Auxiliary Car Alarm/Battery

Hi All

I have a car alarm system which was recently defeated by a crook, by simply breaking off one of the battery cables from underneath the car. I’d like to shore this ridiculous vulnerability up by adding either
1. An auxiliary battery that would kick in and provide power to the alarm system if the main battery was disconnected OR
2. A second alarm system with battery and siren that would be triggered by a drop in voltage (to zero) from the main battery.

At first I thought a really simple solution would just be to plug a small SLA into the electrical system in the cabin, with a diode to prevent it from being charged at excessively high rates by the car’s electrical system. However, I quickly realised that the engine would probably draw from this small battery as well as the larger one when cranking. I suppose I could get around this with a switch that would allow me to disconnect the battery when starting the engine, but I know I’d forget half the time, which I guess destroy the battery pretty quickly.

Can anyone recommend a circuit that would monitor the main battery voltage, and trigger either a secondary alarm circuit or a relay or something to bring the auxiliary battery on-line if this voltage drops to zero? Preferably it should also draw as little current as possible in a stand-by state.

Thanks,
James

 

Sorry automotive modification topics are a "banned" item on this message board per the owners of the site.

 

Ok righto - I read the 'PLEASE READ - Posting Questions in the Projects Forum' before posting and didn't see any such rules. If a site admin reads this - perhaps a link to a broader rules page should be included in this readme?

I'm now looking at an unspecified 12VDC circuit on a different post

I just can't seem to post the post... Keep getting a 500 - Internal server error. :-/ Will try again later.

 

Playing devil's advocate, I am technically assuming that the OP isn't talking about modifying anything on a vehicle, merely creating a standalone self powered circuit which could potentially draw power from an automobile during recharging and how to isolate said circuit from said vehicle.

However if such an alarm system was factory installed by the vehicles OEM then it would probably cross the line and the thread should be locked.