car alarm system with (re-)chargeable backpup power source

Hello, I'm new here, but this forum has already helped me much with some project research.

So the thing is, somebody tried to steal my car at night the other week, and I've decided to create a fairly simple car alarm/anti-theft system using the Arduino Uno, which will nonetheless have a few little stings in its tail. Installing an OEM system would involve too much fiddling with the car's internal CAN bus architecture, and aftermarket anti-theft systems wouldn't quite give me what I have in mind.

The plan is to have the device activated as needed (and not automatically everytime the car is locked!) by a not so obvious control button in the dashboard. Not pressing the button again and deactivating the system before turning the ignition over will result in vital parts of the engine electrics simply being shut off by the Arduino through switching a series of engine management relays, so that the car will not move an inch. Also, it will set off the horn and the hazard lights. This should make my car a bit harder to steal off the curb without any commotion at 3am >:(

So far, so good, the programming side of it shouldn't be a big problem, I've got a solid background there. I've also got a working knowledge of electronic components and circuitry that will probably have to be expanded, but again won't be a big problem.

At present, the only real challenge with this project is that I would like the system to have a "zombie mode", which means that if a car thief triggers the alarm and tries to shut it off by disconnecting the car battery, the horn and lights will be unfazed and continue blaring and blinking for some time.

I just saw a relatively inexpensive 12V 1300 mAh rechargeable lead battery online (a Panasonic LC-R121R3PG) which should have enough juice to power a 12V horn and hazard lights for half an hour (although the plan is indeed to shut the alarm back off after 30 seconds either way to save me and my neighbors from potential sleepless nights if the system malfunctions :wink: ). The problem I've got now though is that my understanding and knowledge of battery charging is kind of limited, and I've got no idea whether or not I could (ideally) simply have the Arduino oversee the charging of that battery and at the same time rely on it as a power source when the car battery is disconnected.

From what I've been able to gather, it won't be enough to just wire the battery up and have the Arduino continuously checking the voltage and terminating the charging process when a certain voltage is reached?

Any thoughts on the feasibility of this part of the project will be greatly appreciated.