backup siren from 2nd truck battery

Alarm system - Audiovox 997a. with Remote Start. Question. Adding siren\ horn back up. I have two full size batteries in my suburban. I wanted to add a siren or horn to the seconded battery when the alarm is triggered. Just in case someone cuts the siren wire. That’s if a thief can get to it.    My seconded battery is isolated from the main battery when the truck is off. So I figured this would make a good 2nd power source. I did install and design this dual battery system for my truck. The GM factory version had to many flaws , no relay and no switch for control of the batteries. I know DEI makes one, but I don’t want to put that in my truck when I have a 2nf full size battery I could use. Also I’m going get power from the 2nd battery for an additional Shock sensor and Glass Break sensor I’m installing. The rest of the devices ( one shock sensor ) and alarm unit is wired up to the trucks main power \ battery. I’m not sure how I would trigger the 2nd alarm siren though ; or do I really need another alarm siren? Because, the alarm also uses the trucks horn at the same time. Maybe, I should use the 2nd battery as backup power for the main alarm if it can be done.if so how?

Use a relay on the 2nd battery to connect the siren. The relay is controlled by the alarm. You could also use an alarm controlled relay to connect the 2nd battery to the alarm. It should be connected to be self latching. The battery interconnection depends on how it is wired etc (but I assume a common ground). And it depends on the reason for the 2nd battery. If it's to guard against disconnection, cool. If it's to be like a satellite "fail-safe" siren, then many NC contacts used to the 2nd siren. If its to guard against a discharged or shorted battery, then you need a low-voltage disconnect. And if it's in case the siren discharges the battery, I assume the siren will cut out in accordance with local regulations (ie, our EPA dictates 10 minutes depending on how it was triggered).

Thanks for the great information. It sure helped me out.