It is my understanding that the siren alarm battery can't be replaced. It is self charging and only activates if the car's power is lost. There is almost no drain on the battery except for when it goes off. If some one was to cut the wires to the alarm, the siren would still sound. I saw this when they replaced my siren. When the power was disconnected, the alarm went off.

Do a search for Panic Button. There is alot of info on the systems involved and the modules involved.