what is your goal? Adding an extra battery shouldn't 'kill' your alternator, unless it is nearly dead now.

If you just want extra electrical storage, you can wire them in parallel, or get a relay like dalton said.

If you are looking for extra reserve capacity, like playing the system with the car not running, and want to ensure that your main battery can still start the car, then you need some sort of real isolation...either a diode based alternator, or a smart relay setup.

I have used all of these methods, and can get you the parts for any of them.

Toby