Battery backup

No you need and isolation charging unit to seperate the backup from the main battery. Please use the search function to review old posts. https://www.the12volt.com/installbay/search.asp?KW=battery+backup&SM=1&SI=TC&FM=3&OB=1Steve G

Well if you want it to be in parallel then it won't be a backup but rather just extra amperage. It won't stay idependent of the main. It would do the same thing as the main would so if headlights were left on all night they would both eventually drain. It would just take a longer time to drain them. Most often people use a backup in case the main dies or is disabled by a would-be thief to continue powering the alarm system no matter what. The setup your talking about would be more for car's with large audio amps that need extra power so they don't rob the normal vehicle's electrical system of it's needs. Usually you'll want to replace your stock alternator as well to pump out the extra juice that the amps need. I guess it all depends on what you are trying to accomplish with the extra battery.Steve G

I understand that being in parallel, both batteries will supply current  IF no wires are cut. However, if a thief gets around the siren and hood trigger, then cuts the battery cable, the battery in the back of the truck (with its own ground) acts as a backup since it is still powering the alarm.....correct? Ken

On my install I used a 4.5ah sealed battery that I got at the hobby shop I work at. what I did was I wired it to key on power, and then put a doide to isolate it from the main battery. this way it will charge when the car is running but physically isolated from the main power system when the car is off (a theif can pull every wire under the dash and the alarm will still continue to go off).