I myself have a mini piezo siren inside of my car. Its 128 dbz of ear bleeding sound. I use to think, ah hell, my stereo easily does over 140 dbs. But its the high pitched 128 dbs that will get you. It confuses the thief. I know it makes me figit a lil. The siren is very very small. It is meant to be inside of the cabin of the car so when the thief actualy makes entry into the car.
As for the back up battery, get it. Its great. I have one myself. Its great when the thief actualy makes entry into the car, pops the hood, and cuts the battery cables. The battery will add about 10 pounds to your ride //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif