Battery backup for alarm, have battery

I have a few 12volt batteries from PC battery backups. I wold like to wire one of these batteries in so that it will power the alarm in the event that the car's battery is damaged.   However, I don't want the battery to power the rest of the car. How should I wire this battery into my alarm? -Chris

you need put a diodes on 12volt  to your alarm  and put your pc battery after the dioes they will work  . dioes only let the elcetrec go one way and your pc battery on power you alarm

Ok. Do I need to put diodes on the pos and neg or just teh neg?

Oh yes.... how the heck do I get this battery to recharge off the car's electrical system? I think this is teh general idea: For power to the alarm: Ground the neg from the battery to the chasis, no diodes there. Run a positive to the car alarm. Put a diode in the line for the car alarm, before where the PC pattery comes in. To recharge the battery: Run a pos line form the car to the battery, putting a dioded in the line to allow power to only flow to the battery for charging. How does that sound?