...

A switch on the LED, frankly the alarm itself draws about 5-10mamps when it's on, not enough to make ANY real world difference, the LED is the power crazy part, the flux capacitor hardly draws anything.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

oldspark wrote:

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Some use a 2nd battery (AGM, typically a 7AH for ~AUD$35) connected to the main alt/battery only when charging - eg, the "UIBI" - a common relay ~$3-$5 controlled by the charge-lamp; plus 2 circuit breakers (AUD$7 each).

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I actually already have a completely separate isolated full-size battery... (it connects to the main battery via a high-power relay triggered from the alternator signal). The alarm already can't kill the starter battery.   But as you noted, this needs protection too. After a few weeks, you can kill the whole thing.

I'll look into the MW728. I think it should be possible to make such a device that draws no current below a threshold.

wpegden wrote:

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...it connects to the main battery via a high-power relay triggered from the alternator signal...

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Wow! Where did you get that system from. (Just in case my FIGJAM isn't warranted, IMO your use of such a system warrants your own FIGJAM!)


Although it gets a bit ridiculous, the MW728 can be powered through a self-latching relay (a normal SPST relay with feedback, and 2 diodes to prevent feedthru) that drops itself AND the MW728 out below the LV threshold - hence NO current drain after the LV cut-out trips.
Simplest is to manually trigger it back on with a momentary closed push-button.

oldspark wrote:

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wpegden wrote:

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...it connects to the main battery via a high-power relay triggered from the alternator signal...

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Wow! Where did you get that system from.

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I installed it... this is a VW camper van so its nice to be able to use the lights, stereo, outlets, etc while camping without endangering the starter battery. The relay is one of these: https://www.newark.com/stancor/120-902/contactor-spst-no-15vdc-100a-bracket/dp/50F3663

The alternator has a little wire that signals to the dash that it's working---that's what's used for the relay.

Anyways, the van is sometimes parked for a couple weeks at a time, which can drain the whole aux battery from the alarm, which is obviously not great for the battery.


I like the self-latching relay idea...

LOL! I'll have to sue you for infringement of the UIBI (Patent Pending)!

That's a complimentary joke...
Why compliment? Because you must be a genius figuring something like that out...
For years people have been selling voltage sensors to "imitate" the same functionality. They consist of the same type of relay (well, usually ~80A rated) plus a simple voltage circuit but with a complex set of voltages, hysteresis, and delays to chose from. (For what? To try to determine WHEN the alternator is charging, but to drop our or not when cranking or turning on headlights or idling with load or....)
Some sell for $300 (150A) claiming "priority charging" of the main battery!!! (Since removed from Australian packaging though still present on their web video.)

Along comes some Aussie goon with the UIBI - the "Ultimate Intelligence Battery Isolator" and wipes the market for those that have a (alternator) charge light.

To add insult to injury, this entrepreneurial antipodean releases the info publicly (hence my jokes about Patent Pending).

I've watched as people try knocking/attacking the UIBI. (I've had my share too. The only winners for voltage sensing "Smart Isolation" systems are stator outputs as using in marine and some RTV/motorcycle applications - ie, no rotor-controlled alternator and no charge lamp circuit or equivalent. Otherwise why spend more than just a relay??


Apologies for the ramble wpegden, but the UIBI Initiator argued others have probably done the same... "it is an obvious solution if one happens upon it".

So now I know of another genius out there!    


20.8 coil resistance - 2/3 Amp. That could be a problem for some regulators (alternators) but not old mechanical units.

FYI - the "Initiator's" latest version - a mere $3 MOSFET that handles 120A and requires only uA from the charge-light circuit - no strain on any system!


I'll see if I can find some suitable LVCOs (LV Cut Outs).
My philosophy - the LVCO itself must cut out if triggered - you want to remove ALL drains from the battery! (Unlike a certain Spec that required "an LVCO LED" to show it was active (ie, has tripped due to LV)! That's what we call a de-energised NC relay/alarm contact!)